5.2 Chemical Cells & Fuel Cells

A student investigated how the metal used as the electrode in a simple cell affected the voltage produced. They used the apparatus shown in Figure 1.

Figure 1

The student's results are shown in Table 1.

Table 1

Which metal is the most reactive?

Explain your answer.

Why does silver have a negative voltage?

The half equation for the reaction that happens at the copper electrode in the simple cell made using copper and magnesium electrodes is:

Cu²⁺ + 2e^- → Cu

Give the half equation for the reaction that occurs at the magnesium electrode and state whether this is oxidation or reduction, giving a reason why.

Predict the voltage that would be produced if the simple cell was made with zinc and nickel electrodes.

A simple cell consists of two different metals in contact with an electrolyte.

Explain what a battery is.

Batteries can be non-rechargeable and rechargeable.

Explain the difference between the two different types of batteries. 

A student  made a battery from lemons as shown in Figure 1.

Figure 1

The student found that if they had less than four lemon halves connected as shown in Figure 1, the bulb did not turn on.

Explain why.

The student changed the bulb for a voltmeter so that they could measure the voltage produced.

They investigated what happened to the voltage produced by the battery when different metals were used instead of copper and zinc.

Give two control variables in this investigation.

The results that the student obtained are shown in Table 1.

Table 1

The results give a positive voltage when metal 1 is more reactive than metal 2.

The results give a negative voltage when metal 2 is more reactive than metal 1.

List the metal in order of reactivity, starting with the least reactive metal.

Predict the voltage that would have been obtained for the reading that was not taken where metal 1 is iron and metal 2 is aluminium.

Explain your answer.

Hydrogen fuel cells can be constructed using either an acidic or alkaline electrolyte.

The half equations at the electrodes in acidic conditions are:

• H₂ ⟶ 2 H⁺ + 2 e^-
• O₂ + 4 H⁺ + 4 e^- ⟶ 2 H₂O

The half equations at the electrodes in alkaline conditions are:

• H₂ + 2 OH^- ⟶ 2 H₂O + 2 e^-
• O₂ + 2 H₂O + 4 e^- ⟶ 4 OH^-

The overall equation for the reaction in alkaline conditions is:

• 2H₂ + O₂ → 2H₂O

Suggest why the voltage produced in a hydrogen fuel cell with an acidic electrolyte is exactly the same as one with an alkaline electrolyte.

You should refer to the overall equation for the reactions in your answer.

The electrodes used in hydrogen fuel cells can be made by using a porous ceramic material which has been coated in platinum.

Suggest two reasons why this method of making electrodes is preferential to using platinum rods as electrodes.

The only product formed in hydrogen fuel cells is water so they are often thought of as sustainable.

Suggest a reason why this is not necessarily true.

Other than any factors which may make fuel cells less sustainable, give two disadvantages of using hydrogen fuel cells compared with rechargeable battery, such as a lithium-ion battery, when used to power an electric car?

Electrochemical cells contain chemicals which react together.   

This reaction involves the movement of electrons which is what produces electricity.

Rechargeable cells are cells that can be recharged. 

Explain how this process occurs.

Give three factors that affect the voltage produced by a cell.

Balance the half-equation for the reaction which occurs at an electrode in one kind of hydrogen fuel cell.

H₂   +  OH⁻  ⟶  H₂O   +  e⁻

Explain why the fuel cell in part (c) can be described as an alkaline fuel cell?

Some chemical reactions can produce electricity. 

Figure 1 shows a simple cell.

Figure 1

Alkaline batteries are non-rechargeable.

Explain why they cannot be recharged.

Rechargeable lithium-ion batteries and hydrogen fuel cells can be used to power electric cars.

Complete the balanced equation for the overall reaction in a hydrogen fuel cell and state what type of reaction it is.

________ H₂ + ________ ⟶ ________ H₂O

Table 2 below shows data based on different methods of powering electric cars.

Table 2

Use the table and your knowledge to evaluate the use of hydrogen fuel cells compared with rechargeable lithium-ion batteries to power electric cars.

Some new models of cars are powered by hydrogen fuel cells.

What type of energy is released by these cells?

Owners of these cars must buy hydrogen from specially equipped hydrogen filling stations. 

The number of these stations in the UK is expected to increase up to the year 2035 as can be seen in Figure 3.

Figure 3

Explain why the UK government should encourage the production of more hydrogen filling stations. 

Use the data in Figure 3 and your own knowledge to answer this question, making reference to the reaction of hydrogen fuel cells.

Give two disadvantages of using hydrogen fuel cells.