Year 12 Chemistry Chapter 6

The overall reaction for an acidic fuel cell is shown below.

2H₂ + O₂ → 2H₂O

Porous electrodes are often used in acidic fuel cells because they

• are highly reactive.

• are cheap to produce

• are more efficient than solid electrodes at moving charges and reactants.

• provide a surface for the hydrogen and oxygen to directly react together.

Which one of the following is the most correct statement about fuel cells and secondary cells?

• Fuel cells can be recharged like secondary cells.

• Fuel cells produce thermal energy, whereas secondary cells do not produce thermal energy.

• The anode in a fuel cell is positive, whereas the anode in a secondary cell is negative.

• Fuel cells deliver a constant voltage during their operation, whereas secondary cells reduce in voltage as they

  discharge.

Hydrogen, H₂, fuel cells and H₂-powered combustion engines can both be used to power cars. Three statements about H₂ fuel cells and H₂-powered combustion engines are given below:

I Neither H₂ fuel cells nor H₂-powered combustion engines produce greenhouse gases.

II Less H₂ is required per kilometre travelled when using an H₂-powered combustion engine than when using

H₂ fuel cells.

III More heat per kilogram of H₂ is generated in an H₂-powered combustion engine than in H₂ fuel cells.

Which of the statements above are correct?

• II only

• I and II only

• III only

• I and III only

Methane can be used as a reactant in a fuel cell. If the cell is operating in acidic conditions, the reaction occurring at the anode will be

Use the following information to answer Questions 6 and 7.

The design of the phosphoric-acid fuel cell (PAFC) is detailed in the diagram shown on the left.

Which of the following correctly identifies the reactant, electrode polarity and type, and the product

produced at the electrode during operation of the fuel cell?

• Hydrogen reacts to produce water at the negatively charged anode.

• Hydrogen reacts to produce water at the positively charged anode.

• Oxygen reacts to produce water at the negatively charged cathode.

• Oxygen reacts to produce water at the positively charged cathode.

Consider the following statements about the PAFC.

I For a set mass of hydrogen gas, the PAFC produces the same amount of electrical energy as the combustion reaction would produce as heat energy.

II Like galvanic cells which have a gaseous reactant, the electrodes in the PAFC must be porous and catalytic.

III A significant disadvantage in the operation of the PAFC is the generation of major greenhouse gases as unwanted products.

Which of these statements are correct?

• I and II only

• II and III only

• I, II and III

• none of I, II or III

Use the following information to answer Questions 8 and 9.

A fuel cell which uses methanol as the energy source has been developed by chemists. The simplified diagram below shows its structure.

The fuel cell is different to a secondary cell because in a fuel cell

• a spontaneous redox reaction is used.

• reactants are continuously replenished.

• the polarity of the electrodes is reversed.

• cations travel towards the cathode.

Methanol could also be used to generate electricity by combustion to produce hot gases which are then used in turning the turbine in an electrical generator.

For a set mass of methanol, which device would produce the greater amount of electrical energy?

• fuel cell

• generator

• Both devices will be equivalent, as the number of mole of methanol used is equal.

• No prediction can be made without knowing the exact mass of methanol used.

The overall equation for a particular methanol fuel cell is shown below.

2CH₃OH(g) + 3O₂(g) → 2CO₂(g) + 4H₂O(l)

The equation for the reaction that occurs at the cathode in this fuel cell is

• CO₂(g) + 5H₂O(l) + 6e^– → CH₃OH(g) + 6OH^–(aq)

• CH₃OH(g) + 6OH^–(aq) → CO₂(g) + 5H₂O(l) + 6e^–

• O₂(g) + 2H₂O(l) + 4e^– → 4OH^–(aq)

• 4OH^–(aq) → O₂(g) + 2H₂O(l) + 4e^–