Subject – CHEMISTRY
As part of your preparation for your A Level Chemistry course, you will need to complete a
research report on The history and development of our understanding of atomic structure.
Your report will be awarded an attitude score (Outstanding, Motivated, Coasting or
Unacceptable). Please see the green box below for the descriptors for O, M, C and U.
Your Chemistry teacher(s) will use the following success criteria to assess your attitude.
Success criteria:
 Report contains ideas that relate to the report title and does not contain plagiarised
content
 Report contains content relevant to the A level Chemistry specification (the specification
can be found here: http://www.aqa.org.uk/subjects/science/as-and-a-level/chemistry7404-7405) Page 11 3.1.1.1 and page 12 3.1.1.3 specifically
 Report is word-processed, Arial font size 12, 1.5 line spacing with margins with normal
widths (2.54 cm)
 The word limit for the report is 1500 words ± 10%
 Correct use of spelling, punctuation and grammar
 Use of annotated diagrams to illustrate key points where relevant
 Diagrams or illustrations should be referenced where applicable
 Inclusion of at least 4 references which may be websites (include full web address)
Please see the exemplar report to help clarify minimum expectations.
Outstanding:
You meet all of the success criteria as described above.
It is evident you have a deep understanding of the topic.
After receiving your ‘next steps’, you have made all
improvements to a high standard.
Attitude grade awarded:
Motivated:
You meet most of the success criteria as described above.
It is evident you have a good understanding of the topic.
After receiving your ‘next steps’, you have made most of
the improvements to a high standard.
Coasting:
You meet some of the success criteria as described above.
It is evident you have a moderate understanding of the
topic.
After receiving your ‘next steps’, you have made some of
the improvements but they are not consistently to a high
standard.
Unacceptable:
You meet none of the success criteria as described above.
It is evident you have a weak understanding of the topic.
After receiving your ‘next steps’, you have not made
improvements to a high standard.
Signed (teacher): ………………………………………..
Date: …………………………..
Date: 1st June, 2017
Name: A N Other
The factors that affect the rate of a reaction
Surface area, temperature, pressure, concentration and adding a catalyst are five factors
that can affect the rate of a reaction. In this report, I will explain how and why each factor
affects the rate of a chemical reaction. An industrial example of catalysis will also be
considered and its importance explored.
The collision theory
Before we can begin to explore what why different factor affect the rate of a chemical
reaction we must first explore the collision theory. This theory explains that for a reaction to
occur certain conditions must be met. In particular, the energy of the collision, and whether
or not the molecules hit each other the right way around (the orientation of the collision). It
should be noted that this theory only applies to reactions where 2 species are colliding.
Decomposition reactions do not follow this theory because this is where a compound
separates into different species. Reactions with more than two species reacting are
extremely unlikely.
The first principle of this theory is that for the molecules to have a chance of reacting they
must first collide, which means that they come into contact with each other. If this occurs
then the molecules may have a chance of reacting, but only if they collide with the correct
orientation and energy.
To explain this theory I will use the reaction between a molecule of ethene and hydrochloric
acid to produce a molecule of chloroethane as an example.
This reaction requires the double bonds to be broken and a hydrogen atom to be attached
to one side, with the chlorine atom at the other. The reaction can only happen if the
hydrogen end of the H-Cl bond approaches the carbon-carbon double bond. Any other
collision between the two molecules doesn't work. The two simply bounce off each other.
Figure 1. http://chemguide.co.uk/physical/basicrates/introduction.html#top
Figure one demonstrates the possible orientations for a collision between HCl and ethene.
Only collision one is a viable option because this is the only way for the hydrogen side of the
HCl molecule to attack the double bond of the ethene molecule.
But it is not enough for collision to have the correct orientation. For a reaction to occur the
collision must also overcome the activation energy. This is the minimum energy required for
the reaction to occur. To represent this graphically we must plot an energy diagram (Figure
2). From this diagram we can see clearly what the activation energy for a reaction is. This is
often different for the forward and reverse reaction depending upon if the reaction is
exothermic or endothermic in the forward direction.
Figure 2: http://www.chem1.com/acad/webtext/dynamics/dynamics-3.html
Now I have discussed the conditions that must be met for two particles to have a reaction; I
will now take each of the 5 factors the in turn and explain how and why they affect the rate
of a chemical reaction.
Surface area
Due to the fact that particles must collide for the chance of a reaction to occur it benefits
the reaction to have more particles exposed. This means that increasing the surface area
must therefore increase the rate of a chemical reaction. This only applies to solid reactants
and can be achieved by cutting or grinding the solid substance into smaller pieces or a
powder. Once this has been achieved we should observe a faster rate of reaction due to
more particles being exposed, which increases the probability of a collision.
Figure 3:
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_gateway/chemical_economics
/reaction3rev1.shtml
A graphic representation of the rate a chemical reaction is shown in figure 3. The rate of a
reaction can be calculated by determining the gradient of a time from the start of a reaction
against mass of the product graph. The red line represents the smaller surface area
reactants and has a less steep gradient than the blue line which represents the reactant
with a larger surface area. From this graph we can also see that it reaches horizontal
plateaux quicker. This means that the reaction reaches completion quicker.
Pressure and Concentration
Increasing the concentration of a liquid solution means that there are more particles in the
same volume of solution. This increases the likelihood of a particle colliding and thus
increases the probability of a successful collision resulting in a reaction.
Altering the pressure of a gas has the same effect. By increasing the pressure of a gas there
are the same amount of particles but in a smaller volume. This again increases the likelihood
of a particle colliding and therefore increases the probability of a successful collision. (Lister
& Renshaw, 2015)
Temperature
Increasing the temperature gives the particles more kinetic energy and therefore they will
have a faster speed. This will increase the frequency of collisions and also increase the
probability of a successful collision.
By analysing two gaseous particles we can deduce that the frequency of their collisions is
proportional to the square root of the kelvin temperature. If you increase the temperature
from 293 K to 303 K (20°C to 30°C), you will increase the collision frequency by a factor of:
That gives an increase of 1.7% for a 10° rise. The rate of reaction will probably have doubled
for that increase in temperature - in other words, an increase of about 100%. A conclusion
can be drawn from this data that the frequency of particles does not need to increase much
to have a big effect of the rate of a reaction.
Catalysts
Catalysts are a substance that can increase the rate of a chemical reaction without being
used up in the process. I have previously discussed that for a reaction to occur the collision
between particles must have the correct activation energy. Catalysts increase the rates of
reactions by providing a new mechanism that has smaller activation energy, as shown in
figure 3 below. The diagram shows that with a catalyst there are a larger proportion of the
collisions that occur between reactants that now have enough energy to overcome the
activation energy for the reaction. As a result, the rate of reaction increases.
Figure 3: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html
It is said that a catalyst decreases the activation energy for a given reaction by providing an
alternative pathway. To illustrate this concept let's consider the mechanism for the
decomposition of hydrogen peroxide catalyzed by the I- ion. Without a catalyst this reaction
occurs in a single step, but in the presence of this ion, the decomposition of H 2O2 doesn't
have to occur in a single step. Therefore it occurs in two easier, faster steps. In the first step,
the I- ion is oxidized by H2O2 to form the hypoiodite ion, OI-.
H2O2(aq) + I-(aq)
H2O(aq) + OI-(aq)
In the second step, the OI- ion is reduced to I- by H2O2.
OI-(aq) + H2O2(aq)
H2O(aq) + O2(g) + I-(aq)
Above we can see that there is no net change in the concentration of the I- ion (1 molecule
reacts in the first reaction and 1 molecule is the product of the second reaction), as a result
of this the I- ion satisfies the criteria for a catalyst.
This example has shown that the addition of a catalyst reduces the activation energy of a
reaction by providing an alternative reaction pathway and that it is not used up in the
process.
An industrial example: The contact process
The contact process is a way to manufacture sulphuric acid. The chemical reaction that
takes place is the conversion of sulphur dioxide into sulphur trioxide. Sulphur dioxide gas is
passed together with air (as a source of oxygen) over a solid vanadium(V) oxide catalyst.
This is therefore an example of heterogeneous catalysis, meaning that the catalyst is a
different state to the reactants.
The fact that this is a reversible reaction makes no difference to the operation of the
catalyst. It speeds up both the forward reaction and the back reaction by the same amount.
The importance of this in industry is to save money and time. A catalyst is a one off fee,
meaning that we do not need to replace the catalyst as it is not used up in the reaction. This
one off fee is counteracted by the fact the reaction is now much more efficient so takes less
money to run the reactor in terms of wages for employees and also in terms of energy bills.
Conclusions
In conclusion increasing the temperature, concentration, pressure or surface area of
reactants will increase the rate of a reaction. Adding a catalyst to a reaction will speed up
the rate of reaction by providing an alternative reaction pathway which lowers the
activation energy.
Word count: 1443 excluding references.
References
All websites were accessed for research purposes between 28th May – 2nd June, 2017.
http://chemguide.co.uk/physical/basicrates/introduction.html#top
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_gateway/chemical_economics
/reaction3rev1.shtml
http://www.chem1.com/acad/webtext/dynamics/dynamics-3.html
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html
http://chemguide.co.uk/physical/catalysis/inorganic.html
T Lister & J Renshaw (2015). AQA Chemistry A level Student Book. Oxford University Press.
Second Edition, Oxford.