Electrolysis (Cambridge O Level Chemistry)

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.

Extended Only

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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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.

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^–

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.

Zinc is extracted from zinc blende, ZnS.

Zinc blende is heated in air to give zinc oxide and sulfur dioxide. Most of the sulfur dioxide is used to make sulfur trioxide. This is used to manufacture sulfuric acid. Some of the acid is used in the plant, but most of it is used to make fertilisers.

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The remaining zinc oxide reacts with sulfuric acid to give aqueous zinc sulfate. This is electrolysed with inert electrodes (the electrolysis is the same as that of copper(II) sulfate with inert electrodes).

Ions present: Zn²⁺ (aq)    SO₄^2- (aq)     H⁺ (aq)     OH^- (aq)

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Give two uses of zinc.

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.

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

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

Explain how sodium hydroxide is formed during this electrolysis.

Complete the diagram by:

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

Extended Only

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:

Extended Only

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.......................................

Electroplating steel objects with silver involves a three-step process.

step 1   A coating of copper is applied to the object.
step 2   A coating of nickel is applied to the object.
step 3   The coating of silver is applied to the object.

A diagram of the apparatus used for step 1 is shown.

The chemical process taking place on the surface of the object is

Cu²⁺ (aq) + 2e^– → Cu (s)

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Give two changes which would be needed in order to coat nickel onto the object in step 2.

Copper, nickel and silver are transition elements.

Typical physical properties of transition elements are a high density and a high melting point.

Give three different properties of transition metals which are not typical of other metals.

Chromium is a transition element.
 

Chromium is used to electroplate steel objects. The diagram shows how this could be done.

Metal objects can be electroplated with silver.

Describe how a metal spoon can be electroplated with silver.
Include:

• what to use as the positive electrode and as the negative electrode
• what to use as the electrolyte
• an ionic half-equation to show the formation of silver.

You may include a diagram in your answer.

Give one reason why metal spoons are electroplated with silver.

Silver is a transition element with proton number 47.

Use the Periodic Table to state the number of occupied electron shells in an atom of silver.

Describe, with the aid of a diagram, the metallic bonding in silver.

Give two physical properties of silver that are only characteristic of transition elements but not of all metals.

Silver nitrate is a white crystalline soluble salt.

Name a suitable combination of an acid and an insoluble base which is used to prepare silver nitrate.

Aqueous silver nitrate, AgNO₃(aq), is electrolysed using inert electrodes.

The products of the electrolysis are silver and oxygen.

Acidified aqueous silver nitrate reacts with aqueous sodium iodide.

State the observations for this reaction.

The diagram shows a simple cell.

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

Explain why all cell reactions are exothermic and redox.

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

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.

Electrolysis can be used to silver-plate metal cutlery.

Give two reasons why metal cutlery is sometimes silver-plated.

The apparatus used to electroplate a metal spoon with silver is shown. The apparatus has not been set up correctly.

The metal spoon erodes and a thin layer of copper forms on the pure silver electrode.

Explain two changes that should be made to this apparatus in order to be able to silver plate the metal spoon. 

Suggest why it is difficult to electroplate a plastic object.

Hallmarked precious metals such as gold, silver, platinum and palladium are often plated on jewellery. It is usually cheaper to have plated jewellery than solid pieces made from these heavy, expensive and precious metals.

A base metal, often copper or nickel, is manipulated into the desired shape for the piece of jewellery. The precious metal is then plated onto the piece of jewellery.

Give the name used to describe the electroplated base metal.

Around 85% of female wedding rings are 1.5 - 2.5 mm wide. The depth of a female wedding ring depends on the shape of the ring. The dimensions of a flat profile ring are shown.

With wedding rings, electroplating to a depth of 8 microns (0.008 mm) is considered thick plating.

• an average female, flat profile wedding ring.
• an average female, flat profile wedding ring with thick plating.

Most wedding rings contain between 4 and 6 g of metal. Wedding rings that are 18-carat gold are plated with gold that contains 750 parts per 1000 of gold.

Using your answer to part (b), calculate the number of moles of gold that is required to plate an average wedding ring with 18-carat gold.

The wedding ring can be electroplated using the apparatus shown.

Label the anode and the cathode on the diagram. Explain your answer in terms of gold ions.

Electroplating is generally done for decorative purposes or for protection.

Food cans are generally made from three different materials:

• aluminium
• tin-coated steel / tinplate
• electrolytic chromium coated steel

Explain why aluminium food cans do not need to be electroplated.

Tin-coated steel food cans and electrolytic chromium coated steel food cans that contain acidic foods can bulge as shown. The bulging is a result of a redox reaction inside the can forming a gas.

Common electroplated gifts used to include items such as jewellery, jewellery boxes, shaving kits, cutlery, hip flasks, candlesticks and other ornamental pieces.

More recent developments in electroplating mean that plastic and organic items, such as the rose shown, can be metal plated.

Plastic and organic items are sprayed or coated with a fine film to allow electroplating to occur.

Name a material that the film must contain. Explain your answer.

There are a number of steps in the process of gold-plating a rose.

• Surface Cleaning
• Surface Preparation
• Electro Cleaning and Steam Cleaning
• Water Rinse
• Flashing / Striking
• Applying Base Coat
• Electroplating
• Final Rinse and Drying
• Final Polish

During the flashing / striking step, a thin layer of high-quality nickel is plated on to the rose. This step improves the bonding between the rose and the gold-plating due to the electrical conductivity of the nickel.

Suggest another reason for applying a thin layer of nickel. Explain your answer.