P- Electrode Potential =)

Question 1

Eo/ V
Li+ (aq) + e– → Li(s) –3.04
2 H2O(l) + 2 e– → H2(g) + 2 OH–(aq) –0.83
Use data in the table above to explain why an aqueous electrolyte is not used for a lithium cell. (2)

Answer 1

• Li would react w/ electrolyte (water)
• E of Li < of H2O

Question 2

An EMF value for a commercial lithium-iodine cell is 2.80 V
Suggest why this value is different from the value calculated in part (b). (1)

Answer 2

non-standard conditions

Question 3

State the meaning of the term electrochemical series. (1)

Answer 3

list of electrode potentials in order

Question 4

Iron can be protected by attaching a sacrificial anode (aluminium in this case) to it, which corrodes in preference to iron, keeping it unaffected.
Give 2 reasons why the value of the cell e.m.f. calculated from standard electrode potentials would not normally be achieved when the cell operates in seawater. (2)

Answer 4

1. non-standard conditions
2. current flow affects voltage
3. Al3+ ions may react in alkaline conditions ( w/ OH- –> Al(OH)3 )

Question 5

When aluminium is used as a sacrificial anode for iron, its mass stopped decreasing after a time.
Explain why. (1)

Answer 5

coating developed on Al –> prevent further reaction

Question 6

In other lithium cells, lithium cobalt oxide electrodes and lithium electrodes are used.
Give an equation for the reaction that occurs at the positive lithium cobalt oxide electrode. (1)

Answer 6

Li+ +CoO2 + e- ⇌ LiCoO2

Question 7

What is the purpose of a salt bridge? (1)

Answer 7

complete the circuit/ allow (mobile) ions to move

Question 8

State the substances and conditions needed in a standard hydrogen electrode. (3)

Answer 8

1. H2 (g) at 100kPa
2. [H+] = 1 moldm-3 (HCl/ HNO3)
3. Pt electrode, temp 298K

Question 9

Suggest why the EMF values of the acidic and alkaline hydrogen-oxygen fuel cells are the same. (1)

Answer 9

same overall reaction
(2H2 + O2 –> 2H2O)

Question 10

When the voltmeter is replaced by a bulb, the EMF of the Cu (s)∣Cu2+ (aq)∣∣Cu2+ (aq)∣Cu (s) cell in the diagram decreases over time to 0 V.
Give one reason why. (1)

Answer 10

[Cu2+] in 2 solutions become equal

Question 11

A salt bridge was used in a cell to measure electrode potential.
Explain the function of the salt bridge. (2)

Answer 11

• completes circuit
• ions in ionic substance move thru salt bridge

Question 12

There are two ways to use hydrogen as a fuel for cars. One way is in a fuel cell to power an electric motor, the other is as a fuel in an internal combustion engine.
Suggest the major advantage of using the fuel cell. (1)

Answer 12

fuel cell converts ↑ of available energy from combustion of H2 into KE of car

Question 13

When the standard electrode potential for Fe3+(aq) / Fe2+(aq) is measured, a platinum electrode is required.
What is the function of the platinum electrode? (1)

Answer 13

allow transfer of e-

Question 14

This question is about a glucose–oxygen fuel cell.
When the cell operates, the glucose (C6H12O6) molecules react with water at the negative electrode to form carbon dioxide and hydrogen ions.
Oxygen gas reacts with hydrogen ions to form water at the positive electrode.
State what must be done to maintain the EMF of this fuel cell when in use. (1)

Answer 14

constantly add reactants/ keep conc of reactants constant

Question 15

The EMF of an alkaline hydrogen–oxygen fuel cell is +1.23 V.
The standard electrode potential for one of the electrodes in the alkaline hydrogen–oxygen fuel cell is
2H2O(l) + 2e–  2OH–(aq) + H2(g) Eo = –0.83 V
Suggest why the EMF values of the acidic and alkaline hydrogen–oxygen fuel cells are the same. (1)

Answer 15

same overall reaction

Question 16

State why a fuel cell does not need to be electrically recharged. (1)

Answer 16

reactants supplied continuously