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CH 217 Analytical Chemistry
POGIL activity 8
Electrochemical Cell
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Objectives:
After this activity, students will be able to:
1. Identify the cathode, anode, slat bridge, reduction half- cell, and oxidation half-cell.
Students can write the reduction reaction, oxidation reaction, and net reaction of an
electrochemical cell.
2. Predict the direction of a redox reaction.
3. Calculate cell voltage from standard reduction potential.
4. Represent an electrochemical cell with line notation.
Reading: Daniel Harris “Quantitative Chemical Analysis” 8 ed. Chapter 13
Model 1:
Zinc copper cell animation: http://www.wwnorton.com/college/chemistry/gilbert2/chemtours.asp
Observations:
1. The mass of the Cu electrode increases and the concentration of Cu2+ decreases
2. The mass of the Zinc electrode decreases and the concentration of Zn2+ increases
Consider this:
During a redox reaction, one species is reduced, and one species is oxidized. The one that is
reduced gains electrons (GER). The one that is oxidized loses electrons (LEO).
In a device where the oxidation occurs at one location and reduction occurs at another location,
one can force the electron to go through a wire to do work, such as powering a light bulb. The
device is called a galvanic cell. The electrodes are the two metal strips that dip into the solutions
to conduct electricity. Very often they participate in the redox reactions. The cathode is where
the reduction takes place. The anode is where oxidation takes place. The salt bridge allows ions
to slowly migrate from one beaker to the other to maintain electrical neutrality in each half-cell.
The voltmeter measures the voltage between the two electrodes. Each beaker with an electrode
and an electrolyte is called a half-cell.
Look at the direction of electron flow and answer the following questions.
Critical thinking questions:
1.1 Does electron flow in or out of Zn electrode?
1.2 What type of reaction occurs on the Zn electrode? Reduction or oxidation?
1.3 Write down a reaction involving Zn metal and Zn 2+ ion. (left half-cell reaction)
1.4 Is Zinc a cathode or anode? Which end of the voltmeter is it connected to?
1.5 Does electron flow in or out of Cu electrode?
1.6 What type of reaction occurs on the Cu electrode? Reduction or oxidation?
1.7 Write down a reaction involving Cu metal and Cu2+ ion. (right half-cell reaction)
Diagram 1: The Zinc-copper cell
1.8 Is the Cu electrode anode or cathode? Which end of the voltmeter is it connected to?
1.9 The total reaction of the Zn-Cu cell is the sum of oxidation and reduction reaction. Write
down the cell reaction.
1.10 Standard reduction potential is the REDUCTION potential measured under STANDARD
conditions. You may find the values in Appendix H. Check the standard reduction potential of
Zn and copper, which one is higher (more positive)? Look at the assignment of cathode and
anode, which one has higher standard reduction potential?
1.11 Label the above cell (Diagram 1) with anode, cathode, reduction half cell, oxidation half
cell, voltmeter, salt bridge. Mark the direction of where the electrons flow.
Draw conclusions: Circle the right answer or fill the blank in the following table
Electrode
Half-reaction
Anode
Reduction/oxidation
Cathode
Reduction/oxidation
Standard half-cell
potential
Electron flow direction
More Positive/more negative
More positive/more negative
In/out
In/out
Voltmeter connection
Positive/negative end
Positive/negative end
One day, Dr. Who must pass a bridge to go to a Geek concert, an ugly dinosaur who lived under
the bridge jumped out and shouted at Dr. Who: “To pass my bridge, you must solve my riddle!”
“There is something that has Red cat on the right and angry ox on the left! What is it?” Will you
help Dr. Who to figure out the riddle and pass the bridge?
(instructor’s hint to the faculty readers:
Red cat on the right = reduction occurs on the cathode which is usually put on the right
Angry ox on the left = oxidation occurs on the anode which is usually put on the left
The answer an electrochemical cell)
Applications: Based on the general trend listed in table 1, use standard reduction potential to
predict the direction of a reaction.
1.12 If you have an Ag+/Ag half cell and a Cu2+/Cu half cell, which one is going to have
reduction reaction? Which one is going to have oxidation reaction? What’s the net reaction?
Hint: The standard reduction potential is a measurement of the tendency of an ion to be reduced.
For half cells, the half cell with higher reduction potential will be reduced. The half cell with
lower reduction potential will be oxidized. A net reaction is spontaneous when the cell potential
is positive.
1.13 A voltaic cell was prepared by dipping a Magnesium strip in a beaker containing 1.0 M of
Magnesium nitrate, and a lead strip in a another beaker containing 1.0 M of lead nitrate. The
metal strips are connected by a wire, and the solutions are connected by salt bridge.
a. Which one (Mg or lead) is the anode? Which one is the cathode?
The standard reduction potential for Mg2+ is:
Mg2+ + 2e-  Mg(s) Eo = -2.360 V
The standard reduction potential for Pb2+ is:
Pb2+ + 2e-  Pb(s)
E° = -0.126 V
b. Write down the spontaneous net reaction.
Model 2: Calculate the cell potential from standard reduction potential in the above cell.
Ecell = ΔV = Eright – Eleft
Ecell = ΔV = Ecathode – Eanode
The voltage is the difference between the potentials of the two electrodes:E  E  E
E+ is the standard potential that is more positive. It is usually attached to the positive terminal of
the voltmeter. E- is the standard potential that is less positive or more negative.
For example:
Right half-cell: Pb2+ + 2e-  Pb(s)
1) E° = -0.126 V
Left half-cell: Mg2+ + 2e-  Mg(s)
2) Eo = -2.360 V
Net reaction: Pb2+ + Mg (s)  Pb(s) + Mg2+ 3)
Reaction 1 – reaction 2 = reaction 3
E0cell
E 3 = E1- E2
E0cell = -0.126 – (-2.360) = 2.234 V
Why not the other way around? Why not take (-2.360) –(-0.126)? You end up with a negative
cell potential, which means the reverse reaction is spontaneous.
Key questions:
2.1 Calculate the cell potential under standard condition of zinc-copper cell shown in model 1.
2.2 Calculate the cell potential under standard condition of silver-copper cell in question 1.12.
Model 3:
The convention of representing cells by line notation
is to write out a cell by using two symbols:
–
| to represent a phase boundary
–
|| to represent a salt bridge
The electrodes are shown at the extreme left and righthand sides of the line diagram. The cathode (reduction
cell) is by convention put on the right-hand side.
Cd(s) | Cd(NO3 ) 2 (aq) || AgNO 3(aq) | Ag(s)
When there are two species in the same phase, use “,” to
separate them. For example:
Key questions:
Pt (s) Fe3+ (aq), Fe2+ (aq) Ag+ (aq)Ag (s)
3.1 Use line notation to represent the model cell #1 (Zinc-copper cell)
3.2 Consider a circuit in which the left half-cell was prepared by dipping a Pt wire in a beaker
containing an equimolar mixture of Cr2+ and Cr3+. The right half-cell contained a Tl rod
immersed in 1.00 M TlClO4. Use line notation to describe this cell.
Conclusions:
Discuss within the group, list five things you have learned from this activity.