Applications of Electrolysis (Cambridge (CIE) IGCSE Chemistry)

Separate: Chemistry Only

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)

i) Explain whether this process is oxidation or reduction.

[1]

ii) Explain why the concentration of copper ions in the electrolyte remains constant throughout step 1.

[2]

Separate: Chemistry Only

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.

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.

Extended Only

Fuel cells are used to propel cars. Give two advantages of a fuel cell over a gasoline-fuelled engine.

Chromium is a transition element.  

i) Predict two differences in the physical properties of chromium and sodium.

[2]

ii) Predict two differences in the chemical properties of chromium and sodium.

[2]

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

i) Give two reasons why steel objects are plated with chromium.

[2]

ii) The formula of the chromium(III) ion is Cr³⁺ and of the sulfate ion is SO₄^2–. Give the formula of chromium(III) sulfate.

[1]

iii) Write the equation for the reaction at the negative electrode (cathode).

 [2]

iv) A colourless gas, which relights a glowing splint, is formed at the positive electrode (anode). Name this gas.

[1]

v) During electrolysis, it is necessary to add more chromium(III) sulfate but during copper-plating using a copper anode, it is not necessary to add more copper(II) sulfate. Explain. 

 [2]

Separate: Chemistry Only

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.

Separate: Chemistry Only

Give one reason why metal spoons are electroplated with silver.

Extended Only

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.

Extended Only

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.

Extended Only

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.

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.

Extended Only

In a hydrogen fuel cell, hydrogen loses electrons at the anode. Write the half equation to show the reaction at the anode.

Extended Only

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)

Extended Only

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.

Extended Only

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.

Extended Only

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.

Separate: Chemistry Only

Electrolysis can be used to silver-plate metal cutlery.

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

Separate: Chemistry Only

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. 

Separate: Chemistry Only

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.

 i) Calculate the cross-sectional area of:

• an average female, flat profile wedding ring.

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

 [2]

 ii) Calculate the percentage of the ring that is plating.

 [1]

Extended Only

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.

Separate: Chemistry Only

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.

Extended Only

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.

Extended Only

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.

 i) Give the ionic equation forming the gaseous product during the reaction between the metal food can and the acidic food.

 [1]

 ii) Explain how this shows that there is a break in the tin coating or chromium plating.

 [2]

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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.