Electrochemistry — IGCSE Chemistry 0620 exam-style questions with answers

Question 1

Core Electrolysis [1]

Which statement is the correct definition of electrolysis?

Question 2

Core Electrolysis [1]

Molten zinc chloride is electrolysed using two graphite rods connected to a battery. What is the name given to the positive electrode?

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Question 3

Core Electrolysis [1]

Molten lead(II) bromide is electrolysed using inert electrodes. Which pair correctly describes the two products and the electrode at which each forms?

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Question 4

Core Electrolysis [1]

In the electrolysis of any molten ionic compound, which statement is always true?

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Question 5

Core Electrolysis [1]

Dilute sulfuric acid is electrolysed using platinum electrodes. What are the products at the cathode and at the anode respectively?

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Question 6

Core Electrolysis [6]

Molten lead(II) bromide is electrolysed in a school laboratory using two graphite electrodes connected to a power supply.

(a)

State which electrode – the anode or the cathode – is the negative electrode, and name the product that forms there.

[2]

(b)

State the product that forms at the anode.

[1]

(c)

Describe two observations that would confirm the identity of the product at the anode.

[2]

(d)

State the name of the molten substance that is decomposed in this experiment.

[1]

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Question 7

Core Electrolysis [4]

Molten lithium chloride is held in a crucible and electrolysed using two carbon electrodes connected to a d.c. power supply.

(a)

Define the term electrolysis.

[2]

(b)

State the name given to:

(i)

the electrode connected to the negative terminal of the power supply

[1]

(ii)

the molten lithium chloride

[1]

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Question 8

Core Electrolysis [1]

Stainless steel cutlery is often electroplated with a thin layer of chromium. What are the main reasons for doing this?

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Question 9

Core Electrolysis [5]

A steel spoon is to be electroplated with silver.

(a)

State two reasons why the spoon is electroplated with silver.

[2]

(b)

Describe how the spoon could be electroplated with silver. Your answer should name which electrode the spoon should be, what the other electrode should be made of, and a suitable electrolyte.

[3]

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Question 10

Core Electrolysis [1]

Molten calcium bromide is electrolysed using inert electrodes. What are the products formed at the cathode and at the anode respectively?

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Question 11

Supplement Electrolysis [1]

Aqueous copper(II) sulfate is electrolysed using two inert carbon electrodes. What is observed at the cathode?

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Question 12

Supplement Electrolysis [7]

Aqueous copper(II) sulfate is electrolysed twice: once using two inert carbon electrodes, and once, separately, using two copper electrodes.

(a)

State the product formed at the cathode. This is the same in both experiments.

[1]

(b)

With carbon electrodes, state the product formed at the anode.

[1]

(c)

With copper electrodes, describe what happens at the anode instead.

[2]

(d)

Explain why, when copper electrodes are used, the blue colour of the copper(II) sulfate solution stays roughly constant, but with carbon electrodes it gradually fades.

[3]

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Question 13

Supplement Electrolysis [1]

Molten sodium chloride is electrolysed using inert electrodes connected to a battery. In the external circuit (the wires connecting the battery to the electrodes), in which direction do electrons flow?

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Question 14

Supplement Electrolysis [6]

Molten magnesium chloride is electrolysed using inert graphite electrodes connected to a battery.

(a)

Write the ionic half-equation for the reaction at the cathode.

[2]

(b)

Write the ionic half-equation for the reaction at the anode.

[2]

(c)

Describe how charge is carried:

(i)

through the electrolyte

[1]

(ii)

through the external circuit

[1]

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Question 15

Supplement Electrolysis [1]

Concentrated aqueous potassium bromide is electrolysed using inert electrodes. What is formed at the anode?

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Question 16

Supplement Electrolysis [7]

Two separate electrolysis experiments are carried out using inert electrodes: Experiment 1 uses a dilute aqueous solution of sodium iodide; Experiment 2 uses a concentrated aqueous solution of sodium iodide.

(a)

State the product formed at the cathode in both experiments, and explain why sodium metal is not formed there.

[2]

(b)

State the product formed at the anode in Experiment 1 (dilute solution).

[1]

(c)

State the product formed at the anode in Experiment 2 (concentrated solution).

[1]

(d)

Explain, in terms of the ions present, why the anode product is different between the two experiments.

[3]

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Question 17

Supplement Electrolysis [1]

Molten potassium chloride is electrolysed using inert electrodes. Which half-equation correctly represents the reaction at the anode?

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Question 18

Supplement Electrolysis [6]

(a)

Molten lithium bromide is electrolysed using inert electrodes. Write balanced ionic half-equations for the reactions at:

(i)

the cathode

[1]

(ii)

the anode

[1]

(b)

Dilute sulfuric acid is electrolysed using inert electrodes. Write balanced ionic half-equations, including state symbols, for the reactions at:

(i)

the cathode

[2]

(ii)

the anode

[2]

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Question 19

Core Hydrogen–oxygen fuel cells [1]

What is the only chemical product formed when a hydrogen–oxygen fuel cell operates?

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Question 20

Core Hydrogen–oxygen fuel cells [1]

Which two substances react together inside a hydrogen–oxygen fuel cell to generate electricity?

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Question 21

Core Electrolysis [1]

Concentrated aqueous sodium chloride is electrolysed using inert carbon electrodes. What gas is produced at the anode?

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Question 22

Core Electrolysis [1]

Molten strontium chloride is electrolysed using inert electrodes. What products form at the cathode and at the anode respectively?

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Question 23

Core Electrolysis [5]

Dilute sulfuric acid is electrolysed in the laboratory using platinum electrodes.

(a)

Name the gas produced at the cathode and describe a test that would confirm its identity, including the result.

[2]

(b)

Name the gas produced at the anode and describe a test that would confirm its identity, including the result.

[2]

(c)

State the electrolyte used in this experiment.

[1]

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Question 24

Core Electrolysis [5]

A car bumper made of steel is electroplated with a thin layer of chromium before sale.

(a)

State two reasons why the bumper is electroplated with chromium rather than left as bare steel.

[2]

(b)

Describe how the bumper could be electroplated with chromium. Your answer should name which electrode the bumper should be, a suitable material for the other electrode, and a suitable electrolyte.

[3]

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Question 25

Supplement Electrolysis [8]

Impure copper is purified using electrolysis. A block of impure copper is used as the anode and a thin strip of pure copper as the cathode. Both are dipped into aqueous copper(II) sulfate.

(a)

Write the ionic half-equation for the reaction at the cathode.

[2]

(b)

Write the ionic half-equation for the reaction at the anode.

[2]

(c)

Explain what happens to the mass of the anode and the mass of the cathode as electrolysis proceeds.

[2]

(d)

Explain why the concentration of the copper(II) sulfate solution stays approximately constant during this process.

[2]

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Question 26

Supplement Electrolysis [8]

In industry, concentrated aqueous sodium chloride (brine) is electrolysed continuously using inert electrodes. This is called the chlor-alkali process.

(a)

State the products formed at the cathode and at the anode.

[2]

(b)

Write the ionic half-equation for the reaction at the anode.

[2]

(c)

Name the solution that remains after electrolysis, and state one industrial use of it.

[2]

(d)

Explain, in terms of the ions present, why chlorine rather than oxygen is formed at the anode, even though oxygen is normally the preferred product at a carbon anode in dilute aqueous solution.

[2]

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Question 27

Supplement Electrolysis [5]

Dilute sulfuric acid is electrolysed using inert platinum electrodes connected to a battery.

(a)

Describe how charge is carried through the dilute sulfuric acid (the electrolyte).

[2]

(b)

Describe how charge is carried through the wires of the external circuit.

[1]

(c)

State what happens to hydrogen ions when they reach the cathode.

[2]

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Question 28

Core Hydrogen–oxygen fuel cells [4]

A hydrogen–oxygen fuel cell is used to power the electric motor of an experimental car.

(a)

State the two gases that are supplied to the fuel cell.

[1]

(b)

State the only chemical product of the fuel cell reaction, and explain why the car's exhaust does not release carbon dioxide, unlike a petrol-engine car.

[3]

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Question 29

Supplement Hydrogen–oxygen fuel cells [6]

(a)

State two advantages of using a hydrogen–oxygen fuel cell, rather than a petrol engine, to power a vehicle.

[2]

(b)

State two disadvantages of using a hydrogen–oxygen fuel cell, rather than a petrol engine, to power a vehicle.

[2]

(c)

Suggest one reason why hydrogen fuel cell vehicles are not yet as widely used as petrol-engine vehicles.

[2]

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Mark scheme

(a) [2]

any two from: (1 each)
produces no carbon dioxide / no polluting exhaust gases, only water (1)
converts chemical energy to useful (electrical) energy more efficiently than a petrol engine, which loses more energy as heat (1)
quieter running / fewer moving parts (1)
hydrogen can be made from water (by electrolysis) so is a renewable fuel if renewable electricity is used (1)
ALLOW no dependence on crude oil / petrol, a non-renewable resource

(b) [2]

any two from: (1 each)
hydrogen is difficult/expensive to store safely, e.g. needs high-pressure tanks or very low temperatures, and takes up more space than the same energy stored as petrol (1)
hydrogen does not occur freely in nature and must be manufactured, often using energy from fossil fuels (1)
hydrogen is flammable and poses an explosion risk (1)
fuel cells / hydrogen vehicles are expensive to manufacture, e.g. platinum catalyst (1)
few hydrogen refuelling stations (1)
ALLOW shorter range between refuelling

(c) [2]

a specific reason, e.g. there are very few hydrogen refuelling stations compared with petrol stations / fuel cell vehicles and hydrogen fuel are currently expensive / safe hydrogen storage technology is still being developed (1)
linked consequence, e.g. so drivers cannot easily refuel / are unwilling to buy the vehicles (1)
ALLOW a reason already credited in (b) may be used again here
IGNORE vague answers with no specific reason (0)

Worked solution

Compared with a petrol engine, a hydrogen fuel cell has real advantages: it produces no carbon dioxide or other polluting gases as it runs, since its only chemical product is water, and it can convert chemical energy into useful energy more efficiently, since a fuel cell does not waste as much energy as heat as a combustion engine does. However, there are real disadvantages too: hydrogen gas is bulky and difficult to store safely, usually needing heavy high-pressure tanks, and it is not found freely in nature, so producing it usually requires energy, which may itself come from fossil fuels. These practical difficulties — combined with the small number of hydrogen refuelling stations currently available and the higher cost of the technology — help explain why hydrogen fuel cell vehicles are not yet as common as petrol vehicles, even though the fuel cell itself is a clean way of generating electricity.

Question 30

Supplement Hydrogen–oxygen fuel cells [6]

A car manufacturer advertises its hydrogen fuel cell car as "100% pollution-free, with zero emissions."

(a)

Explain why this claim is accurate in terms of what comes out of the car's exhaust while it is being driven.

[2]

(b)

Evaluate whether the claim that the car is "100% pollution-free" is a fair description of its overall environmental impact, considering how the hydrogen fuel might be produced.

[4]

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Mark scheme

(a) [2]

only water (vapour) leaves the exhaust (1)
no carbon dioxide, carbon monoxide or other polluting gases are released while driving, because no carbon-based fuel is burned (there is no combustion) (1)

(b) [4]

the claim is only true for emissions "at the tailpipe" while driving, not for the whole life cycle of the fuel (1)
hydrogen does not occur freely in useful quantities and must be manufactured, e.g. by the electrolysis of water, or from natural gas/hydrocarbons (1)
if the electricity or process used to manufacture the hydrogen comes from burning fossil fuels, carbon dioxide and other pollutants are released during production (1)
so the "zero emissions" claim is misleading for the fuel's whole life cycle unless the hydrogen is made using a low-carbon/renewable energy source, even though the car itself is genuinely pollution-free while driving (1)
ALLOW valid alternative reasoning, e.g. energy/emissions involved in manufacturing or disposing of the fuel cell and storage tank

Worked solution

While the car is being driven, the claim is genuinely true: hydrogen and oxygen combine in the fuel cell to form only water, so the exhaust really does release no carbon dioxide or other pollutants — the car itself is pollution-free in use. However, "100% pollution-free" is a much bigger claim than that, because it ignores where the hydrogen fuel comes from. Hydrogen is not a naturally occurring fuel source; it has to be manufactured, commonly by electrolysing water or by processing natural gas, both of which require energy. If that energy comes from burning fossil fuels, then pollutant gases, including carbon dioxide, are released during hydrogen production, even though none come from the car itself. So the advertisement's claim is fair only for the car's own emissions while driving, not for the environmental impact of the whole process of making its fuel — a more honest claim would depend on how "green" the hydrogen supply actually is.