Electrochemistry (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

Aluminium is obtained from purified alumina, Al₂O₃, by electrolysis.

Alumina is obtained from the main ore of aluminium.

State the name of this ore.

Describe the extraction of aluminium from alumina. Include the electrolyte, the electrodes and the reactions at the electrodes.

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Aluminium is resistant to corrosion. It is protected by an oxide layer on its surface.The thickness of this oxide layer can be increased by anodising.

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During electrolysis, ions move in the electrolyte and electrons move in the external circuit.
Reactions occur at the electrodes.

The diagram shows the electrolysis of molten lithium iodide.

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The results of experiments on electrolysis are shown in the following table.

Complete the table. The first line has been done as an example.

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The diagram below shows the electrolysis of dilute sulfuric acid. Hydrogen is formed at the negative electrode (cathode) and oxygen at the positive electrode (anode) and the concentration of sulfuric acid increases.

The ions present in the dilute acid are H⁺ (aq), OH^– (aq) and SO₄^2– (aq).
 

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4OH^– (aq) → O₂ (g) + .....H₂O (l) + ....... 

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The electrolysis of concentrated aqueous sodium chloride, between inert electrodes, is used to make four important chemicals.

hydrogen
chlorine
sodium hydroxide
sodium chlorate(I)

The ions present in the electrolyte are Na⁺, H^+, Cl^– and OH^–

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H₂O     H⁺ + OH^–

   Suggest an explanation why the concentration of hydroxide ions increases.

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The results of experiments on electrolysis using inert electrodes are given in the table.

Complete the table; the first line has been completed as an example.

Copper has the structure of a typical metal. It has a lattice of positive ions and a “sea” of mobile electrons. The lattice can accommodate ions of a different metal.

Give a different use of copper that depends on each of the following.

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Aqueous copper(II) sulphate solution can be electrolysed using carbon electrodes. The ions present in the solution are as follows.

Cu²⁺      SO₄^2–      H⁺     OH^–

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Aqueous copper(II) sulphate can be electrolysed using copper electrodes. The reaction at the negative electrode is the same but the positive electrode becomes smaller and the solution remains blue.

Molten potassium bromide can be electrolysed.
Predict the products of this electrolysis at:

the anode
the cathode

Electrolysis of concentrated aqueous sodium chloride using inert electrodes forms chlorine, hydrogen and sodium hydroxide.

What is meant by the term electrolysis?

Name a substance that can be used as the inert electrodes.

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Write an ionic half-equation for the formation of hydrogen during this electrolysis.

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Give the formulae of the four ions present in concentrated aqueous sodium chloride.

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Explain how sodium hydroxide is formed during this electrolysis.

Complete the diagram by:

• labelling the anode and cathode
• showing how the gases are collected.

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When concentrated hydrochloric acid is electrolysed, gases are produced at the electrodes.

The incomplete apparatus is shown.

Predict the products of this electrolysis at the:

positive electrode:

negative electrode:

When concentrated hydrochloric acid is electrolysed, gases are produced at the electrodes.

The incomplete apparatus is shown.

Graphite (carbon) electrodes are used in this electrolysis.

Suggest one other element that can be used as an electrode and give a reason, other than that it can conduct electricity.

element:

reason:

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When concentrated hydrochloric acid is electrolysed, gases are produced at the electrodes.

The incomplete apparatus is shown.

Hydrogen chloride is produced when chlorine reacts with hydrogen.

Complete the chemical equation for this reaction.

Cl₂ + .......... → .....HCl

Predict the products of the electrolysis of molten aluminium oxide at:

the positive electrode ........................................

the negative electrode.......................................

A fuel cell produces electrical energy by the oxidation of a fuel by oxygen.

The fuel is usually hydrogen but methane and methanol are two other fuels which may be used.

A diagram of a hydrogen fuel cell is given below.

When the fuel is hydrogen, the only product is water.

What additional product would be formed if methane was used?

Write the equation for the chemical reaction that takes place in a hydrogen fuel cell.

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Fuel cells are used to propel cars.

Give two advantages of a fuel cell over a gasoline-fuelled engine.

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Some cars use hydrogen–oxygen fuel cells as a source of energy.

Explain one advantage to the environment of using a hydrogen–oxygen fuel cell instead of a petrol engine.

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Fuel cells can be used with alternative fuels. One alternative fuel is the liquid fuel, methanol, CH₃OH.

Suggest an advantage of using methanol instead of hydrogen in a fuel cell to power vehicles.

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When methanol reacts with oxygen in a methanol-oxygen fuel cell, carbon dioxide and water are produced. Draw the reaction pathway diagram for the reaction on the axes below.

Label the following:

• reactants
• products
• activation energy
• overall energy change

Write the balanced symbol equation for the reaction between methanol and oxygen in a methanol-oxygen fuel cell.

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The diagram shows a simple cell.

Write an equation for the overall reaction occurring in the cell.

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Explain why all cell reactions are exothermic and redox.

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Which electrode, zinc or iron, is the negative electrode? Give a reason for your choice.

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Zinc alloys have been used for over 2500 years.

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Describe how zinc metal can be obtained from zinc sulfate solution by electrolysis.

A labelled diagram is acceptable. Include all the products of this electrolysis. The electrolysis is similar to that of copper(II) sulfate solution with inert electrodes.

The ore of aluminium is bauxite which is impure aluminium oxide. Alumina, pure aluminium oxide, is obtained from bauxite.

Aluminium is formed at the cathode when a molten mixture of alumina and cryolite, Na₃AlF is electrolysed.

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The purification of bauxite uses large amounts of sodium hydroxide.

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Zinc is an important metal. Its uses include making alloys and the construction of dry cells (batteries).

Name an alloy which contains zinc. What is the other metal in this alloy?

The main ore of zinc is zinc blende, ZnS.

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Zinc can be obtained from zinc oxide in a two step process. Aqueous zinc sulfate is made from zinc oxide and then this solution is electrolysed with inert electrodes. The electrolysis is similar to that of copper(II) sulfate with inert electrodes.

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A dry cell (battery) has a central rod, usually made of graphite. This is the positive electrode which is surrounded by the electrolyte, typically a paste of ammonium chloride and manganese(IV) oxide, all of which are in a zinc container which is the negative electrode.

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Lithium bromide is an ionic compound. It can be electrolysed when it is molten or in aqueous solution. It cannot be electrolysed as a solid.

Solid lithium bromide is a poor conductor of electricity. The ions cannot move to the electrodes, they are held in an ionic lattice by strong forces.

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The diagram shows the electrolysis of molten lithium bromide.

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When aqueous lithium bromide is electrolysed, a colourless gas is formed at the negative elctrode and the solution becomes alkaline.

Explain these observations and include an equation in your explanation.

A fuel cell produces electrical energy by the oxidation of a fuel by oxygen.
The fuel is usually hydrogen but methane and methanol are two other fuels which may be used.
A diagram of a hydrogen fuel cell is given below in Figure 1.

Figure 1

When the fuel is hydrogen, the hydrogen is oxidised to form water, which is the only product.

Suggest what additional product would be formed if methanol was used.

Give the chemical equation for the reaction that takes place in a hydrogen fuel cell.

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In a hydrogen fuel cell, hydrogen loses electrons at the anode.

Write the half equation to show the reaction at the anode.

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Hydrogen and oxygen can also react in a combustion reaction forming water vapour.

Calculate the mass of water vapour produced when 500 cm³ of hydrogen combusts in excess oxygen.

Mass of water vapour ...................................... (g)

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The Gemini and Apollo moon probes use hydrogen fuel cells to generate an electric current and do not need to be electrically recharged. The product of this reaction can be used to supplement the drinking water for astronauts.

Deduce the half equations for the reactions at each electrode in a hydrogen fuel cell, and then the overall equation for the hydrogen-oxygen fuel cell, to show the product used to supplement drinking water.

A hydrogen fuel cell is an electrochemical device that converts chemical energy into electrical energy. A continuous supply of fuel is supplied to one electrode and a continuous supply of air is supplied to the other electrode.

Suggest why a fuel cell does not need to be recharged.

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The General Motors Electrovan was built in 1966. It was the first vehicle powered by a hydrogen fuel cell and could travel at up to 70 mph for 30 seconds.

Suggest two environmental advantages of using a hydrogen fuel cell rather than an internal combustion engine.

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Hydrogen fuel cells are commonly considered to be carbon neutral by the general public because they do not release any carbon dioxide or other greenhouse gases when they are used. This is not technically correct.

Suggest why hydrogen fuel cells cannot be classed as carbon neutral.