The Second Stage Is to React Sulphur Dioxide with Oxygen to Form Sulphur Trioxide

Contact Process

CPG Paper

Over 150 million tonnes of sulphuric acid (H2SO4) are produced annually making it one of the world’s most manufactured chemicals. Sulphuric acid is produced industrially using the contact process. The first stage is to react sulphur with oxygen to produce sulphur dioxide gas.

S + O2 → SO2

a)  Calculate the mass of sulphur required to produce 100kg of sulphur dioxide. Relative atomic masses: sulphur = 32, Oxygen = 16

(3 marks)

The second stage is to react sulphur dioxide with oxygen to form sulphur trioxide.

b) i) Describe the operating conditions used for this reaction (2 marks)

ii) What catalyst is used in this reaction? (1 mark)

c) The formation of sulphur trioxide is exothermic, which means a low temperature would increase the yield of SO3. Explain why a low temperature is not used. (1 mark)

In the final stage, oleum (H2S2O7) reacts with water to produce sulphuric acid.

d) Write a balanced symbol equation for this reaction. (1 mark)

Sample Paper

2. The “Really Useful Chemicals” Factory makes sulphuric acid using the Contact

Process. The Contact Process involves three stages. The first stage involves producing sulphur dioxide by burning sulphur in excess air.

(a) Write a word equation for the first stage of the contact process. (1 mark)

(b) The second stage involves producing sulphur trioxide from the sulphur dioxide.

This reaction is carried out in the presence of a vanadium(V) oxide catalyst.

(i) Write a symbol equation for this second stage of the contact process.

(2 marks)

(ii) What does a catalyst do? (1 mark)

(c) The conversion of sulphur dioxide into sulphur trioxide is a reversible, exothermic reaction. At the Really Useful Chemicals factory this conversion takes place at a temperature of 450°C and results in a 95% conversion rate of sulphur dioxide into sulphur trioxide. A change in temperature affects both the rate of reaction and the yield of sulphur trioxide.

(i)  State and explain one disadvantage of using a temperature higher than 450°C. (3 marks)

(ii)  State and explain one advantage of using a temperature higher than 450°C in this conversion. ( 3 marks)

(d)  Sulphuric acid is used as a catalyst in the preparation of esters, a group of compounds found in flavourings and perfumes. Complete the symbol and word equations for the preparation of the ester methyl ethanoate.

______+ CH3COOH → + CH3COOCH3 + ______

methanol + ______→ methyl ethanoate + ______

(3 marks)

June 2009

3.  Sulphuric acid is an important substance. It is used to make many other substances.

a) Name two substances that are produced from sulphuric acid. (2 marks)

b) The first step in the manufacture of sulphuric acid is the burning of sulphur in air to

form sulphur dioxide.

S + O2 → SO2

Suggest why air rather than pure oxygen is used. (1 mark)

c)  The sulphur dioxide is then reacted with more oxygen from air in the presence of a hot catalyst. The reaction is reversible.

i) Name the catalyst used. (1 mark)

ii) Write the balanced equation for the reversible reaction (3 marks)

iii) Describe how the gas from (c) is made into sulphuric acid. (1 mark)

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