electrode potentials

Question 1

Explain, in terms of electrode potentials and equilibrium, why the pH of the solution in the hydrogen half-cell decreases as this cell delivers current.

Answer 1

H redox system is more negative
(e.g. has a more –ve E OR less +ve E OR is –ve electrode)
OR H redox system releases electrons
(May be in equation, e.g. H2 → 2H+ + 2e–) 
Equilibrium shifts to increase [H+] OR H+
OR standard hydrogen equation shifts to increase [H+] OR H+ 

Question 2

Explain, in terms of equilibrium, why acidic conditions should not be used with cells containing CN–(aq) ions.

Answer 2

H+ reacts with CN– OR HCN forms
OR equation: H+ + CN– → HCN (ALLOW ⇌)
OR CN– accepts a proton/H+
OR equilibrium shifts right AND CN– is removed 

Question 3

State one important difference between a fuel cell and a modern storage cell.

Answer 3

Fuel reacts with oxygen/oxidant to give electrical energy/voltage

Question 4

Suggest one advantage of using ethanol, rather than hydrogen, in a fuel cell for vehicles.

Answer 4

ethanol is a liquid OR is less volatile
OR ethanol is easier to store/transport/stored more safely
OR hydrogen is explosive/more flammable
OR ethanol has more public/political acceptance 

Question 5

The overall reaction in a DEFC is the same as for the complete combustion of ethanol.Write the equation for the overall reaction in a DEFC.

Answer 5

C2H5OH + 3O2 → 2CO2 + 3H2O

Question 6

Deduce the half-equation for the reaction at the oxygen electrode in a DEFC.

Answer 6

O2 + 4H+ + 4e– → 2H2O 

Question 7

State two other large-scale uses of the hydrogen.

Answer 7

1. fuel cells
2. manufacture of margarine
   OR hydrogenation of alkenes/unsaturated fats/unsaturated
   oils/unsaturated molecules
3. making of ammonia OR Haber process
4. making of HCl/hydrochloric acid
5. making of methanol

Question 8

Define the term standard electrode potential.

Include all standard conditions in your answer.

Answer 8

Definition
The e.m.f. (of a half-cell) compared with/connected to a
(standard) hydrogen half-cell/(standard) hydrogen electrode 
Standard conditions Units essential
Temperature of 298 K / 25ºC
AND (solution) concentrations of 1 mol dm–3
AND pressure of 100 kPa OR 105 Pa OR 1 bar 

Question 9

Explain, in terms of equilibrium, why the cell potential increases.

Answer 9

Assume Cu2+|Cu OR Cu half cell unless otherwise stated.
[Cu2+] decreases OR < 1 mol dm–3
AND
Equilibrium (shown in table) shifts to left 
more electrons are released by Cu 
The cell has a bigger difference in E 

Question 10

State one advantage of using hydrogen as a fuel compared with conventional fuels.

Answer 10

no/less CO2 OR H2O is only product OR greater efficiency

Question 11

State one other way that hydrogen can be stored as a fuel for vehicles.

Answer 11

liquefied/as a liquid AND under pressure/pressurised

Question 12

Give two reasons why it is uncertain whether reactions predicted from Eovalues may actually take place.

Answer 12

High activation energy OR slow rate

Conditions not standard OR concentrations not 1 mol dm–3


Question 13

Draw a labelled diagram to show how the student could have set up this cell to measure its standard cell potential.

Answer 13

complete circuit with voltmeter and salt bridge linking
two half-cells 
Pt electrode in Fe3+/Fe2+ half-cell with same
concentrations 
Cr electrode in 1 mol dm–3 Cr3+ half-cell 

Question 14

Explain, in terms of their electrode potentials, why ‘chromium is more reactive than copper’ in this reaction.

Answer 14

E o for chromium (redox system) is more
negative/lower/less (than copper redox system) ORA 
chromium system shifts to the left /
 Cr(s)  Cr3+(aq) + 3e–
AND
copper system shifts to the right /
 Cu2+(aq) + 2e–  Cu(s) 

Question 15

When this experiment was carried out, the student observed some bubbles of a gas.Suggest an explanation for this observation.

Answer 15

Cr reacts with H+ ions/acid to form H2 gas 

Question 16

Suggest two advantages of using methanoic acid as the fuel in a fuel cell rather than hydrogen.

Answer 16

1. Methanoic acid is a liquid AND easier to
   store/transport
   OR hydrogen is a gas AND harder to store/transport
   OR hydrogen as a liquid is stored under pressure
2. Hydrogen is explosive/more flammable
3. HCOOH gives a greater cell potential/voltage
4. HCOOH has more public/political acceptance than
   hydrogen as a fuel

Question 17

The student made the following observations.
• In Cell A, the nickel electrode lost mass.
• In Cell B, the nickel electrode gained mass.
• In both cells, the measured cell potential slowly changed.
Explain these observations. Include equations in your answer.

Answer 17

Cell A (based on 1 and
2)
Ni + 2Fe3+  Ni2+ + 2Fe2+

Cell B (based on 1 and
3)
2Cr + 3Ni2+  2Cr3+ + 3Ni

concentrations (of the ions in each cell) change
OR
concentrations are not standard


Question 18

State a method, other than absorption, that is being developed to store hydrogen for possible use as a fuel in cars.

Answer 18

adsorbed (on a solid) OR on the surface (of a solid)
OR as a liquid under pressure


Question 19

State two large-scale uses for the hydrogen produced.

Answer 19

Two from points below:
1. fuel OR fuel cells
2. manufacture of margarine
OR hydrogenation of alkenes/unsaturated fats
3. manufacture of ammonia OR ‘Haber process’ 
4. manufacture of HCl/hydrochloric acid
5. reduction of metal ores/metal oxides

Question 20

As the cell approaches full charge, the aqueous KOH electrolyte starts to decompose, forming hydrogen gas at one electrode and oxygen gas at the other electrode.
Predict half-equations that might take place at each electrode for the decomposition of the electrolyte to form hydrogen and oxygen.

Answer 20

4OH– ⎯→ O2 + 2H2O + 4e– 

2H2O + 2e– ⎯→ H2 + 2OH– 

Question 21

Write an equation for the complete combustion of methanol.

Answer 21

CH3OH + 1½O2  CO2 + 2H2O 

Question 22

State two advantages of vehicles using fuel cells compared with the combustion of conventional fossil fuels.

Answer 22

less CO2 OR less greenhouse gases 

greater efficiency 

Question 23

Suggest one advantage of using methanol, rather than hydrogen, in a fuel cell for vehicles. Justify your answer.

Answer 23

methanol is a liquid
AND
methanol is easier to store/transport 

Question 24

State the charge carriers that transfer current through the wire, ………………………………………………………………………………………………….
through the solution. ………………………………………………………………………………………..

Answer 24

electrons AND ions

Question 25

Fuel cells are a type of electrochemical cell being developed as a potential source of energy in the future.
• State one important difference between a fuel cell and a conventional electrochemical cell.
• Write the equation for the overall reaction that takes place in a hydrogen fuel cell. • State two ways that hydrogen might be stored as a fuel for cars.
• Suggest why some people consider that the use of hydrogen as a fuel for cars consumes more energy than using fossil fuels such as petrol and diesel.

Answer 25

A fuel cell converts energy from reaction of a fuel
with oxygen into a voltage/electrical energy

2H2 + O2  2H2O
 ALLOW multiples, e.g. H2 + ½O2  H2O
IGNORE state symbols
Two from:
 under pressure OR at low temperature OR as a
liquid
 adsorbed on solid
 absorbed within solid 
Energy is needed to make the hydrogen
OR energy is needed to make fuel cell
