| Date | May 2016 | Marks available | 2 | Reference code | 16M.3.hl.TZ0.20 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Suggest | Question number | 20 | Adapted from | N/A |
Question
Fuel cells and rechargeable batteries are useful sources of energy.
One type of fuel cell contains a proton exchange membrane between electrodes and uses aqueous methanol as the fuel.
State half-equations for the reactions which occur at the negative and positive electrodes.
Negative electrode (anode):
Positive electrode (cathode):
Suggest one advantage and one disadvantage of a fuel cell over a lead–acid battery as an energy source in a motor vehicle.
Advantage:
Disadvantage:
Markscheme
Negative electrode (anode):
CH3OH(aq) + H2O(l) → CO2 (g) + 6H+ (aq) + 6e-
Positive electrode (cathode):
O2 (g) + 4H+ (aq) + 4e- → 2H2O(l)
Award [1 max] if correct half-equations are given but at the wrong electrodes.
Accept e for e-.
Accept any correct half-equation with fractional coefficients.
Advantage:
produces continuous supply of electricity «on addition of more raw materials»
OR
does not need to be recharged
OR
less hazardous if broken/exposed to the environment
OR
weighs less for same energy output/has higher energy density/has higher specific energy than lead-acid battery
Do not accept “water is non-polluting”.
Do not accept “weighs less” without reference to energy output/power/capacity etc.
Disadvantage:
«more» expensive
OR
needs constant supply of fuel
OR
methanol/ethanol fuel cells difficult to use in cold weather
OR
methanol/ethanol fuel cells produce carbon dioxide
OR
storage/transport of gases/hydrogen a problem in hydrogen fuel cell
OR
does not produce high current
OR
potentially explosive/hydrogen is flammable
Do not accept “fuel cells are prone to poisoning by impurities in fuel”.
