Chemical Cells and Fuel Cells

1. Give two variables that affect the voltage produced by a chemical cell.

Answer: The electrodes used and the electrolyte used.

2. Explain the difference between a cell and a battery.

Answer: A cell carries out a chemical reaction to produce a voltage. A battery is two or more cells connected together (in order to increase the voltage supplied).

3. Explain why rechargeable cells and batteries can be recharged.

Answer: Rechargeable cells and batteries use chemical reactions that can be reversed to reform the reactants.

4. Describe the difference between a chemical cell and a fuel cell.

Answer: A chemical cell will eventually run out of reactants and stop producing a voltage. A fuel cell has a continuous supply of reactants so can produce a continuous voltage supply.

5. Give an advantage and a disadvantage of a hydrogen fuel cell over a rechargeable battery.

Answer: ADVANTAGES: they produce a continuous supply of electricity ; the only waste product from the fuel cell is water.
DISADVANTAGES: they are very expensive; hydrogen is a flammable gas ; hydrogen is often made from natural gas so is non-renewable ; there are not many places to buy hydrogen gas from.

6. A student wanted to investigate how the type of electrodes used in a simple chemical cell affected the voltage produced. They kept a copper electrode attached to the positive terminal of the voltmeter and connected five different metals to the negative terminal. The figure below shows the apparatus used.

a) Describe the purpose of the potassium nitrate solution in the chemical cell.

Answer: Potassium nitrate is the electrolyte.

b) Identify the type of particles responsible for the flow of electricity through the potassium nitrate solution and through the electrical wires.

Answer: ions and delocalised electrons

c) Give one variable that the student must keep the same to ensure that their results are valid.

Answer: the type of electrolyte or the concentration of the electrolyte

d) The table below shows the student’s results. Predict the voltage produced if the student were to use two copper electrodes in their chemical cell.

Answer: There is no difference in reactivity between the two electrodes, so the cell would not work (0 V).

e) Which of the five metals is the most reactive? Give a reason for your answer.

Answer: Aluminium is the most reactive. The greater the voltage reading the bigger the difference in reactivity of the two metals.

f) An electrical device requires a voltage as close to 1.00 V as possible. Suggest two metals that would be the most suitable for the chemical cell for this device. Predict the voltage produced by this cell.

Answer: lead and vanadium ; 1.52 V - 0.47 V = 1.05 V

7. Hydrogen gas is becoming an increasingly important fuel. Fuel cells react hydrogen with oxygen to produce electrical energy. Hydrogen gas can be produced by the electrolysis of water.

a) At the anode of a hydrogen fuel cell, hydrogen gas is converted into hydrogen ions and electrons: 2H2  →  4H+ + 4e. Explain why this is an oxidation reaction.

Answer: Oxidation is the loss of electrons and H2 is losing electrons.

b) At the cathode of a hydrogen fuel cell, oxygen gas reacts with the hydrogen ions and electrons. Give the half equation to show this change.

Answer: O2 + 4H+ + 4e  →  2H2O

c) Another source of hydrogen gas is from the reaction between natural gas (CH4 ) and steam. Carbon monoxide (CO) is also produced. The figure below shows the displayed formulae for this reaction, while the table shows the bond energies and the overall energy change for this reaction. Calculate the bond energy X for the C≡O bond in carbon monoxide.

Answer: bonds broken = 4(412) + 2(463) = 2574 ; bonds formed = 3(436) + X = 1308 + X ; so 189 = 2574 - (1308 + X), and X = 1077 kJ/mol

d) "Cars powered by hydrogen fuel cells are sustainable vehicles with zero carbon dioxide emissions." Use the information in this question and your own knowledge to evaluate this statement.

Answer: Sustainable hydrogen can be produced by the electrolysis of water using renewable sources of electricity such as wind or solar power but if the electricity for the electrolysis of water is generated by burning fossil fuels, then this is not sustainable. Natural gas is a fossil fuel, so producing hydrogen from natural gas is also unsustainable. The only waste product from the fuel cell is water, however a lot of carbon emissions are likely to be caused by the production of hydrogen gas.