Chapter 27
Question 6 (02 Chem2)
b. Some ethanol may also be used for electric power for lighting, refrigeration, computing and other
electronic
equipment. This can be provided by a fuel cell with an acidic electrolyte, whose cell reaction is
identical to
the equation given above. In the fuel cell the cathode reaction is
O2(g) + 4H+(aq) + 4e– 2H2O(l)
The voltage across the fuel cell is 1.15 V.
i. Give the half reaction occurring at the anode where the ethanol is oxidised in the fuel cell.
ii. Calculate the electrical energy provided per mole of ethanol consumed in the fuel cell.
1 + 2 = 3 marks
c. An alternative way of generating electricity from ethanol is to use it as the fuel for an internal
combustion
engine driving a generator.
Suggest one important reason why the fuel cell would be better than the generator for this purpose.
1 mark
Questions 4, 5 and 6 refer to the following information.
The oxidation of methane (natural gas) can be used to produce electricity in a gas-fired power station.
Methane can also be oxidised to produce electricity in a fuel cell. The overall equation for the
oxidation of methane is
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔH = –900 kJ mol–1
Question 5 (04 Chem2)
In a fuel cell based on the oxidation of methane, the equation for the anode half reaction is
CH4(g) + 2H2O(l) → CO2(g) + 8H+(aq) + 8e–
The corresponding equation for the half reaction at the cathode is
A. 2H2O(l) + 4e– → 4H+(aq) + O2(g)
B. 4H+(aq) + O2(g) → 2H2O(l)
C. 2H2O(l) → 4H+(aq) + O2(g) + 4e–
D. 4H+(aq) + O2(g) + 4e– → 2H2O(l)
Question 6 (04 Chem2)
A fuel cell is set up based on the oxidation of methane. Again, the equation for the anode half reaction
is
CH4(g) + 2H2O(l) → CO2(g) + 8H+(aq) + 8e–
Assuming that all the energy of the oxidation reaction is converted to electricity, the amount of
electric charge, in coulomb, obtained from the oxidation of one mole of methane is closest to
A. 8 × 102
B. 1 × 103
C. 8 × 105
D. 1 × 106
Question 4 (04 Chem2)
A small electric vehicle is powered by rechargeable lead-acid car batteries. The energy that drives the
vehicle comes from the cell reaction
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
The equation for the anode half reaction is
Pb(s) + H2SO4(aq) → PbSO4(s) + 2H+(aq) + 2e–
a. Write the equation for the cathode half reaction.
1 mark
b. In this vehicle, each battery operates at 12.0 V. When fully charged, the vehicle can travel on a
level surface for 2.5 hours with an average energy consumption of 1.0 kJ s–1, after which time the
batteries must be recharged.
i. How much energy, in kJ, is used by the vehicle in travelling for 2.5 hours?
1 mark
ii. Write the equation for the overall chemical reaction that occurs when the battery is being
recharged.
1 mark
iii. What voltage should be used to recharge each battery?
1 mark
Total 4 marks
Question 7 (05 Chem2)
The rechargeable nickel-cadmium cell is used to power small appliances such as portable
computers. When the
cell is being used, the electrode reactions are represented by the following equations.
NiO2(s) + 2H2O(l) + 2e. Ni(OH)2(s) + 2OH.(aq)
Cd(s) + 2OH.(aq) Cd(OH)2(s) + 2e.
Which of the following occurs during the recharging of the nickel-cadmium cell?
I cadmium is deposited on the negative electrode
II the pH of the electrolyte increases
III the direction of electron ß ow in the external circuit is from the anode to the cathode
A. I only
B. I and II only
C. II and III only
D. I and III only
Question 6 (06 Chem2)
Natural gas can be burnt in a power station to provide electrical energy. Alternatively, natural
gas can be fed
into a fuel cell for electricity generation.
At the present time, which one of the following best compares the efÞ ciency and cost of
generating electrical
energy by these two methods in a city like Melbourne?
effi ciency of fuel cell cost per kJ of electricity from fuel cell
A. higher lower
B. higher higher
C. lower lower
D. lower higher
Question 5
An Internet site reporting the latest developments in fuel cell technology describes a cell that
uses a solid ceramic
material as the electrolyte and hydrogen gas and oxygen gas as the reactants.
Key features of this cell are
. water is the only product from the cell reaction
. the ceramic material allows the movement of oxide ions (O2.)
. the reaction at the anode is H2(g) + O2.(in ceramic) → H2O(l) + 2e.
. operation at very high temperatures of over 1000°C means that precious metal catalysts
are not required.
A representation of the cell providing electricity for an appliance is shown in the diagram
below.
a. What distinguishes a fuel cell from a galvanic cell such as a dry cell or lead-acid battery?
1 mark
b. On the diagram above
i. in circles A and B, indicate the polarity of the cathode and anode
ii. show, by using an arrow, the direction of electron ß ow in the external circuit.
1 + 1 = 2 marks
c. Write an equation for each of the following reactions. You are not required to show states
in these two
equations.
i. the overall cell reaction
ii. the reaction at the cathode
1 + 1 = 2 marks
d. A ceramic fuel cell delivers a current of 0.500 A for 10.0 minutes at a potential of 0.600
volts.
i. How much electrical energy, in joules, would be provided by the cell?
ii. Calculate the charge, in coulomb, produced by the cell.
iii. If this particular cell operated at 60.0% efÞ ciency, what amount of hydrogen gas (H2), in
mole, would
be consumed by the fuel cell?
1 + 1 + 3 = 5 marks
Total 10 marks
Questions 15 and 16 refer to the following information.
A rechargeable cell, used in laptop computers, contains a metal alloy (designated M) which
has hydrogen atoms
adsorbed on its surface, and nickel in the form of NiO(OH)(s) and Ni(OH)2(s).
The half reactions, written as reduction reactions, are
H2O(l) + e. ! H (adsorbed on M) + OH.(aq)
NiO(OH)(s) + H2O(l) + e. ! Ni(OH)2(s) + OH.(aq)
While this cell is generating electricity, the metal alloy acts as the negative electrode.
Question 15 (07 Chem2)
When this cell is generating electricity
A. NiO(OH) acts as the oxidant.
B. the concentration of OH. ions in the cell increases as the cell discharges.
C. OH. ions produced at the negative electrode migrate to the positive electrode.
D. electrons ß ow in the external circuit from the positive to the negative electrode.
Question 16 (07 Chem2)
When the cell is recharged, which one of the following processes occurs at the electrode
connected to the
positive terminal of the external power source?
A. reduction of H2O(l)
B. reduction of NiO(OH)(s)
C. oxidation of Ni(OH)2(s)
D. oxidation of H (adsorbed on M)
Question 17 (07 Chem2)
A fuel cell currently under development for powering small electronic devices is based on the
reaction of
methanol and oxygen using an acidic electrolyte.
The reductant in the cell reaction and the half reaction at the anode are
reductant anode reaction
A. methanol O2(g) + 4H+(aq) + 4e. → 2H2O(l)
B. oxygen O2(g) + 4H+(aq) + 4e. → 2H2O(l)
C. methanol CH3OH(g) + H2O(l) → CO2(g) + 6H+(aq) + 6e.
D. oxygen CH3OH(g) + H2O(l) → CO2(g) + 6H+(aq) + 6e.