chemical dominoes
Section 6
Electrochemical Cells and
Half-Reactions
Section Overview
In this section, students will use their red LED
to build a conductivity tester. They test several
solutions to determine which ones conduct
electricity, and therefore contain electrolytes.
They will construct a zinc-copper battery and
use the LED to determine the direction of the
electricity flow. To conclude the section, students
will connect batteries in a series to create a circuit
with more voltage. This should enable them to
light LEDs that require greater voltage.
Background Information
Electricity Conduction
Through a Solution
In most electrochemical cells, part of the electrical
circuit runs through a solution. Electrical
conductivity has two conditions that must be
satisfied simultaneously:
1) There must be charged particles in the
solution – anions and cations.
2) The charged particles must be able to
move around.
It follows that the more charged particles
there are, the better the medium will conduct
electricity. Similarly, the more easily the charged
particles can move, the better the medium will
conduct electricity. This places a limit on the
increase in concentration in many circumstances.
For example, it has been found that as the
concentration of sulfuric acid increases, its
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conductivity decreases when it reaches very high
concentrations (about 97%) because at this point
the solution is so viscous that the charged particles
cannot move as easily.
There are many examples of media that do not
conduct electricity well. These examples can
confuse students, largely because applying two
conditions simultaneously can be conceptually
challenging. Pure distilled water is not a very
good conductor of electricity, and its electrical
conductivity is certainly not measurable with the
low-tech conductivity meters students construct
in this activity. Pure water is an example of a
medium that satisfies the second criterion listed,
but does not satisfy the first one. (Actually, there
are charged particles, namely H3O+ and OH−,
in pure water, but they are present in such low
concentrations [about 10−7 M], that electrical
conductivity is not measurable in this section.)
Molecular solids do not conduct electricity
because they do not satisfy either of the criteria
(molecules are neutral particles). Therefore,
when molecular solids are dissolved in water,
the resulting solutions do not conduct electricity
either, because there are no charged particles.
Crystals of salts, which are ionic solids, can also
confuse students if they have learned that ionic
solids are composed of charged ions arranged
in lattices. This is an example of a medium that
satisfies the first but not the second criterion.
However, when ionic solids are dissolved in water,
the ions become solvated and can move through
the liquid phase. Therefore, solutions containing
dissolved ions do conduct electricity.
Section 6 Electrochemical Cells AND HALF-REACTIONS
An electrochemical cell can be thought of as a
way of moving electrons through a wire for an
oxidation-reduction (or redox) reaction to take
place. The simplest version of an electrochemical
cell is represented by the following diagram:
Electron
Flow
Anode
–
Salt
Bridge
+
Cathode
chapter 4
Electrochemical Cells
the answer to the first question lies in the
thermodynamic quantity ΔG, as usual. It can be
shown that:
ΔG = −nFE°cell
where n = least common multiple of electrons
transferred (see Background Information in
Section 4)
F = t he Faraday constant =
96,500 Coulombs/mole of electrons, and
E°cell = t he standard cell potential,
measured in volts,
Each beaker represents one-half of the redox
reaction. The beaker on the left, with the anode,
is where the oxidation half-reaction takes place
(the anode cell is defined as the location of the
oxidation half-reaction), while the beaker on the
right, with the cathode, is where the reduction
half-reaction takes place. The circuit is complete
when electrically charged particles can make a
complete circuit, though the same particles do not
travel around the entire circuit. (See section titled
The Path of Electricity in a Battery in the Chem
Talk for this section in the Student Edition.) In
particular, it is important to note that removing
the salt bridge, or altering it so that electrically
charged particles cannot move through it (such as
filling it with distilled water), breaks the circuit
and stops the flow of electricity.
Reduction Potentials
and Predicting Spontaneity
Two questions that apply to all of reaction
chemistry are: Can the change occur? and, How
fast does it occur? In the case of electrochemistry,
and the “naught” superscript (o) on ΔGo and
E°cell indicates “standard” conditions (of 298 K
temperature, 1 atm pressure, and 1.0 M
concentrations of all solutions). The product nF
is simply the quantity of electric charge transferred
from anode to cathode. The main point to keep
in mind regarding this equation is that spontaneity
of a particular cell setup is determined by the
sign of E°cell. Specifically, a cell setup will be
spontaneous (ΔG<0) when the standard cell
potential is positive (E°cell > 0).
Determining the value of the standard cell
potential is a matter of knowing how to use
tables of standard reduction potentials and
knowing the oxidation and reduction halfreactions in the system. Tables of standard
reduction potentials provide voltage values
for half-cells. The standard cell potential is the
sum of the voltages for each half-cell. When
looking up a reduction half-reaction in the tables,
simply use the voltage provided in the table.
When looking up an oxidation half-reaction
in the tables, reverse the sign of the voltage
provided, since the oxidation is the reverse of
the reduction. The logic is no different from the
logic applied when using the Metal Activity Series
to judge whether a given redox reaction will be
spontaneous. In fact, it is the order of the metals
in the standard reduction potentials that gives us
the Metal Activity Series.
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For example, consider the following problem:
What is the spontaneous direction in which
electricity will flow in the following electrochemical
cell, and what voltage would register if a voltmeter
is connected across the wire?
voltage = ?
v
Al
Case 1 leads to a negative sum of the half-cell
potentials (E°cell = −1.66 + 0.25 = −1.41 V), and
case 2 leads to a positive sum for E°cell = +1.41 V.
Therefore, case 2 is the spontaneous direction,
and the voltage that should register on a voltmeter
is 1.41 V. The cell in spontaneous operation
should look like this:
voltage = 1.41 V
Salt
Bridge
Ni
Electron
Flow
Anode
1.0 M
solution of
Al2(SO4)3
1.0 M
solution of
NiSO4
To determine the spontaneous direction, look at
the half-reactions and determine which cell will
be anode and which will be cathode to provide a
positive value for the standard cell potential. The
two half-reactions are either:
case 1: reductionAl3+ + 3e− → Al
oxidationNi → Ni 2+ + 2e−
or
case 2: oxidationAl → Al3+ + 3e−
reductionNi 2+ + 2e− → Ni
(This should look similar to the approach in
Section 4.) According to tables of standard
reduction potentials, the standard reduction
potential for the aluminum reduction is –1.66
volts, and the standard reduction potential for the
nickel reduction is –0.25 volts. Therefore, the two
possibilities translate to the following potentials
(sign switches when it’s an oxidation):
case 1: reductionAl3+ + 3e− → Al Eo = −1.66 V
oxidation Ni → Ni 2+ + 2e− Eo = +0.25 V
or
case 2: oxidationAl → Al3+ + 3e− Eo = +1.66 V
reductionNi 2+ + 2e− → Ni Eo = −0.25 V
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v
Salt
Bridge
–
Al
1.0 M
solution of
Al2(SO4)3
+
Ni
Cathode
1.0 M
solution of
NiSO4
Non-Standard Conditions
There are two ways to deviate from standard
conditions: change the temperature, and change
the concentrations of the solutions (or pressure,
if it’s a gas). Laboratory conditions are very
close to standard temperature (and pressure),
so only changing solution concentrations will
be considered.
The value of n = 6 is the least common multiple
of electrons required to arrive at the balanced
redox reaction. The expression for the reaction
quotient is:
[Al3+]2
 ​ 
Q = ______
​  2+ 3 
[Ni ]
Under standard conditions, Q = 1, because all
solution concentrations are 1.0 M. Continuing
with the same aluminum-nickel cell example,
the value of Q will decrease (be a value less than
1.0) from standard when either the aluminum
ion concentration is less than 1.0 M or when the
nickel ion concentration is greater than 1.0 M.
Section 6 Electrochemical Cells AND HALF-REACTIONS
Generalizing from this, to increase the
voltage, make the solution in the anode cell
(Al3+ ions in our example) more dilute, or
else make the solution in the cathode cell
(Ni2+ ions in our example) more concentrated.
This makes sense because the potential to react
is greater when there is more drive to arrive at
products (Al3+ ions) or more drive to use up
reactants (Ni2+ ions). This is an illustration of
a general principle called Le Châtelier’s principle,
which is useful in explaining much of
equilibrium chemistry.
chapter 4
When Q is less than 1, the logarithm of it
takes a negative value, and therefore the nonstandard cell potential is greater than E°cell.
A battery begins with concentrations in half-cells
in a non-equilibrium state. The reactions run
spontaneously in the direction that approaches
equilibrium. As the battery runs, the voltage
continually decreases. Once the concentrations
in the half-cells reach equilibrium, there is no
more drive for the battery to run spontaneously
in only one direction. This means, in effect, that
the voltage drops to zero. Batteries eventually
cease to produce voltage because they “run out.”
Rechargeable batteries can be connected to an
electrical source and forced to run backwards
(with the input of electrical energy), building up
concentrations again in the half-cells to a nonequilibrium state where the half-reactions will run
spontaneously again as if the battery were new.
learning outcomes
learning outcomes
Location in Section
Evidence of Understanding
Determine if a substance will conduct
electricity when dissolved in water.
Investigate
Part A
Chem Talk,
Checking Up
Questions 1-2
Chem to Go
Questions 1-4
Students successfully complete Part A, and provide
answers that match those provided in this
Teacher’s Edition.
Construct a galvanic cell and explain
the function of its components.
Investigate
Part B
Checking Up
Questions 3-5
Chem Essential Questions
Students successfully complete Steps 1-8 of Part B, and
can itemize the battery’s components and functions.
Describe the effect of adding cells in
series.
Investigate
Part B
Chem to Go
Questions 5-7
Inquiring Further
Students successfully complete Steps 9-12 of Part B,
and can provide answers that match those in this
Teacher’s Edition.
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Section 6
Materials, Chemicals, Preparation, and Safety
(“per Group” quantity is based on group size of 4 students)
Materials and Equipment
Materials
(and Equipment)
Quantity
per Group
(4 students)
Tongue depressor
1
Resistor, 100-ohm
1
D-cell battery
1
Battery holder, D-cell
1
Red LED
1
Copper strip, 5 mm x 5 cm x 1 mm
Chemicals
Quantity
per Class
(24 students)
Sodium chloride solution, NaCl,
0.1 M
200 mL
Potassium nitrate solution, KNO3,
0.1 M
200 mL
Sucrose solution, C12H22O11, 0.1 M
200 mL
1
Fructose solution, C6H12O6 ,0.1 M
200 mL
Zinc strip, 5 mm x 5 cm x 1 mm
1
200 mL
Steel wool or sandpaper
1
Zinc nitrate solution, Zn(NO3)2 ,
0.1 M
U-tubesalt bridge
1
Copper (II) nitrate solution,
Cu(NO3)2 ,0.1 M
200 mL
Cotton balls
4
200 mL
Voltmeter
1
Sodium nitrate solution, NaNO3,
0.1 M
Graduated cylinder, 10 mL
1
Graduated cylinder, 100 mL
1
Beakers, 50 mL
2
Beakers, 100 mL
5
Wires with alligator
clips on each end.
4
Materials (and Equipment)
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Chemicals
Quantity
per Class
Scissors
1
Electrical tape, roll ½”
1
Beakers, 250 mL
7
Section 6 Electrochemical Cells AND HALF-REACTIONS
0.1 M NaCl—Dissolve 1.2 g of sodium chloride
(NaCl) in 180 mL deionized water and then
adjust volume to 200 mL. Store in a labeled
250-mL beaker for class use.
0.1 M KNO3—Dissolve 2.0 g of potassium nitrate
(KNO3) in 180 mL of deionized water and then
adjust volume to 200 mL. Store in a labeled
250-mL beaker for class use.
0.1 M Sucrose—Dissolve 6.8 g of sucrose
(C12H22O11) in 180 mL deionized water and then
adjust volume to 200 mL. Store in a labeled
250-mL beaker for class use.
0.1 M Fructose—Dissolve 3.6 g of fructose
(C6H12O6) in 180 mL of deionized water and then
adjust volume to 200 mL. Store in a labeled
250-mL beaker for class use.
1.0 M Zn(NO3)2—Dissolve 59.5 g of zinc nitrate
hexahydrate (Zn(NO3)2•6 H2O) in 180 mL of
deionized water and then adjust volume to
200 mL. Store in a labeled 250-mL beaker for
class use.
1.0 M Cu(NO3)2—Dissolve 48.4 g of copper (II)
nitrate trihydrate (Cu(NO3)2•3 H2O) in 180 mL
of deionized water and then adjust volume to
200 mL. Store in a labeled 250-mL beaker for
class use.
chapter 4
Teacher Preparation
Look at the packaging from the red LEDs.
If the package indicates a forward voltage
maximum that is higher than 3 volts, you will
not need to use resistors. If no maximum is
indicated, you can still avoid resistors if you have
extra LEDs and can replace those that burn out
(life spans of LEDs will vary when operated at
3 volts). If you need or prefer to use resistors,
the calculation necessary to determine which
resistor(s) to use is below. Look for the forward
voltage value and the forward current value on
the package. Assuming a 3-volt battery, calculate
the resistance needed by the following equation:
3-V
(battery voltage)-(LED voltage)
​  LED
 ​ = ​ 
 
  ____________________________
   
  
 ​
Rneeded = _______
ILED
(LED current)
Resistors are only available in certain increments
(10-ohm, 47-ohm, etc.). Use the resistors that you
can, in series, to sum up to a resistance value near,
but not more than, the resistance calculated.
To save time, test this in advance.
Safety Requirements
•Goggles and aprons are required in the
laboratory area.
•Solutions can be disposed of down the drain.
•Wash hands and arms before leaving the
laboratory area.
1.0 M NaNO3—Dissolve 17.0 g of sodium nitrate
in 180 mL of deionized water and then adjust
volume to 200 mL. Store in a labeled 250-mL
beaker for class use.
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Meeting the Needs of All Students
Differentiated Instruction
Augmentation and Accommodations
Learning issue
740
Reference
Augmentation and Accommodations
Constructing a
conductivity tester,
fine motor skills,
following a diagram
Investigate
Part A, 1.
Accommodations
• S ome students may have the ability to construct something real with their
hands from given parts and a diagram, but for others it may be daunting. One
way to divide the groups for this activity is to identify those students who enjoy
putting things together and place one of them in each group.
•A
simple way to help students with this task would be to provide a model they
can refer to at their lab tables. Using multiple models that illustrate each step
would provide an even better scaffold.
•T
he simplest scaffold would be to construct a conductivity tester step-by-step
with the class.
Organizing
observations in a
table
Investigate
Part A, 5.
Augmentation
•C
heck students’ tables before they begin their observations to make sure they
have the space for all the information they need. Help them correct their table
format before they proceed.
Accommodations
• L ist the components of the data table for students. They need to chart the
name of the solution, the chemical formula, the appearance at the beginning
and end, and the results of the conductivity test. Be sure to include a line for
distilled and tap water.
• Give the students a table already formatted that they can complete.
Draw pictures to
represent particles in
distilled water and in
tap water
Investigate
Part A, 6.a)
ChemTalk
Augmentation
• S how students sample pictures of particles in solution. Ask them to discuss
which picture represents particles in distilled water, which could be particles in
tap water, and which might be a third category altogether. Discuss the reasons
the solutions may be different.
•H
ave students compare the drawings of solutions and the conditions necessary
for a solution to conduct electricity with their observations in Investigate, Part A.
Point out what they should have been observing according to the text. Allow
them to modify their observations to improve the accuracy of their descriptions
and the vocabulary used, but do not allow them to copy the observation unless
their actual reaction was similar.
Multi-tasking,
attention to task,
sequencing
Investigate
Part B
Augmentation
• In Step 2, students are asked to measure copper nitrate, place it into a specificsized beaker, label the beaker, place their copper electrode, observe what
happens and record it. Students who have difficulty following directions,
sequencing a series of steps and paying attention could bring chaos to this
activity. The requirements for simultaneously processing all those tasks at once
is significantly diminished when the activity is structured so that students can
complete one task at a time and work from a model. A quiet, orderly classroom
will help students maintain their focus. The activity remains quite complex in
the next steps. Monitor closely to see that all groups stay on task and follow
correct procedures. Built-in checks will prevent the procedure from failing.
Section 6 Electrochemical Cells AND HALF-REACTIONS
Reference
chapter 4
Learning issue
Augmentation and Accommodations
Applying previously
learned concepts
and skills in a new
problem
Investigate
Part B
Chem Talk
Augmentation
•M
any students may not remember how to write the half-reactions and
oxidation-reduction equations and may not be able to find the information in
the previous section. Review and model balancing these equations with other
chemicals before students get started. Make sure all students have the essential
concept of how ions can be expected to react.
•R
efer students to The Path of Electricity in a Battery in the Chem Talk section
for a model of the equations and an explanation of the reaction. Have them
compare their observations to the ones in the text.
Following directions
Investigate
Part B, 5.
Augmentation
• Model this step for students.
Defining vocabulary
and connecting it to
key concepts
Chem Talk,
Checking Up
Augmentation
•H
ow do a metal’s reactivity and its number of electrons affect its charge and
the direction in which electricity flows? How is this related to the anode and
cathode of a battery? Students will need to synthesize the information in the
Chem Talk section and refer back to Investigate, Part B, Step 8 where anode
and cathode are first mentioned. They may need help putting this information
together to understand the concepts.
Strategies for Students with Limited English Language Proficiency
Learning issue
Reference
Augmentation and Accommodations
Background
knowledge
What Do You
Think?
Ensure that students understand the meaning of “eventually” as an adverb.
Share derivatives.
Vocabulary
Following directions
Investigate
Check understanding and pronunciation of new words such as “distilled.” Have
students work in small groups to explain what the questions are asking for.
Clarify the direction in Part B, Step 1., when students are asked to “construct.”
Also, clarify how students are to “record” answers.
Background
knowledge
Vocabulary
Comprehending text
Chem Talk
Have students add words that are in block print to their personal dictionaries. Refer
to the word “conduct” as a derivation of “conductivity.” Students might benefit
from multiple meanings of the word “conduct.” Check for understanding of signal
words such as “since,” and “while.” Also, check for understanding of the word
“spontaneously,” and share derivatives.
The teacher may want to separate sections of the text and either share the reading
aloud or have small groups read through the sections. Asking literal questions will
allow the teacher to assess understanding.
Supporting details
Research skills
What Do You
Think Now?
Refer to another use of the word “eventually.”
Comprehension
Vocabulary
Chem Essential
Questions,
Chem to Go
Share derivatives of the word “phenomenon.” Ensure that students understand
the word “resemble.” Make sure that students know the distinction between
“compare,” and “contrast.”
Allow students to discuss the possible answers to the questions. Have students
work in groups to achieve a consensus on the possible answers.
Following directions
Inquiring Further
Check for understanding of the word “affect” and explain its difference from
“effect.” Students can answer questions in small groups or with a partner. Clarify
what is expected when they are asked to “determine” something.
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Section 6
Teaching Suggestions
and Sample Answers
What Do You See?
The primary purpose here is
to arouse students’ interest
and encourage them to share
their perceptions and ideas based
on the illustration. You can
expect all kinds of responses,
ranging from “the cat and the
mouse are dancing” to more
interpretive observations. For
example, it appears that the
phonograph is being powered
by something coming from two
beakers. Both responses are
fine and it is your task to elicit
responses without judgment.
Some of the features in the
illustration that students will later
find significant are: a galvanic
battery, a zinc strip, a copper
strip, and a diagram explaining
How to Make Batteries.
Students’ Prior Conceptions
Some stumbling blocks to learning can include the
following misconceptions:
1. Electrons can flow through solutions.
Most people think of electricity as involving the
flow of electrons, and for metals, this is correct.
This probably comes from the root of the word
electricity being electron, and both teachers and
students capitalize on this when first learning
about electricity. However, in solutions, it is not
the electrons that are in motion. Instead, charge
balance is maintained in the solution by the
movement of cations and anions toward the
electrodes where charge transfer takes place at the
interface between an electrode and the solution.
Without ions, there is little or no electrical current.
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2. Water is a good conductor of electricity.
Water is, in fact, a very poor conductor of
electricity. The reason it is dangerous to insert
a light bulb while standing in a puddle of water,
or to use a hair dryer while in the bathtub, is that
water is a great solvent for ionic compounds. Tap
water and fresh water typically contain dissolved
ions in sufficiently high concentrations to enable
the solution to be electrically conductive. It is the
ions in the solution that carry the charge, however,
not the water itself. The water simply provides a
medium through which the charged ions can move.
Section 6 Electrochemical Cells AND HALF-REACTIONS
These are broad questions that
will have varied answers.
They should lead to a good
discussion. The purpose of
these questions is to elicit
students’ prior understanding
and to uncover misconceptions.
Correct answers are not
important, only engagement.
This is also an opportunity for
you to gauge students’ answers
as indicators of their prior
knowledge and conceptions.
What Do You Think?
a chemist’s response
A battery uses a spontaneous
oxidation-reduction reaction
separated into half-cells so that
electrons are not transferred
directly from atom to ion but are
forced to flow through a circuit
which generates a potential
difference (voltage).
A battery eventually dies because
the reactants for the reaction are
consumed.
3. The salt bridge is unnecessary.
This preconception is incorrect because a circuit
must be complete in order for electricity to flow.
Electricity cannot flow in an electrochemical cell
unless charge balance can be maintained in the
solutions. Solutions cannot build up extra positive
or negative charges, due to the oxidation and
reduction half-reactions taking place separately
in the two half-cells. In the oxidation half-cell
(or anode), a metal is oxidized and the resulting
electrons move through the circuit, leaving behind
positively charged metal ions in the solution.
Investigate
chapter 4
What Do You Think?
Part A:
Solutions That Conduct
Electricity
1.
You may not need a resistor
(see Teacher Preparation section)
but if in doubt, use a 100-ohm
resistor in series with the other
components. Instruct students
to use electrical tape to hold the
tester together as well as hold
the connections in place.
In order to balance the overall charge, negative
ions must be able to flow into the solution. These
negative ions travel through the salt bridge from
the reduction half-cell (or cathode). Similarly,
positive ions travel through the salt bridge toward
the reduction half-cell, to balance the charge in
that solution.
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2.a)
5.
The LED will not light, or only
very weakly. If it does light,
then the water is contaminated.
Repeat the test with pure water
without ions.
The solutions that should
conduct electricity are NaCl
and KNO3. Solutions that
should not conduct electricity
are sucrose and fructose. If the
sucrose and fructose solutions
do conduct, either the students’
samples have been contaminated
by insufficient washing of the
leads between uses, or the stock
solution is contaminated.
2.b)
Make sure students understand
it is necessary to test distilled
water to establish that it is not
the substance that is conducting
electricity. The testing of the
distilled water serves as a
control for the testing of other
substances.
3.
Students should have a good
sense of which compounds are
ionic in nature. They may be less
familiar with the two sugars,
which are organic compounds.
4.a)
The solutions will be identical
in appearance clear, colorless
liquids.
4.b)
Students should be able to
predict that the substances most
likely to create charged ions are
the NaCl and KNO3, which are
the ones with metal ions.
Notes
744
6.
Tap water should conduct
electricity, because it has
dissolved electrolytes in it
(such as sodium fluoride and
sodium chloride).
7.
All solutions can be disposed of
by pouring down a drain. The
beakers should be rinsed well.
teaching tip
Students should be able to
complete Part A in a 45 minute
class period. If you are running
short of time, you could provide
students with commercial
conductivity testers. However,
constructing their own device will
provide them with better insight
that will help them complete the
challenge.
Part B:
Making a Battery
1.
To save time, you may want to
clean the electrodes before the
lab. If students do it, be sure
they use the steel wool on a
durable or protected surface.
This would be a good time to
discuss why it is necessary to
clean the electrodes and identify
the coatings (ZnO and CuO).
This section provides a review of
what students learned previously
about the Metal Activity Series.
They should be able to predict
that the Zn2+(aq)/Cu(s) reaction
will not occur, but that there
should be evidence of the
occurrence of a reaction in the
case of Cu2+(aq)/Zn(s).
Section 6 Electrochemical Cells AND HALF-REACTIONS
chapter 4
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chemical dominoes
2.
If this is the students’ first
exposure to the terminology
and concept of solution
concentration, it might be
helpful to show students a
volumetric flask and talk about
the preparation procedure of
making a 1 molar solution.
2.a)
Nothing should happen to the
copper electrode. Students may
observe some air bubbles on the
surface of the metal but this is
not a sign of a reaction.
3.a)
Nothing should happen to the
zinc electrode. If a reaction is
observed, the students might
have placed the zinc electrode
into the copper nitrate (blue)
solution.
3.b)
Since nothing happened to
the electrodes, the students do
not have much information
available to answer this question.
However, if they understand the
Metal Activity Series, they may
correctly infer that the two metal
electrodes need to be in solutions
of their own ions in order for
the battery to work properly. If
the zinc electrode is placed in the
copper solution, the oxidation
reaction and the reduction
reaction will occur at the zinc
electrode and no electricity will
pass through the circuit. In short,
if you put the zinc in the copper
solution, a reaction will happen
that will produce Zn2+ and Cu in
the same beaker.
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4.a)
5.
Zinc atoms lose electrons and
copper atoms gain electrons. The
two half-reactions are:
Students should use tweezers to
handle the glass wool (or the
cotton ball). This serves as a
barrier to slow the migration of
Na+ ions. A strip of paper towel
soaked in the salt solution may
also serve as the salt bridge.
At the anode: oxidation
Zn → Zn2+ + 2e−
At the cathode: reduction
2+
Cu + 2e− → Cu
No change should occur.
4.b)
2+
6.a)
2+
Cu + Zn → Zn + Cu and the
electrons cancel out.
Section 6 Electrochemical Cells AND HALF-REACTIONS
chapter 4
with the two connected in series,
the 2.5–2.6 volts available should
be enough to light the LED.
In the diagram in the Student
Edition, the LED is deliberately
shown with two leads of equal
length, so that students will have
to use their knowledge of how to
connect the LED.
10.a)
With the battery in operation,
students should begin to see
copper metal coating the
Cu electrode, or dropping into
the Cu2+ solution beneath the
electrode. It is difficult to see
the decrease in mass of the
Zn electrode, however.
Students should note that the
Cu electrode is gaining mass as
the Cu2+ions become Cu atoms.
They might say that the Zn
atoms are becoming Zn2+ ions
and dissolving in the solution
although they cannot observe
that change. They could also
say that the zinc electrode is
not disappearing.
11.a)
7.a)
Drawings should look like
the picture in their textbook
without the voltmeter. The parts
that should be labeled are: Cu
electrode, Zn electrode, copper
(II) nitrate solution, zinc nitrate
solution, salt bridge.
8.a)
The voltmeter should record the
voltage in the positive direction.
If it appears to be negative,
switch the wires attached to it.
It should read 1.0–1.1 volts. If
the voltage is appreciably less,
check all connections. The zinc
electrode is the anode and the
copper electrode is the cathode.
9.
From Part A of this Investigate,
students may recall that the LED
will light when the short lead
on the LED is connected to the
negative terminal of the battery.
The Cu/Zn cell only produces
about 1–1.1 volts, so the LED
probably will not light using the
Cu/Zn cell alone. Nor will it light
using a D-cell battery, which only
produces 1.5 volts. However,
The LED light should go out
when the salt bridge is removed
from the circuit.
11.b)
The LED light should go on
again when the salt bridge is
replaced.
12.a)
Students may predict that the
LED will not light with distilled
water in the salt bridge because
distilled water does not conduct
electricity. On the other hand,
they may predict that the LED
will light because there is a
connection between the two cells.
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chemical dominoes
12.b)
When the NaNO3 (or KNO3)
solution in the salt bridge is
replaced with distilled water, the
cell will not operate and the LED
will not light up.
Chem Talk
In the first section, students
are introduced to the
requirements for a conductive
solution. The Path of Electricity
in a Battery defines and analyzes
the design of a battery. In the
final section, students learn
how the Metal Activity Series
is related to the selection of
materials for a battery.
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Section 6 Electrochemical Cells AND HALF-REACTIONS
chapter 4
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chemical dominoes
Checking Up
1.
A salt solution can conduct
electricity because there are
electrically charged particles
(the Na+ and Cl− ions) that can
move through the solution.
A sucrose solution cannot
conduct electricity because
there are no charged particles
to conduct the electricity.
2.
An electrolyte is a substance that
has charged particles (ions) when
dissolved in water, forming a
solution that conducts electricity.
A non-electrolyte is a substance
that dissolves in water but does
not form charged particles,
meaning that the solution does
not conduct electricity.
3.
In the zinc-copper battery,
the electrons come from the
oxidation half-reaction (when
Zn metal gives up two electrons
to become zinc ions). The
electrons go through the wire and
arrive at the copper electrode,
where they are taken up by Cu2+
ions in the solution through a
reduction half-reaction. The
copper ions then become copper
metal atoms at the cathode.
4.
The anode of a battery is
the terminal from which the
electrons in an electrical circuit
flow. It is where the oxidation
half-reaction takes place.
5.
The cathode of a battery is the
terminal where the reduction
half-reaction takes place.
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What Do You Think
Now?
This would be a good time to
have the students go back to the
What Do You See? illustration
and review their initial
observations. Some aspects of
the illustration should be more
significant to them now.
How does a battery work?
Students’ answers will vary but
they should make reference to
two electrodes and two different
metals. Electrons arise from the
oxidation taking place at the
anode and they are consumed
by the reduction process taking
place at the cathode. The
difference in activity of the two
metals should be mentioned
as the driving force for the
oxidation-reduction reaction.
Section 6 Electrochemical Cells AND HALF-REACTIONS
As the reactions proceed at
each terminal of the battery,
the reactants are used up. The
voltage produced drops off
slowly and finally reaches a level
too low to operate a light or
motor.
You may want to provide
students with the answer found
in A Chemist’s Response at the
beginning of this section and
open a discussion to compare
that response with their answers.
chapter 4
Why does a battery
eventually “die”?
Notes
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Chem Essential
Questions
What does it mean?
SYMBOLIC — The drawing should
be similar to the one shown in
Investigate, Part B, Step 10, in the
Student Edition.
MACRO — Descriptions of the
construction of a battery should
be similar to the procedure
outlined in the Investigate section.
How do you know?
NANO — Electrons flow from
the zinc anode through a wire,
the LED, a second wire, and to
the copper cathode where they
react with Cu2+ ions in the
cathode solution, forming Cu
atoms. The negatively charged
nitrate ions in solution migrate
toward the positively charged
anode to balance the change in
positive ions at each electrode.
Notes
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The voltmeter and the lighting of
the LED provide evidence that it is
a battery.
Why do you believe?
Students’ answers will vary.
Suggested items might range
from MP3 Players, CD Players
and cameras to remote controls,
calculators and flashlights.
Why should you care?
Three ways to change the design
of a battery system to produce
more voltage:
1. Using metals that are farther
apart on the activity series
would increase voltage because
the difference in the metals’
abilities to lose electrons would
increase, resulting in greater
voltage.
2. Linking batteries together in a
series increases voltage.
3. Decreasing the concentration
of the cathode cell and/or
increasing the concentration of
the anode cell would change
the reaction from standard
conditions and thereby
generate higher voltage.
Section 6 Electrochemical Cells AND HALF-REACTIONS
chapter 4
Reflecting on the
Section and the
Challenge
Students should read this
section for a specific, direct
connection between the section
and the Chapter Challenge.
While students do not answer
any questions in this section,
it will provide them with
valuable direction in the Chapter
Challenge. You may want to
provide some class time for
students to read this paragraph
silently or aloud.
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Chem to Go
1.
The solutes that were electrolytes
were KNO3 and NaCl. The
solutions that conducted
electricity could be distinguished
by the light coming on, indicating
a complete circuit was formed.
2.
a) and d): KF and Na2SO4
should make solutions that
would conduct electricity
because they are ionic
compounds and produce ions
when dissolved in water.
3.a)
It would be dangerous to use
an electrical appliance while
bathing because tap water
(bath water) conducts electricity.
If you were to drop the
appliance into the water, you
could be electrocuted.
3.b)
If you took a bath in distilled
water, you would still not be
safe to operate an electrical
appliance, because the salts from
your body would dissolve in
the bath water, making the bath
water conductive.
4.a)
Yes, a sports drink will conduct
electricity because it contains
electrolytes.
4.b)
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The experiment design should
include an indication of how
concentrated the electrolytes are
in solution. One way to do this
would be to continually dilute
the sports drink solution and test
the dilutions for conductivity
until the conductivity falls
below a detectable level. The
drink requiring the most
dilutions before ceasing to be
conductive would be the one
with the greatest concentration
of electrolytes in the original
sports drink. Testing the sports
drink for voltage (i.e., attaching
a voltmeter to the conductivity
tester) would not work, because
the voltmeter would simply
measure the battery’s voltage,
regardless of the concentration
of the solution that completes
the circuit.
4.c)
You would need a control in
the experiment, to prove that
there is no conductivity in a
solution that has no electrolytes.
Distilled water should be used as
the control.
5.
Students should choose a metal
farther up on the activity series
(i.e., more reactive) because those
atoms lose electrons more easily,
which would increase voltage.
Tin, aluminum, and magnesium
would be the realistic choices.
Section 6 Electrochemical Cells AND HALF-REACTIONS
8.
chapter 4
c) Au3+ + 3e− → Au
For reduction, the electrons will
always be on the left side of an
equation.
9.
b) The number lost is always
equal to the number gained.
10.
Preparing for the
Chapter Challenge
Students may use the
information in this section to
design a battery for the Chapter
Challenge. Because of the
difficulty in producing adequate
voltage, students may elect to
use a commercial battery. This
will require them to somehow
complete a circuit to light
the LED. The first part of the
section in which students build
a conductivity tester may give
them some good ideas about
how to design such a device.
Inquiring Further
Factors affecting
voltage generated
6.a)
6.c)
The voltage should decrease
to zero because when the salt
bridge is removed, the ions
cannot flow through it to
keep the anode and cathode
electrically neutral.
The voltage should decrease to
zero because electrons cannot
flow out of the anode.
6.b)
The voltage should decrease
to zero because distilled water
cannot conduct the electrical
current between the anode
and cathode and therefore
cannot maintain their electrical
neutrality.
Be sure to check students’
proposed procedures for safety,
efficacy, and practicality before
approving.
6.d)
The voltage should decrease
because fewer atoms and ions
will be left to react since they
have already reacted over the
preceding hour.
7.
d) c oncentration of ions
decreases
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Section 6 – QUiz
4-6a
Blackline Master
1. For a solution to conduct electricity, which of the following conditions must be met?
a) Charged particles must be present and able to move around.
b) Charged particles must be present but not able to move around.
c) Charged particles cannot be present and molecules must be able to move around.
d) Charged particles cannot be present and molecules must not be able to move around.
2. Which half-reaction correctly represents oxidation?
a) 2Cl− + 2e− → Cl 2
b) Fe2+ → Fe3+ + e− c) 2e− + Sn 4+ → Sn2+
d) Na1+ + e− → Na
3. Why was it necessary to test distilled water for conductivity before you tested the solutions for
conductivity? Explain why the experiment would not be valid if you didn’t test distilled water.
4. The following picture shows a typical electrochemical cell, similar to ones that you built in the
activity. Refer to the picture to give explanations for the following questions.
Electron flow
Salt bridge
Anode
—
Cathode
+
a) W
hy is it necessary to fill the salt bridge with a solution containing dissolved ions?
b) If you connect a lamp in the circuit and let the electrochemical cell run for a while, the light will
become dimmer. Why?
Section 6 – QUIZ ANSWERS
❶
a) Charged particles must be present and able to move around.
2+
3+
−
❷ b) Fe → Fe + e
❸It serves as the control. Since all of the substances are dissolved in distilled water, you have to
test distilled water to make sure that it is not responsible for the conductivity of a solution.
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❹
a) The circuit cannot be complete without mobile charged particles that can move inside
the salt bridge.
b) The voltage will decrease because the ion concentration in the cathode cell will decrease,
which makes the reaction run more slowly.
Section 6 Electrochemical Cells AND HALF-REACTIONS
chapter 4
Notes
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