INT. J. SCI. EDUC.,
2000, VOL. 22,
NO.
11, 1171± 1200
RESEARCH REPORT
Students’ reasoning about basic chemical
thermodynamics and chemical bonding: what
changes occur during a context-based post-16
chemistry course?
Vanessa Barker, Science and Technology Group, University of London
Institute of Education, London WC1H 0AL; e-mail: [email protected] and
Robin Millar, Department of Educational Studies, University of York, York
YO10 5DD; e-mail: [email protected]
A longitudinal study of 250 students following the Salters Advanced Chemistry (SAC) course probed a
range of chemical ideas including the exothermicity of bond formation and the development of thinking
about covalent, ionic and intermolecular bonds. Students responded to the same diagnostic questions on
three occasions: at the start, after eight months and sixteen months of a twenty-month course. At the
start, many students demonstrated misunderstandings about these chemical ideas, but in general their
understanding improved as the course progressed. By the end of the study, about half knew that bond
making is exothermic. Initially, few described covalent bonds accurately or understood hydrogen bonding. A majority gave responses at the final survey which were in line with ideas and language a chemist
may use. Students attributed changes to the use of context-based materials including a drip-feed
approach which allowed their understanding to develop over time. However, some aspects of chemical
bonding, including ionic bonding and intermolecular bonds other than hydrogen bonds remained
problematic for students despite explicit teaching. The findings have implications for post-16 chemistry
teaching, suggesting that a review of teaching strategies is needed in some areas.
Introduction
In recent years, courses adopting a context-based approach to science teaching at
the secondary (11- 18 year old) level have been developed in several different
countries attracting international attention. By presenting scientific concepts in
everyday situations, course authors aim to promote students’ enthusiasm and
motivation for science. The Salters project in the UK has involved development
of Salters’ Chemistry (SC), Salters’ Science (SS), Salters Advanced Chemistry
(SAC) and currently under development, Salters Horners Advanced Physics
(SHAP) (University of York Science Education Group 1989, 1990± 2, Burton
et al. 1994a, 1994b, 1994c and 1994d, Swinbank 1997, Edexcel Foundation
1998). Other examples include The Supported Learning in Physics Project
(Whitelegg and Edwards 1997), ChemComm (American Chemical Society
1988), the Chemical Education for Public Understanding Programme (CEPUP)
(1991) and PRIME Science (The PRIME Science Education Group 1998) in the
US, and the Physics Curriculum Development Project (PLON) (Eijkelhof and
International Journal of Science Education ISSN 0950- 0693 print/ISSN 1464-5289 online # 2000 Taylor & Francis Ltd
http://www.tandf.co.uk/journals
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V. BARKER AND R. MILLAR
Kortland 1988) in the Netherlands. The approach raises questions about the effectiveness of learning science in this context-led way rather than through the more
usual form of curriculum organization and sequencing based on the major divisions and sub-divisions of the science subject. This study, completed in 1995
(Barker 1994 and Barker and Millar 1996a, 1996b and 1996c), focused on students
following the Salters Advanced Chemistry (SAC) course and sought answers to
two research questions: What level of understanding do beginning A level students
have about basic chemical ideas? and in what ways is student learning influenced
by the context-theory approach?
Their thinking about a variety of basic chemical ideas was investigated. This
article presents data on students’ understanding of basic thermodynamics and
chemical bonding. An earlier paper (Barker and Millar 1999) reported findings
concerning students’ thinking about other aspects of chemical reactions.
The Salters Advanced Chemistry (SAC) course
At the age of 16 students in England and Wales select usually three subjects for
study at Advanced (A) level. A level courses take about 20 months to complete and
are regarded as preparation for university entrance. SAC (Burton et al. 1994a,
1994b, 1994c and 1994d) was developed by the University of York Science
Education Group to provide a stimulating account of chemistry for 16± 18 year
olds by emphasizing industrial and real-life applications of chemistry. This, it was
hoped, would help to increase the numbers of students electing to study chemistry
beyond the end of compulsory education at age 16 by helping to retain students’
motivation for studying chemistry and so aid the quality and future supply of
research and industrial chemists.
SAC adopts a novel, context-led approach to A level chemistry teaching. The
course comprises thirteen theoretical units each made up of a ‘storyline’, ‘chemical
ideas’ and ‘activities’. Each unit requires approximately twenty hours of teaching
time. These units include all the nationally agreed subject core for Chemistry
(SEAC 1993). The course also includes a structured visit to the chemical industry
and an extended individual practical investigation. The chemical ` storylines’ provide contextual settings for chemistry reflected in the unit titles, for example
` Developing Fuels’ . Three important consequences arise from this structure:
first, chemical ideas are introduced only as contexts demand, thus breaking
down the traditional physical, inorganic and organic divisions of chemistry. This
encourages students to draw several aspects of the subject together to understand a
specific chemical context. Second, students only learn the chemistry required to
understand each storyline, so any one chemical topic is delivered in a ` drip-feed’
fashion through several units which are taught in a prescribed order. This feature
has an advantage from a research perspective in that a population of students
drawn from different schools and colleges receive the same material at approximately the same time. Third, chemical ideas are revisited as the course proceeds,
allowing students’ understanding to develop over a longer time period than is
possible with a ` traditional’ type course. These tend to deliver each chemical
topic in one block of lessons, with contexts used as illustrations after the ideas
have been taught rather than as pretexts for introducing them. The data collected
during the study explore the strengths and illustrate some weaknesses associated
with these strategies.
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
1173
The treatment of chemical bonding and thermodynamics in
SAC
Treatment of chemical bonding and thermodynamics illustrate the key features of
the SAC approach. Different aspects of chemical bonding feature in many units, so
this is an excellent example of ‘drip-feeding’ information. In the first unit, ‘The
elements of life’, students learn how covalent and ionic bonds form, how to represent simple covalent molecules using dot/cross diagrams and the meaning of bond
polarity. In unit 4, ‘The Atmosphere’, bond polarity is revisited in the context of
heterolytic fission. Unit 5, ‘The Polymer Revolution’, uses intermolecular bonds
including hydrogen bonds to help explain polymer properties; and in unit 8,
` Engineering Proteins’ , students learn about the shapes of simple covalent molecules. Knowledge about covalent and ionic bonds is assumed in several other
units, where ideas such as molecular stability, lattice enthalpy, enthalpy of solution
and variation in boiling point are discussed. Thus, students revisit chemical bonding on many occasions allowing knowledge to be reinforced and developed through
application to a variety of contexts. New ideas or aspects of the topic are added at
each point, so students build up the knowledge required over a long period of time.
Basic thermodynamics concepts are introduced in unit 2, ` Developing Fuels’ ,
which is taught within the first three months. The storyline describes how petrol
became the prime world fuel and provides a strong context-based link for teaching
and learning basic thermodynamics ideas. Students learn that bond breaking is
endothermic and bond making is exothermic, how to perform Hess’ Law calculations and meet entropy in a qualitative sense. Aspects of thermodynamics feature
in several other units. Ideas about energy changes are developed in unit 3, ` From
minerals to elements’ . Bond breaking is met again in unit 4, ` The Atmosphere’ ;
and enthalpy change of combustion is used to compare hydrogen and petrol in unit
7, ` Using Sunlight’ . These units are taught within the first twelve months of the
course. In unit 13, ` The Oceans’ , students learn about entropy in more detail.
These later references offer students the opportunity to revisit ideas presented
in earlier units and permit consolidation of understanding.
Students’ understanding of chemical bonding and
thermodynamics
We provide a brief review of literature relating to these areas of chemistry. We note
that both have received relatively little attention from researchers compared to
other topics.
Thermodynamics
The basic chemical idea associated with thermodynamics probed in this study is
that energy is released when chemical bonds form. Ross (1993) notes that many
students think the opposite, that energy is released when chemical bonds break. He
suggests this arises because of the strong association students develop between
fuels and energy. This prompts them to learn the phrase ` fuels contain energy’
almost by rote, and then associate chemical bonds with an energy-storing role.
Work by Andersson (1984), Schollum (1981) and BouJaoude (1991) among others
show that students aged 14± 15 tend to describe burning in very primitive ways,
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V. BARKER AND R. MILLAR
and may associate energy release with ` flames’ or simply ` burning’ . Early teaching
about chemical bonding provides a more scientific-sounding notion: that the
energy comes from breaking bonds. Boo (1998) suggests that this idea may be
linked to the macroscopic everyday experience that energy is needed to make or
do something, and so to ` make bonds’ must therefore require energy which is
released when the bonds are broken.
Ross (1993) suggests that describing combustion as a ` fuel-oxygen system’
may assist students, as this means that the fuel and the energy generated on its
combustion must be perceived in the context of a chemical reaction. Students can
be encouraged to think about how the products are formed rather than focusing on
the starting molecules alone.
Chemical bonding
In this study we report students’ ideas about covalent, ionic and intermolecular
bonding. We consider earlier work in three separate sections.
Covalent bonds
Taber (1993a and b) reports an intensive case study carried out with one student,
` Annie’ , which charts her progress towards understanding that chemical bonds
involve electrostatic attractions between atomic nuclei and electrons. Initially,
Annie described covalent bonding as atoms ` pulling together’ . At the mid-point
of the study, she associated covalent bonds with electron sharing and acquisition of
full electron orbitals. These ideas reflect increasing sophistication of her ideas and
may point to stages of development among students more generally.
Peterson and Treagust (1989, Peterson 1993) probed the understanding of 17
year old chemistry students and found that about 23%of them thought that electrons were shared equally in all covalent bonds. As almost without exception all
covalent bonds exhibit some degree of polarity, this is an important misconception.
These authors also note that 60% of this group and 55% of first year university
chemists could not correctly position the electron pair between hydrogen and
fluorine.
Boo (1998) notes that some students think a single covalent bond comprises
one electron alone, in the same way an apple may be shared between two people.
Ionic bonds
Misconceptions about ionic bonds common to several studies are noted. The
notion that ionic substances exist as discrete molecules was found by Butts and
Smith (1987), Taber (1993a) and Boo (1998) among chemists aged 17± 19 years.
Boo also notes that this idea influences students’ thinking about how ionic compounds behave; for example, a solution of sodium chloride comprises water molecules and sodium chloride molecules, while hydrochloric acid contains hydrogen
chloride molecules. In each case the molecules are made from ions bonded
together.
Butts and Smith (1987) and Boo (1998) also report some students describe the
bond between sodium and chlorine as covalent. This finding is corroborated by
Boo (1998).
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
1175
Boo (1998) notes that students who view ionic compounds as molecular think
that the intramolecular ionic bond will be stronger than other bonds if the molecules are placed close together. This finding is similar to that reported by Taber
(1994), who describes a ` molecular framework’ characterizing the development of
thinking about ionic bonds among five interviewees. This features three key misconceptions: that as a sodium atom can donate only one electron, so it can form
only one ionic bond; that the sodium ion formed is bonded only to the chloride ion
produced on receiving the electron; that the ions may be attracted by forces to
other ions, but that these attractions are not ionic bonds.
Intermolecular bonds
‘Annie’ was asked (Taber 1993a) to describe the bonding she perceived between
hydrogen fluoride molecules drawn in a chain arrangement, showing distorted
electron clouds which were touching one another. She did not think there was
any bonding between the molecules. By the end of Taber’ s study, completed postteaching, Annie demonstrated much clearer understanding of hydrogen bonding.
She also knew about temporary induced dipole-dipole bonds (van der Waals’
` forces’ ) and incorrectly thought these would be present in sodium chloride.
Peterson and Treagust (1989) also investigated students’ thinking about
intermolecular bonds. They found that about 23%of students thought that intermolecular bonds were positioned within a covalent molecule. They also note that
one-third of their sample confused the relative strengths of inter- and intramolecular bonds, saying that ` strong intermolecular forces exist in a continuous
covalent network’ (Peterson and Treagust 1989: 460). In his later work,
Peterson (1993) reports that 36% of first year university chemists thought
that silicon carbide had a high melting point because of ` strong intermolecular
forces’ .
Peterson et al. (1989) noted that some 17 and 18 year old chemists thought that
intramolecular bonds break on change of state, rather than intermolecular bonds.
The longitudinal study
The progress of 250 SAC students drawn from thirty six different schools and
colleges in the UK was tracked by asking them to complete the same diagnostic
questionnaire comprising twenty three questions on three occasions - at the start,
after eight months and after sixteen months of the twenty month course.
Questionnaires were sent to schools for completion by students under examination
conditions. Teachers administered the questionnaires without warning in a lesson
within a specified two week period. Students were not permitted to take questionnaires home or discuss or debate the paper with each other or their teacher. They
were not given any feedback on their responses. The questionnaire was designed to
require a maximum of one hour to complete and was presented in a booklet format
on coloured paper to ensure a distinctive appearance. Each question was given a
name rather than a number to help give a ‘user-friendly’ and non-threatening
style. Question names are used throughout this paper. Response rates to most
questions were over 80% on all three occasions, indicating that students were
able to complete the questionnaire within the timescale suggested. Data were
also collected relating to the students’ progress through SAC. These indicated
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V. BARKER AND R. MILLAR
that by the time of the second and third surveys the great majority of students in
the study had completed the same course units and so could reasonably be
expected to have had similar learning experiences.
Students’ responses to eight of the twenty three questions are reported here.
‘Methane’ and ‘Energy change’ investigated ideas about basic thermodynamics.
The question ‘Chemical bonds’ investigated ideas about covalent bonds and
hydrogen bonds, so data are reported separately under these headings. The formation of stable covalent molecules was explored using ‘Methane molecules’. The
question ` Sodium and chlorine’ probed ionic bond formation, while ` Hydrogen
chloride’ explored ideas about the formation of ions from a covalent molecule. The
questions ` Boiling and Chlorides’ permitted investigation of ideas about the roles
of intermolecular bonds in changes of state. Six questions were devised for this
study, while ` Boiling’ was adapted from Osborne and Cosgrove (1983) and
` Chlorides’ from a University of London A level Chemistry examination paper
set in June 1990. Figures 1± 4 give representations of the questions as presented
to the respondents. Data relating to the questions are presented in tables 1± 9.
The expected responses and chemical ideas probed are given as questions are
discussed.
Twenty four students whose thinking appeared to change markedly after the
second survey were selected for interview to validate written responses and to
probe reasons for these changes. Students selected for interview had given written
responses which were representative of the cohort as a whole. They were
interviewed one-to-one with the researcher and their permission was sought to
record the interviews for research purposes. Respondents were told in advance
about the interview, but were not given any warning about the content. They
were invited to re-read their answers given in the first and second surveys and
to comment on changes in their thinking. The interviews confirmed that students’
responses had been interpreted correctly and so ensured that the analysis of written
answers was accurate. The interviews were not carried out as a free-standing study
and so were not subjected to rigorous analysis. Selected extracts from these
interviews are reported here to help highlight and clarify points noted in the
tabulated data.
Describing changes in students’ understanding
In analysing the data a novel coding strategy was devised which was applied consistently across all questions in the test paper. This section begins by noting
differences between ‘Methane and Energy change’. These are used to inform the
comments about development of the coding system.
In ‘Methane’, the chemical equation is given together with a detailed enthalpy
level diagram showing the formation of products from the reactants. Students were
asked specific questions focusing on aspects of the information presented.
Responses were given in terms of the chemical idea being probed. The chemical
reaction was not used in students’ answers.
In ` Energy change’ , respondents were given the equation for the reaction
together with much less detailed enthalpy level diagrams. The question was
more open than ` Methane’ and accordingly a wider range of responses was generated. In some cases students responded in terms of the chemical reaction, rather
than using ideas about thermodynamics.
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
1177
The existence of responses using ideas about the chemical reaction rather than
the chemical idea under test is significant and provided a key to devising the
codings. A distinction was made between students’ understanding of two aspects
of the questions: the chemical idea being probed (aspect 1) and the chemical
reaction or event used (aspect 2). Application of this distinction helped to clarify
responses which were neither completely in accordance with the expected answers
nor indicative of misunderstanding the chemical idea being probed.
Hence, responses were grouped into five main categories denoted P, Q, R, S
and T in tables 1 and 2. A summary of the types of evidence placed in each follows:
. P Evidence for understanding aspect 1, no evidence for misunderstanding
aspect 2;
. Q Partial evidence for understanding of aspect 1, no evidence for misunderstanding of aspect 2;
. R Evidence for understanding aspect 2, no evidence for misunderstanding
aspect 1;
. S Evidence for misunderstanding aspect 2, evidence for understanding
aspect 1;
. T Evidence for misunderstanding aspect 1; and
. U Uncodeable responses, including no responses.
In the case of ‘Methane’, responses were placed only into categories P, Q, T and
U. This also applies to ‘Covalent bonds’, ‘Methane molecules’, ‘Hydrogen bonds’,
‘Boiling and chlorides’. In answering these, categories R and S were not needed, as
students always responded in terms of aspect 1, in most cases because no chemical
reaction featured in the question. For ‘Energy change’, all categories were used,
because some responses focused on the chemical reaction between sodium and
chlorine and did not mention aspect 1. In ` Sodium and chlorine’ and ` Hydrogen
chloride’ , students also gave responses involving the chemical reactions. In category Q were placed responses which were incomplete or inconclusive and so gave
only partial evidence. For ` Methane’ , for example, some students did not answer
all three parts of the question, while for ` Energy change’ students may have
selected a correct diagram, but failed to explain their choice. The scheme was
verified by chemical educators and their comments on placing responses in specific
categories were noted in preparation of the final codings.
The data tables give the levels of understanding represented in the five main
categories and indicate the proportion of uncodeable responses at each stage.
Examination of the first column indicates the understanding exhibited by students
beginning SAC. These students had taken the General Certificate of Secondary
Education (GCSE) examinations several months prior to the data collection. Use
of the three columns together permits us to report shifts in response levels between
categories as the study proceeded. To aid this, codings were entered on a computer
database, which was used to explore patterns of response across the three surveys.
These data point to influences of the context-theory approach on students’
learning as it is possible to tie changes in responses to the material presented
prior to the administration of the surveys. À2 tests were used to determine whether
changes in the relative proportion of P-coded responses in the third survey were
significant at the 0.05 or 0.01 level. Comments on values are made as questions are
discussed.
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V. BARKER AND R. MILLAR
Results
Thermodynamics
Methane is shown in figure 1 and data relating to this question is given in table 1.
The chemical idea being tested is that energy is needed to break bonds between
atoms, but is released when bonds form. The answers expected to each part were:
. Methane and oxygen are stable as a mixture, but some molecules will break
apart if energy is supplied. This starts a reaction between them.
. Energy is released when carbon dioxide and water form as products of
combustion. Some of the energy is used to break up more molecules of
reactants. This will continue until the supply of one reagent is exhausted.
. The energy comes from bonds forming between atoms of oxygen, carbon
and hydrogen to make molecules of carbon dioxide and water.
In analysis, the responses to part c were considered first as this probed the chemical idea under investigation. Responses given to the first and second parts are
shown in each box in the table.
A very large increase in P-coded responses is found, significant at the 0.01
level (À2 ˆ 118:2). One interviewee explained her changed thinking at interview.
She was asked to explain where energy comes from in the reaction:
S:
I:
S:
From the bonds that have been formed . . . you break a few bonds, they form
other products, they give out more energy, break a few more bonds and that
keeps on going.
. . . Here [1st survey] you tell me that it’s those bonds [in methane] that are
broken which give out energy, and here you say that it’s energy released when
- [interrupted]
. . . I came to this college not understanding this completely . . . I thought if you
broke the bonds you’d give out energy, which wasn’t true, because it confused
me at GCSE . . . because everyone said ‘Energy is stored in bonds’ . . .
She went on to identify the ‘Developing Fuels’ unit as the source of her changed
thinking. In company with around 33% of the group, this student changed
response from a misunderstanding-type answer to the correct idea. An additional
12% made the same change between second and third surveys. Inspection of the
response database shows that about 32% give correct responses at the second and
third surveys, suggesting that most retain their knowledge.
However, while these data are encouraging, inspection of the response database shows that almost 27% gave T-coded responses on all three occasions, suggesting that these students learned no new material to prompt changed thinking.
Also, about 39% were T-coded for the first and second surveys and 30% for the
second and third. This suggests that the misunderstandings are tenacious. One
possible reason for the resistance to change is demonstrated in the following interview extract in which a student is arguing that bond breaking is both exo- and
endothermic:
I:
S:
I:
S:
. . . how can energy be put in to break bonds, and yet you say energy comes from
breaking bonds?
[Pause] How do you - [stops]
. . . what you’re saying [in your written answers] is that activation energy means
you’ve got to put energy in to break bonds.
Yes.
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
Figure 1. Questions ‘Methane’ and ‘Energy change’.
1179
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V. BARKER AND R. MILLAR
Table 1.
RC
Changes in SAC students’ responses to ‘Methane’.
1st %
2nd %
3rd %
Energy is from bond formation
Products are more stable than reactants
Part a: EA supplied / Energy splits molecules
Part b: reaction is exothermic / chain reaction / O2
is unlimited
Total
5.6
0.4
43.6
1.6
46.4
3.2
6.0
45.2
49.6
Answers as above to a and b only or to part c:
Energy is from bond making and breaking
Total
4.0
8.4
9.2
4.0
8.4
9.2
T1a Energy is from bonds in CH4
T1b Energy is from bond breaking | Energy splits molecules 4.0
T1c Energy is stored in CH 4
12.8
Part a: as above (P) / Energy speeds up reaction
Part b: as above (P) / CH4 releases energy / excess
energy is used
T1
Total
16.8
1.2
6.8
16.0
1.6
4.4
18.8
24.0
24.8
6.4
1.2
0.8
5.6
0.8
0.4
14.4
5.2
1.6
4.8
6.8
2.8
1.2
2.4
1.2
26.0
9.2
5.2
8.4
2.0
2.4
18.8
82.0
7.2
45.6
6.8
40.4
2.0
6.0
8.0
0.8
0.8
0.8
0.8
100.0
100.0
100.0
P1
P2
P
Q1
Q
Description
T2
CH4 is energy store made from animals/sun
Part a: heat required / flammable H2 present
Part b: CH4 is fuel / flammable / O2 available
T3
Energy is from CH4 | Need O2 , fuel &
heat | Fire ‘triangle’ kept going
T4a From burning CH4 | CH 4 always burns in air / until
it runs out
T4b Heat energy from burning
T4c Heat is given out / from the flame
Part a: as above (P) / Heat required
Part b: as above (P) / Heat of burning / flame keeps
reaction going
T4
Total
T5
T6
T
From exo reaction | O2 needs spark | gas has 2
flammable elements in it / is a hydrocarbon
Only 1 part answered
Total
U1
U2
U
Uncodeable for all three parts
No response
Uncodeable
Overall total
n ˆ 250
Key: The symbol / denotes ‘‘or’’; that is, alternative answers which are equally acceptable.
The notation (P) means that responses were identical to those coded P.
I:
S:
If you’ve got to put energy in to break bonds, how can you also say here [in part
3] that energy comes from broken bonds?
. . . when you put something into it, it breaks the bonds releasing energy, and
that energy that’s been released breaks bonds next and continues the reaction. . .
This student thinks that energy is needed to break bonds initially, but once broken, energy is released. This causes him problems when later in the interview he
attempted to explain what happens when bonds form. He realizes that burning
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
1181
fuels releases energy into the environment, but is unable to link the exothermic
character of the reaction with bond formation. After this he is asked:
I:
S:
So you still think that when you break bonds you give out energy?
Yes.
The key problem is that students find it difficult to appreciate that energy is
released by bond formation, not bond breaking. This erroneous reasoning seems
to begin pre-16, as at this stage students are frequently taught that fuels are ‘energy
stores’. When at A level students are faced with the idea that bonds break to enable
a reaction to occur, the most plausible explanation is that energy comes from the
bonds and is ‘released’ on breaking in the way one breaks an eggshell to release the
contents. These data indicate that about one-fifth of SAC students do not move
from this thinking. Table 1 shows that about 19% give the response ` energy is
stored in methane’ (T1c) at the third survey.
Energy change
‘Energy change’ (figure 1) uses the reaction between sodium and chlorine to
explore the chemical idea that an upwards arrow on an enthalpy level diagram
represents energy absorbed when bonds are broken, while a downwards one represents energy given out when bonds are made and the difference in lengths measures the enthalpy change of reaction. The expected answer is that diagram A best
represents the reaction. This shows the largest difference between the arrows
representing a highly exothermic reaction. Diagram C was also accepted as correct,
as no scale is included. Data relating to this question are given in table 2.
These data imply that the energy release on formation of an ionic bond is not
well understood by many students by the end of their A level course. Comparing
these data with those for covalent bond formation explored by ‘Methane’ suggests
that ionic bond formation is more problematic for students. Nevertheless, the
increase in correct responses is significant at the 0.01 level … À2 ˆ 19:6†. One
student explained his changed thinking at interview, explaining that by the second
survey he had a meaning for the arrows: ‘I’d seen the diagrams before and this is a
violent reaction giving energy out so the arrow would be going down.’ As with
‘Methane’, most of the increase in correct responses occurs between the first and
second surveys following the early work on thermodynamics in SAC. Here,
inspection of the response code database indicates that most of the increase arises
from students moving from Q-coded responses to fully accurate explanations.
Q-coded responses are the most popular type through all three surveys. One
reason for this may be that students learn that activation energy is involved in
chemical reactions and so may select a diagram showing what they think represents
the activation energy needed to break a chlorine-chlorine bond. This idea is taught
in the ‘Developing Fuels’ unit. The response-code database indicates that about
6% who gave this response (Q1) at the second survey change to a fully correct
explanation at the third. For these students, the ‘activation energy’ answer appears
to be a half-way point in their thinking.
A relatively high proportion seem able to select a correct diagram but offer no
explanation, or one which is uncodeable (Q4). The response code database shows
that these are different students at each survey. One reason for this response may
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V. BARKER AND R. MILLAR
Table 2.
Changes in SAC students’ responses to ‘Energy change’.
RC
1st %
2nd %
3rd %
1.2
10.8
4.8
18.4
8.0
19.6
0.4
0.4
0.4
P
A/ C Low EA , energy released on bond formation
A/ C Low EA , exothermic reaction /stable
compound forms
A/ C electrons lost & gained / low ionisation energy
and high lattice enthalpy
Total
12.4
23.6
28.4
Q1
Q2
Q3
Q4
Q
A/ C Energy required to break Cl2 bond / start reaction
A/ C Energy required to heat Na / two states react
A/ C Violent reaction / react easily / low EA required /
A/ C Uncodeable / No explanation
Total
4.4
1.2
16.0
21.6
21.2
2.4
17.2
40.8
14.4
0.4
0.4
18.4
33.6
R1
R2
R3
R
A/ C Na & Cl are reactive
A/ C It is an exothermic reaction
B Energy is conserved in the reaction
Total
1.6
6.8
0.4
8.8
2.8
8.4
11.2
2.4
8.0
10.4
S1a
S1b
S2
S3
S
B/ C Reaction doesn’t give out much energy
B Reaction needs lots of energy to start
B electron transfer / equation misunderstood
A/ C 2:1 moles reactants / 1 bond broken, 2 formed
Total
1.6
4.4
3.2
5.2
14.4
1.6
4.0
0.4
1.2
7.2
2.4
3.6
1.2
3.6
10.8
T1
T2
B Bond breaking gives out energy
B Reaction is in equilibrium / energy levels are
equal / no. of bonds broken = no. of bonds formed
3.2
2.8
0.8
B Long arrows => lots of energy made / used / reaction 6.0
is violent / Small gap => low energy input
T3b A / C A shows high energy barrier / lots of energy
0.8
required to start reaction
T3c A / C Bond breaking => energy is given out
T3
Total
6.8
2.0
1.2
2.8
4.0
4.8
0.4
5.6
P1
P2
P3
Description
T3a
T4
B Uncodeable / No explanation
10.8
3.2
4.8
T
Total
20.8
10.8
11.2
U1
U2
U
Uncodeable
No response
Total
0.4
21.6
22.0
0.4
6.0
6.4
0.4
5.6
6.0
Overall total
100.0
100.0
100.0
n ˆ 250
be the absence of numerical data in the question, which prompts students to select
diagram C as a guess, for example:
S:
I:
S:
. . . I didn’t know how to class this - this reaction whether it was a large amount
of energy or small, or medium.
. . . so you just opted for medium?
Yes! [laughs]
Relatively few students selected the completely incorrect diagram at any survey.
Most who think that the long arrow means ‘lots of energy’ (S2) change their minds
after the first survey. This suggests that by the second survey, most associate
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
1183
energy release with a difference in arrow lengths in energy level diagrams, even if
this is at best on an intuitive basis. In contrast to ‘Methane’, few students respond
that bond breaking gives out energy.
Covalent bonding
Covalent bonds
The question on covalent bonds was part of ‘Chemical bonds’ shown in figure 2.
The chemical idea being tested is that a covalent bond comprises two electrons,
Figure 2.
Questions ‘Chemical bonds’ and ‘Methane molecules’.
1184
V. BARKER AND R. MILLAR
one from each atom, sharing the same electron orbital. If two electrons are
involved from each atom, a ‘double’ bond is formed. Covalent bond formation
confers stability on the atoms. The expected answer, coded P, is that line 1 represents a single covalent bond formed when a carbon and a hydrogen atom each
donate one electron to form an electron pair between the two nuclei. Line 2
represents a double covalent bond in which two electron pairs are shared between
two carbon atoms.
Table 3 shows an increase in frequency of P-coded answers over the three
surveys significant at the 0.01 level (À2 ˆ 114:2). By the third survey a majority of
students described single/double covalent bonds in terms of the numbers of electrons involved.
The less precise Q-coded responses ‘single/double bond’ or ‘chemical bond’
decreased in frequency, although this was still given by 25% by the third survey.
Coding this response separately permitted monitoring of the use of more accurate
chemical language. Around 17% changed from the ‘single/double bond’ answer
(Q1) to a more detailed response between the first and second surveys. At interview, several students said their first answers used language learned at GCSE.
A relatively small proportion gave T-coded responses. The most frequent are
shown in table 3. By the third survey, no student thought that an ionic bond was
represented (T3a) and few who used the term ‘electrons’ made errors in the numbers involved.
Examination of the response code database suggests that most of the correct
second and third survey responses arise from students learning new information.
Table 3.
RC
Changes in SAC students’ responses to ‘Covalent bonds’.
Description
1st %
2nd %
3rd %
P1
P2
P
2 / 4 electrons shared
Single / double covalent
Total
8.8
9.6
18.4
43.2
10.0
53.2
51.6
14.0
65.6
Q1
Q2
Q
Single / double bond
Chemical bond
Total
43.6
1.2
44.8
33.2
0.8
34.0
24.8
24.8
T1
T1
1 / 2 electrons shared
Total
14.8
14.8
7.6
7.6
7.2
7.2
T2a
T2b
T2c
T2
Saturated bond
Simple bond
Weak / strong bond
Total
2.8
1.2
4.0
0.4
0.4
0.8
0.4
0.4
0.8
T3a
T 3b
T3
T
Ionic bond
Link between two compounds
Total
Total
4.4
2.0
6.4
25.2
1.2
3.2
4.4
12.8
0.8
0.8
8.8
U1
U2
U
Uncodeable
No response
Total
9.6
2.0
11.6
-
0.4
0.4
0.8
Overall total
100.0
100.00
100.0
n ˆ 250
1185
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
Although students’ responses changed markedly, at interview most did not attribute this to particular course materials or specific units, perhaps reflecting the fact
that SAC features covalent bonds so frequently that students do not recall learning
about them at any specific point. Also, the wide variety of contexts in which
covalent bonds are met means no one unit acts as a memory aid. The drip-feed
strategy seems to be the key factor prompting change.
Methane molecules
The question on methane molecules is shown in figure 2. The chemical idea being
tested is that stability is associated with the formation of covalent bonds by which
electron orbitals are filled by sharing a pair of electrons between two atoms. The
expected answer, coded P, is that CH 4 is the most stable of the formulae listed.
This arrangement confers the greatest stability on both atoms as their outer electron orbitals are filled by sharing electrons. Data relating to this question are given
in table 4.
Table 4 indicates that progression among students towards using ideas about
molecular stability appears to be relatively limited, with 30% giving this response
by the third survey. However, the increase in P-coded responses is significant at
the 0.01 level (À2 ˆ 22:8). Inspection of the response code database shows that
about 6% change to the correct response between first and second surveys. This
suggests that the SAC approach in which molecular stability features in the fourth
unit, The ` Atmosphere’ , in the context of atmospheric chemistry, has had some
effect. About 12 %change to P-coded responses between second and third surveys.
This corresponds with the revisiting of molecular stability in unit 8, ` Engineering
Table 4. Changes in SAC students’ responses to ‘Methane molecules’.
RC
P1a
P1b
P2
Description
1st %
2nd %
3rd %
6.4
6.0
0.4
10.0
8.4
1.6
16.4
11.6
P
CH4 is energetically most stable
C and H are more stable as CH4
C & H need 4 & 1 more electrons for noble gas
configuration
Total
12.4
18.8
29.6
Q1
Q2
Q
C needs four bonds / 4 more electrons
C has valency / oxidation no. of 4 / 4 links
Total
46.8
8.8
55.6
60.4
10.8
71.2
51.6
9.6
61.2
T1
1.6
2.4
0.4
T2
T3
T4
T
C has 4 bonding pairs / lone pairs 2/ 6 / 8 / 10 / 18
electrons
C / H is saturated
More C in air / H travels in pairs /1:4 is equal mass
Because there are 4 Hs and 1 C / other statement
Total
2.0
9.6
13.2
1.6
4.0
2.0
2.4
U1
U2
U
Uncodeable
No response
Total
6.8
12.0
18.8
1.6
4.4
6.0
3.6
3.2
6.8
Overall total
100.0
100.0
100.0
n ˆ 250
1186
V. BARKER AND R. MILLAR
Proteins’ , where students learn about the shapes of simple molecules. This section
features methane specifically.
Despite this increase, Q-coded responses type remain the most popular at all
three surveys. The database shows that about 29% give a valency or carbon-only type answers (Q1 or 2) at all three surveys, suggesting that these students knew
about the number of bonds made by a carbon atom prior to starting the course and
that SAC material did not prompt any change. Use of the term ‘valency’ increased
during the study, and as this term does not feature in SAC, this must arise from
teacher (or other teaching material) use.
Few responses were coded T. The response T4 is the most frequent of these,
suggesting that students giving this answer think ‘because it just is’. The frequency
of this answer decreased markedly by the third survey. The persistent popularity
of Q-coded answers implies that for many, the limited answer ‘Carbon needs four
bonds’ is sufficient.
If the levels of P-coded response to ‘Covalent bonds’ and ‘Methane molecules’
are considered jointly, we see that many more students can describe single and
double covalent bonds in discrete molecules than can explain why four bonds
should form in methane. The idea probed by ‘Methane molecules’ is an extension
of that in ‘Covalent bonds’. The relatively low level of P-coded responses to the
extension question suggests that the notion of molecular stability seems difficult to
grasp.
Ions and ionic bonds
Sodium and chlorine
‘Sodium and chlorine’ is shown in figure 3. The chemical idea being tested is that
an ionic bond may form between atoms when one or more electrons are transferred
to make charged particles called ions. The ions bond together by electrostatic
attraction, releasing energy and forming an ionic lattice. The chemical event is
the reaction between sodium and chlorine. The P-coded answer is that in the
reaction, one electron is transferred per sodium atom to each chlorine atom, resulting in the formation of ions. Energy is released when the ions interact forming the
sodium chloride lattice.
The frequency of the P1 response, that energy is released in bond formation,
increases by 11% over the three surveys. The change is significant at the 0.01 level
(À 2 ˆ 12:4). The relatively low level of this answer implies that most students may
find it difficult to relate the observable events occurring in a chemical reaction to
the energetics of bond formation. This is despite the formation of sodium chloride
receiving explicit treatment in SAC in unit 1, ` The Elements of Life’ , and unit 3,
` From Minerals to Elements’ . The response code database indicates that the picture is more complex than first inspection of table 5 suggests.
At the first survey, a majority of students state simply that sodium and chlorine are ‘reacting’ or ‘forming a compound’ (R1). About 17% maintained this
response for all three surveys, suggesting that this group has not learned new
material, or that these students cannot perceive a link between material studied
and the question. About 36% changed from R1 to a different response at the second
survey. Some moved towards a P-coded answer, including this student:
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
1187
Figure 3. Questions ‘Sodium and chlorine’ and ‘Hydrogen chloride’.
The chlorine and sodium atoms are rushing violently to get to each other to release or
gain electrons and bond with each other. (1st survey, R1)
Na is reacting with the Cl2 forming bonds releasing energy which keeps the reaction
going on vigorously. (2nd survey, P1)
Others gave an S-coded answer on the second occasion, for example:
There is a displacement reaction occurring where the sodium is reacting with the
chlorine. (1st survey, R1)
1188
V. BARKER AND R. MILLAR
[Drawing of Na and Cl forming a covalent bond] Cl needs one more electron to fill its
outer shell and Na has that electron. (2nd survey, S1)
At interview, this student maintained his belief that the bond was covalent rather
than ionic, and used the same idea to explain his response to another question,
‘Solution’, which involved the dissolution of an ionic lattice (reported in Barker
and Millar 1999). The respondent believed that sodium and chlorine form discrete
molecules. The increase in P-coded responses at the third survey was contributed
mainly by students coded S or U at the second stage, rather than any further
changing from the R category.
At both stages, the response code database indicates that some students move
away from the P-coded response. About 7% changed from P-coded to R-type
answers, as they did not use energy ideas at the second survey. A further 7%
changed in the same way between second and third surveys. These students
may have given P-coded responses while relevant teaching was in their minds.
By the next survey, this was no longer immediate, so the answer ‘sodium and
chlorine are reacting’ became the next most appropriate response.
The low level of S and T-type responses at all three surveys indicates the
reaction between sodium and chlorine is well-known and unproblematic. It may
also suggest that more explicit probing of students’ thinking is needed to explore
misunderstandings in greater depth.
Hydrogen chloride
Students’ understanding of the formation of ions in solution was probed by
‘Hydrogen chloride’, shown in figure 3. Several chemical ideas are probed here:
Table 5.
RC
P1
Changes in SAC students’ responses to ‘Sodium and chlorine’.
Description
1st %
2nd %
3rd %
2.4
2.4
12.8
P2
P3
P
Energy is liberated in bond / ionic lattice formation
reaction
e transfer Na to Cl, stable compound forms / redox
Ionic bond forms between Na and Cl
Total
13.6
4.8
19.8
15.6
8.4
29.2
10.8
10.4
34.0
R1
R2
R
Na and Cl are reacting / form a compound
The element(s) is / are reactive hence violent reaction
Total
52.8
2.8
53.6
45.2
0.8
46.0
46.8
1.2
48.0
S1
S2
S
Hot Na has EA required / reaction is quick / endothermic 3.2
Covalent bond forms
2.8
Total
6.0
6.8
4.4
11.2
4.8
1.6
6.4
T1
T2
T3
T
Heat breaks bonds / sodium is burning
Particles expand / contract / collide / break / split
Heat energy used to make bonds
Total
0.8
2.8
3.6
0.8
1.6
2.4
0.4
0.4
0.8
U1
U2
U
Uncodeable
No response
Total
2.0
12.0
14.0
3.6
7.6
11.2
0.8
10.0
10.8
Overall total
100.0
100.0
100.0
n ˆ 250
1189
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
acids contain hydrogen ions; in hydrochloric acid these form when hydrogen
chloride molecules split into hydrogen ions and chloride ions (by heterolytic fission) when the gas dissolves in water; the hydrogen ions are displaced as hydrogen
gas when magnesium metal is added.
The context of this question mirrors that used in unit 3 of SAC, ‘From
Minerals to Elements’, to explain acid formation. The notion of ions in solution
is met again in unit 7 ` Using Sunlight’ , where students study electrochemical cells
and the reactivity series in detail. Data relating to hydrogen chloride are shown in
table 6.
Table 6 shows that the proportion of P-coded responses increases to 25% by
the third survey, a value significant at the 0.01 level (À2 ˆ 25:7). We also find that
the level of T-coded responses remains almost unchanged.
The positive influence of SAC in changing students’ thinking is confirmed by
interview data. This student, for example, explained his change to a P-coded
answer at the second survey as follows: ‘It was after we did all that stuff on
electrochemical cells. We did all about ions in aqueous solution’.
Table 6.
Changes in SAC sample responses to ‘Hydrogen chloride’.
RC
Description
+
1st %
2nd %
3rd %
P1a
P1b
P2
P
H3 O ions | displacement reaction / ionic equation
H+ and Cl- ions | displacement reaction / equation
Hydrogen ions present | No response
Total
6.0
2.0
8.0
3.2
10.4
2.0
15.6
5.2
11.6
8.0
24.8
R1
R
No response | displacement reaction / equation
Total
0.4
0.4
0.8
0.8
1.2
1.2
S1a
S1b
S
H3 O+ | Mg reacts with or displaces Cl2 / Cl / O2 / H2 O
H+ and Cl ions | Mg reacts with / displaces Cl2 / Cl
Total
4.0
4.0
2.0
5.6
7.6
2.8
9.6
12.4
T1a
T1b
T1c
T1
HCl molecules | displacement reaction / equation
HCl molecules | Mg reacts with Cl2
HCl molecules | other explanation / no response
Total
8.4
7.2
12.0
27.6
13.6
11.6
12.8
38.0
11.6
11.6
16.4
39.6
13.2
1.6
11.2
2.8
3.2
2.0
14.8
14.0
5.2
1.6
2.0
0.8
-
T2a HCl / water complex | any explanation
T2b HCl & H2 O shown separately / H2 O alone | any
explanation
T2
Total
T3
+
-
Cl & H ions | incorrect equation / explanation
-
T4a
T4b
T4
T
No response | Mg reacts with acid / Cl / O2
No response | H2 released
Total
Total
10.4
6.8
17.2
61.2
3.6
2.0
5.6
59.6
1.6
2.4
4.0
49.2
U1
U2
U
Uncodeable responses to either part
No response
Total
4.0
22.4
26.4
2.0
14.4
16.4
1.6
10.8
12.4
100.0
100.0
100.0
Overall total
n ˆ 250
Key: The symbol | denotes divisions between answers to parts of the question.
1190
V. BARKER AND R. MILLAR
By adding P and S-coded answers, we see that 37% use ions in their third
survey answers, compared with 12% initially. Nevertheless, given the explicit and
detailed treatment the topic receives in SAC, this is a low figure compared to that
seen for other questions in the survey and indicates that the ideas probed here may
be problematic for students. The request for a diagram may have made the question seem unusual and difficult, dissuading students from giving their best possible
answer. Also, the question was located last in the test paper, so students may have
concentrated less closely on this than other questions. They may have instinctively
copied the molecule diagrams shown in the first gas jar, rather than drawing ions.
Nevertheless, these data show that about 40% drew molecular hydrogen chloride at the third stage (T1), while inspection of the response code database indicates
that about 29% did so in all three surveys. One explanation is that students adopt
molecular hydrogen chloride as a model for acid behaviour, which may be too
persuasive to relinquish. By this model, displacement reactions are explained by
the willingness of the metal to ‘swap partners’ with the chlorine, making molecular
magnesium chloride and releasing hydrogen gas. Students add new information
such as Standard Electrode Potential values to reinforce their model. The model
works without any need to use ions and supports the finding from ‘Sodium and
chlorine’ that students may perceive ionic compounds as discrete molecules.
Response S1 merits discussion. These respondents appear to understand the
principle that acids contain oxonium ions, but think the reaction displaces a gas
other than hydrogen. Table 6 shows that 24% (S and T1b) give the incorrect gas at
the third survey. The notion is also persuasive, as chlorine and magnesium are
widely known as two reactive elements, so students reason that this occurs because
‘magnesium is more reactive than chlorine’ instead of considering hydrogen.
Intermolecular bonds
Students’ understanding of intermolecular bonds was explored using three questions; ‘Hydrogen bonds’, ‘Boiling’ and ‘Chlorides’.
Hydrogen bonds
This is the companion question to ‘Covalent bonds’, described earlier (figure 2).
The chemical idea being probed is that hydrogen bonds form between molecules in
which hydrogen is covalently bonded to a highly electronegative atom, namely
nitrogen, oxygen or fluorine. Hydrogen bonds increase the boiling and melting
points of substances to higher values than expected from relative molecular mass
values. The P-coded answers are that line 3 represents a hydrogen bond which
arises because oxygen is highly electronegative and so a dipole exists in water
molecules. The negative and positive regions of different water molecules form a
bond. Differences between lines 1 and 3 include: line 1 is an intramolecular bond
and line 3 is intermolecular; line 1 is longer and weaker than line 3. Data relating to
this question are given in table 7.
This question shows a very large increase in the proportion of correct
responses at the second survey which is retained at the third survey. The change
is significant at the 0.01 level (À2 ˆ 133:5) and arises because students change their
responses from all other categories. These data suggest that most students learn
about hydrogen bonds in the first few months of SAC.
1191
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
Initially, a relatively small proportion give a P-coded response, while a high
proportion offer no response. This indicates that few students met hydrogen bonds
at GCSE. This is to be expected, as GCSE does not normally feature hydrogen
bonds. About one-quarter of first survey responses showed some understanding
(category Q), probably based on guesswork; the description of the bond as ‘weak’,
‘liquid’, or involving ‘cohesion’ is approaching correct chemical idea. These
responses are not completely correct, nor do they express a misunderstanding,
so are coded separately. A small proportion gave T-coded answers at this point.
Table 7 indicates that up to 24% of students at the third survey suggest that the
bond ‘is an attraction, not a bond’ (T1a). This is illustrated by one student who
gave these responses:
The weak bond between molecules which signifies liquid or solid substances. (1st
survey, Q2)
Hydrogen bonding. A small ionic force Line 1 is covalent, line 3 is a bit ionic. (2nd
survey, P2)
A hydrogen bond. [The line] is an attraction, not a bond. (3rd survey, T1a)
At interview this student explained her second survey response: ‘ I think I meant
that it’s not really a bond, it’s a bit ionic but it does attract each other but it’s not
really a bond’ and continued to explain that a bond is ‘when they’re joined together
Table 7.
RC
Changes in SAC students’ responses to ‘Hydrogen bonds’.
Description
1st %
2nd %
3rd %
P1
P2
P3
P
Hydrogen bond + explanation
H-bond only - no explanation
Intermolecular bond / polar attraction
Total
6.0
4.0
7.6
17.6
34.8
23.6
6.0
64.4
40.4
28.0
0.4
68.8
Q1
Q2
Q3
Q4
‘‘Liquid’’ bond + explanation
Weak bond between molecules + explanation
van der Waals’ / dipole-dipole bond
Cohesion / magnetic attraction / semi-permanent bond /
attraction
Total
9.2
11.2
1.6
4.4
0.4
0.8
3.2
1.2
0.4
1.6
0.4
26.4
5.6
2.4
T1a Line 3 is an attraction force not a bond / not a real bond
T1b Doesn’ t exist / is imaginary / temporary / repelling force
T1
Total
7.2
1.2
8.4
19.2
0.8
20.0
24.0
±
24.0
T2
6.0
2.8
2.0
T2
Triple / covalent / ionic / dative / delocalised / double /
1.5 bond
Total
6.0
2.8
2.0
T3a
T3b
T3
T
Shows water moves around / is in suspension
Shows water is rigid / is stronger than a covalent bond
Total
Total
2.0
2.0
16.4
0.4
1.6
2.0
24.8
0.8
0.8
26.8
U1
U2
U
Uncodeable
No response / Don’t know
Total
Overall total
7.2
32.4
39.6
100.0
1.6
3.6
5.2
100.0
0.4
1.6
2.0
100.0
Q
n ˆ 250
1192
V. BARKER AND R. MILLAR
and wouldn’t want to come apart unless there was some chemical reaction or
other’. This student appears to mean that hydrogen bonds cannot be considered
to be bonds in the same sense as covalent and ionic bonds and that they should
really be classified as ‘attractions’. The increase in frequency of this response is
significant. The words ‘attractive force’ are used on only one occasion where SAC
discusses hydrogen bonding, so the course does not emphasise this terminology.
Students therefore seem to acquire this phrase from their teachers rather than
course materials. Another interviewee supports this:
I:
S:
I:
S:
How do you know a hydrogen bond doesn’t involve sharing of electrons?
Because we were told!
So there is a difference in your mind between a covalent bond and a hydrogen
bond then?
Yes. A hydrogen bond is attractions between charges, different charges.
Nevertheless, table 7 suggests that a majority of students recognize and describe
hydrogen bonds accurately by the third survey. One student who gave these two
responses at the first and second surveys highlights this:
Ionic bond . . . (1st survey, T2)
There are hydrogen bonds when the H atom is slightly Ave (s ‡) and O s¡. The ‡ve
and ¡ve charges attract one another and make intermolecular bonds. (2nd survey, P1)
In explaining the change at interview she commented:
S:
. . . this is the sort of thing I don’t think in my whole life I’ll ever forget after
doing A level chemistry, because you just can’t help knowing so much about
bonds.
The course materials had clearly created a great impact on this student, suggesting
that the continued revisiting and renewed application of knowledge assisted her
learning.
Figure 4.
Questions ‘Boiling’ and ‘Chlorides’.
1193
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
The role of hydrogen bonds in change of state was explored using ‘Boiling’ and
‘Chlorides’ (figure 4). Changes of state do not receive explicit treatment in SAC,
but students learn that hydrogen bonds influence boiling points of compounds like
alcohols, hydrogen fluoride and water.
Boiling
The chemical idea being tested is that boiling is a change of state; involving, in the
case of water, breaking hydrogen bonds between molecules. Gaseous water is
called steam. The chemical event here is the change in state from liquid water
to steam. The P-coded answer is that the bubbles contain steam, since the hydrogen bonds break allowing the water molecules to separate from each other. Data
relating to this question are given in table 8 and the question is shown in figure 4.
The change in the proportion of correct responses is significant at the 0.01
level (À2 ˆ 17:3). Inspection of the response code database shows that about 20%
give the P-coded response on all three occasions and that about 32%did so at both
the second and third surveys.
The responses coded Q do not express change of state ideas. Oxygen and air
are dissolved in water, so these responses are not incorrect. However, these
answers may hide misunderstandings in students’ thinking, as one interviewee
illustrates. His written answers were:
Table 8. Changes in SAC students’ responses to ‘Boiling’.
RC
Description
1st %
2nd %
P1
P
Steam, water vapour, gaseous water
Total
27.6
27.6
39.2
39.2
45.6
45.6
Q1a
Q1b
Q1c
Q
Oxygen
Dissolved or evaporating gas
Air
Total
25.6
2.0
7.2
34.8
20.0
2.0
8.4
30.4
18.4
2.0
10.0
30.4
T1
T1
Heat, energy
Total
0.4
0.4
-
0.4
0.4
T2a
T2b
T2c
T2
Hydrogen
Oxygen and hydrogen
Oxygen or hydrogen
Total
6.4
19.6
2.8
28.8
8.0
13.6
2.0
23.6
7.6
8.4
2.4
18.4
2.8
2.8
5.6
1.2
2.8
4.0
1.6
1.2
2.8
34.8
0.4
0.4
28.0
0.4
0.4
22.0
0.8
2.0
2.8
100.0
0.4
2.0
2.4
100.0
0.8
1.2
2.0
100.0
T3a Carbon dioxide
T3b Gas
T3
Total
T4
T4
T
Nothing / Vacuum
Total
Total
U1
U2
U
Uncodeable
No response
Total
Overall total
n = 250
3rd %
1194
V. BARKER AND R. MILLAR
Hydrogen and oxygen gas. (1st survey, T2b)
AIR (Atmospheric, dissolved in the water). (2nd survey, Q1c)
At interview, the student explained: ‘hydrogen and oxygen, that’s the steam that’s
coming off which is as hydrogen and oxygen rather than the bubbles’. He added
that air would come out of solution as the increased temperature made it less
soluble. Then he was asked:
I:
S:
So besides air, once you’ve boiled off all the air that was in it, what will the
bubbles be then?
[pauses] I don’t know! Hydrogen?
The response ‘air’ hid his thinking that water splits up on boiling. The level of Tcoded responses shown in table 8 may therefore be inaccurate, the true figures
being higher.
The proportion giving T-coded responses decreases to 20%by the third survey. The response database suggests that these third survey responses are given by
students whose thinking has changed from the Q-coded ` oxygen’ /’ air’ answer at an
earlier survey, supporting the point made above. The notion that water molecules
break up on boiling and reform on condensing seems to be persuasive and difficult
to change.
Chlorides
‘Chlorides’ probes ideas about intermolecular bonds other than hydrogen bonds.
The chemical idea investigated is that small dipole-dipole attractions (van der
Waals’ forces) between covalent molecules require much less energy to break
than an ionic lattice. The P-coded answer is that the vapour consists only of
titanium (IV) chloride molecules because the intermolecular bonds between
these require relatively little energy to break them, whereas magnesium chloride
has an ionic lattice structure which requires much more energy to break up. Data
relating to this question are given in table 9.
Although the increase in the proportion of P-coded responses is significant at
the 0.01 level (À2 ˆ 7:1), these data also show a rise in the level of T-type responses
over the three surveys. Before exploring reasons for this, we will look first at one
student who illustrates the move towards the P-coded response. She gave these
written answers:
Because ionic bonding is stronger than covalent bonding . . . it will take more heat
energy to break the ionic bonds . . . (1st survey, T1)
MgCl2 has a much higher boiling point because ionic bonding is stronger as the ions
bond in a continuous lattice . . . in covalent compounds . . . only the weaker intermolecular bonds need to be broken (covalent bonds are not broken). (2nd survey, P1)
At interview she explained that her first survey answer was based on recall of a
table listing ‘Type of bond’ and ‘Boiling point’. Thus, she associated covalent
bond rather than ‘weak intermolecular bond’ with ‘low boiling point’, saying: ‘I
probably thought here that you broke up the titanium [chloride] into that, and then
it reforms in the vapour form . . .’ This student’s answer illustrates two points.
First, ideas learned pre-16 seem to contribute to the high level of T-coded
answers, as students associate covalent compounds in general with lower boiling
point figures than ionic ones. Hence, covalent bonds break when these substances
1195
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
boil, and covalent bonds are ‘weaker’ than ionic ones. Second, this student confirms the model for evaporation/condensation suggested above in ‘Boiling’, that
molecules break up on changing to the gaseous state and reform as molecules
during condensation. The breaking up is an essential part of the state change.
Some students apply this model to ionic solids too, as they picture ionic substances
as discrete molecules with uni-directional ionic bonds between ions. As these are
‘harder’ to break than covalent bonds, ionic compounds have higher boiling points.
They thus make a direct comparison between ionic and covalent compounds,
thinking of both as simple molecular structures. Such students are likely to struggle with the notion of a lattice structure for ionic solids in which there are no
‘intermolecular bonds’ of the hydrogen bonding or dipole-dipole type.
Inspection of the response database suggests that a significant proportion of A
level students may complete their course with this faulty state change model.
About 12% who offered Q-coded answers at the first survey changed to Tcoded responses at the second survey. New material on bonding studied between
surveys may have prompted a shift from chemically acceptable thinking. Around
19%give T-type answers at all three surveys and 38%do so at the second and third,
suggesting that these responses remain persuasive. These data imply that students
learn ` covalent bonds are weaker than ionic bonds’ rather than ` covalent bonds
require more energy to break than intermolecular bonds’ . SAC makes this distinction explicit in unit 12, ` Aspects of Agriculture’ , but this was taught after the third
survey was carried out.
Equally, though, this question may have been problematic because the need to
use intermolecular bonds in responses was not explicit. Respondents are given
‘ionic’ and ‘covalent’ in the test, so adopted these as clues to the correct response.
Also, titanium (IV) chloride may be unfamiliar, so students were not prompted to
Table 9.
RC
Changes in SAC students’ responses to ‘Chlorides’.
Description
1st %
2nd %
3rd %
P1
P2
P
Intermolecular bonds between TiCl4 molecules break
Intermolecular bonds in ionic solids are stronger
Total
0.8
0.4
1.2
4.4
3.6
8.0
13.2
3.2
16.4
Q1
21.2
14.0
13.6
Q2
Q
Covalent subs have lower boiling points / more heat
required
MgCl2 only melts / lattice needs to break down
Total
0.8
22.0
2.0
16.0
4.0
17.6
T1
T2
T3
T4
T
Ionic bonds can’t be broken by heating
MgCl2 ionises / is less reactive / already vapourised
Covalent bonds are weaker than ionic ones, so break
Covalent bonds are stronger than ionic ones
Total
12.8
1.6
24.0
5.6
44.0
18.8
2.8
28.4
8.4
58.4
14.8
1.6
30.8
7.6
54.8
U1
U2
U
Uncodeable
No response
Total
12.4
20.4
32.8
8.8
8.8
17.6
4.4
6.8
11.2
Overall total
100.0
100.0
100.0
n ˆ 250
1196
V. BARKER AND R. MILLAR
consider intermolecular bonds as responsible for the boiling point variation.
Choosing more familiar compounds may have affected responses.
Changes to students’ thinking about intermolecular bonds as probed by these
questions shows mixed progress. Responses to ‘Hydrogen bond’ suggest that most
students can at least recognize these and explain their formation. However, only
about half of the sample could apply this knowledge to explain water boiling.
Further, ‘Chlorides’ suggests that the effects of any other intermolecular bonds
are difficult to understand, as most students cannot explain differences between
two compounds in which hydrogen bonds do not feature. These questions also
reveal that students may complete SAC with a faulty model of state change which
involves covalent bond fission and reformation. This represents a contradiction:
students appear to know that hydrogen bonds are responsible for high boiling
points, but argue that it is covalent bonds, not hydrogen bonds, which break
during state changes. They are unable to link knowledge about hydrogen bonds
or other dipole-dipole bonds to boiling points of chlorides.
Discussion
The evidence presented here indicates that the SAC approach to teaching basic
thermodynamics has had significant positive impact on students’ learning,
enabling half of the students to demonstrate an understanding about where energy
comes from in fuel-oxygen systems. SAC materials were cited in very positive
terms by a number of interviewees who could recall specific contexts and chemical
ideas in a clear and lucid way indicative of the impact of their learning experiences.
Thermodynamics
This study probed the basic idea that energy is released in chemical bond formation. Evidence indicates that about half of the cohort involved learned this effectively, but that about one-quarter thought that bonds or molecules ‘store’ energy
which is released when the bonds are broken, perhaps in the same way that the
contents of an egg are released on fracturing the shell. This idea appears to resist
change. One reason for this is the persuasive earlier teaching that fuels store
energy, and the implication that oxygen is not really involved in the energy release.
These findings suggest that teachers should avoid using this notion, as it appears to
cause confusion among students when they are taught more sophisticated ideas at a
later stage. Ross (1993) suggests that combustion reactions are presented to
students as ` fuel-oxygen systems’ to help students associate energy release with
bond formation.
Chemical bonding
Although basic ideas about covalent and hydrogen bonding appear to be learned by
a majority of students, ions and ionic bonding continue to cause difficulties. Some
students seem to imagine ionic compounds exist as discrete molecules like as
covalent compounds and therefore think of ionic bonds as uni-directional and
subject to the same ‘rules of behaviour’ as covalent bonds. These ideas seem to
originate in pre-16 teaching, which encourages students to link the low boiling
points of covalent compounds to the idea that covalent bonds are ‘weak’ compared
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
1197
to ionic bonds and so break more easily. The ‘discrete molecules’ model for ionic
compounds also influences students’ thinking about displacement reactions. They
reason that hydrogen chloride exists as discrete molecules in acid solution and
when a metal is added a bond is being formed between the metal and the chlorine
atom, ‘swapping partners’ with the hydrogen. The model is plausible and avoids
considering ions. The model also implies that students will find it difficult to
understand or appreciate the nature of the ionic lattice, which requires understanding of multi-directional attraction. This could influence their views about
change of state of ionic substances.
Links between thermodynamics and chemical bonding
Data presented here suggest that students do not readily link basic chemical thermodynamics and chemical bonding. ‘Methane molecules’ shows that students
consider satisfaction of the carbon valency is the reason why methane has the
formula CH4 . ‘Sodium and chlorine’ indicates that most respondents think only
that elements are ‘forming a compound’ rather than releasing energy. ` Energy
change’ shows that about one-fifth select an appropriate energy diagram for the
sodium/chlorine reaction on instinct alone and cannot explain why this may be
right. These instances indicate that students consider thermodynamics and chemical bonding as separate topics. They are not encouraged to make links between
them so as a result seem to end their A level studies with erroneous ideas. SAC
integrates chemical ideas from the traditional areas of chemistry, teaching ` organic’
and ` physical’ aspects within the same unit. However, students do not seem to
make links between difference conceptual areas readily, and by the end of the
course they have perhaps not integrated their thinking as effectively as may be
hoped. Further, the context in which the basic thermodynamics is taught is extremely effective and clearly develops a very strong association in students’ minds
between energy release and covalent bond formation in a fuel-oxygen system. This
is commendable, but may also mean that students find it hard to transfer their
knowledge to other reactions. Indeed, evidence presented here suggests that they
do not nearly as readily associate energy release with ionic bond formation as for
covalent bond formation.
A faulty model for change of state
Responses to ‘Boiling’ and ‘Chlorides’ indicate that between one-fifth and onethird of A level students may complete their course with a faulty model to explain
change of state. In this ‘bond fracture’ model, covalent bonds break and then
reform when a substance vapourizes and condenses. The model accounts for the
‘oxygen and hydrogen’ responses to ‘Boiling’ and the ‘covalent bonds are weaker so
break’ answers to ‘Chlorides’. This reasoning may originate from pre-16 teaching,
describing covalent compounds as having ‘low’ boiling points. Pre-16 students do
not study intermolecular bonds, but are taught about covalent and ionic bonds, so
perhaps inevitably use intramolecular bond fission to explain the change from solid
to liquid or liquid to gas. The model applies equally to ionic solids, as these can
also exist in students’ minds as discrete molecules, and indeed their earlier teaching supports this, because ionic compounds have ‘high’ boiling points and so
bonds in these ‘molecules’ must be ‘strong’. The key problem students seem to
1198
V. BARKER AND R. MILLAR
express here is application of intermolecular bonding. Although responses to
‘Hydrogen bond’ show that most have learned about hydrogen bonds, relatively
few students apply this information to account for state changes in water. Even
more striking is the finding that despite teaching, no other intermolecular bond
type was mentioned in responses to ‘Chlorides’. The notion of intermolecular
bonds therefore does not seem to prompt changes to this model in perhaps
about one-quarter of students.
Changes to teaching approaches
We propose four specific areas in which teaching approaches may need to be
reviewed in the light of these findings. First, the exothermicity of bond formation
needs further attention. Within SAC, this receives very explicit, context-related
treatment which, as the data presented above indicates, is clearly effective.
However, given that the ideas of one-quarter of students remain unchanged,
further work on reinforcing this is needed. Second, the teaching of ionic bonding
is problematic. Students need to be taught that ionic bond formation releases
energy in the same way as covalent bond formation. This could be aided by
emphasizing the similarities between Hess’ law cycles and Born-Haber cycles.
Related to this is the poor understanding demonstrated of the nature of ionic
compounds - we should revise strategies to ensure students appreciate the nature
of the ionic bond as multidirectional not unidirectional. Third, pre-16 teaching
about the relative strengths of covalent and ionic bonds and the link to boiling
points must be reviewed. The current trend, described above, may lead students to
the ‘bond fracture’ model for change of state discussed earlier. This links with the
fourth area, approaches to teaching intermolecular bonds. We need to work on
ways of developing students’ ideas about intermolecular bonds other than hydrogen bonds, and to apply this knowledge correctly in appropriate situations.
As a final point, we draw together several references made above about teacher
language. The dependence on ‘valency’ may lead students to ignore energetics in
considering molecular stability and prompt anthropomorphic explanations like
‘carbon wants to make four bonds’. Students in this study also picked up the
notion that hydrogen bonds are ‘attractions’ not ‘bonds’ from their teachers,
which introduces unnecessarily confusing terminology. Pre-16 language also has
significant impact. ‘Covalent compounds have low boiling points’ may create the
impression that therefore covalent bonds are ‘weak’; while the description of displacement as involving ‘swapping partners’ may support the discrete molecules
model for ionic compounds discussed above.
Conclusions
These data indicate that the SAC approach is clearly effective in teaching the basic
chemical ideas explored here. The context-related approach for thermodynamics
has a significant impact on a high proportion of students, while the drip-feed
strategy encourages a majority to develop clear ideas about aspects of chemical
bonding. However, some misunderstandings remain difficult to change. We have
discussed reasons for this and suggest finally that these problematic areas require
changes in current teaching practice at pre-16 and A level.
CHANGES IN A CONTEXT-BASED POST-16 CHEMISTRY COURSE
1199
Acknowledgements
The work described in this paper was carried out as a DPhil study at the
University of York. The work was made possible by grants from ICI and Shell
(UK). We are grateful for their support.
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