Question 17N.3.hl.TZ0.20
| Date | November 2017 | Marks available | [Maximum mark: 5] | Reference code | 17N.3.hl.TZ0.20 |
| Level | hl | Paper | 3 | Time zone | TZ0 |
| Command term | Deduce, Outline | Question number | 20 | Adapted from | N/A |
A fuel cell converts chemical energy directly to electrical energy.
Deduce the half-equations and the overall equation for the reactions taking place in a direct methanol fuel cell (DMFC) under acidic conditions.
[3]
Negative electrode (anode):
CH3OH (aq) + H2O (l) → CO2 (g) + 6H+ (aq) + 6e–
Positive electrode (cathode):
O2 (g) + 4H+ (aq) + 4e– → 2H2O (l)
Overall equation:
2CH3OH (aq) + 3O2 (g) → 2CO2 (g) + 4H2O (l)
Accept any whole or fractional coefficients in balanced equations.
Award [1 max] for correct half-equations at wrong electrodes for M1 and M2.
Outline one advantage and one disadvantage of the methanol cell (DMFC) compared with a hydrogen-oxygen fuel cell.
[2]
Advantage:
Any one of:
liquid methanol is easier to transport/store than gaseous hydrogen
OR
hydrogen is explosive
longer membrane life «as it operates in aqueous environment»
methanol has greater energy density than hydrogen
Disadvantage:
Any one of:
lower voltage
lower power per unit mass «of the cell»
lower efficiency
toxic/can be mistaken for ethanol
lower specific energy
Ignore any cost references throughout.
Accept “CO2/greenhouse gas produced” OR “requires a more highly efficient catalyst”.
Do not award marks for converse statements for the advantage and disadvantage.