Doc Brown's KS4 Science GCSE/IGCSE Industrial Chemistry Revision Notes

3. Sulphuric Acid - its properties, uses and manufacture by the Contact Process

What is sulfuric acid used for? Sulfuric acid is a very important industrial chemical compound that is manufactured by the Contact Process. This page describes and explains this process and goes on to describe the chemical properties and reactions of sulphuric acid and the many important uses of sulphuric acid e.g. in fertilisers and manufacturing processes. The uses of sulphuric acid include lead-acid batteries, manufacture of fibres, titanium dioxide, dyestuffs, explosives, acid baths to clean metal surfaces, fertilisers e.g. the salt ammonium sulphate.

Index of sections: 1. Limestone, lime - uses, thermal decomposition of carbonates, hydroxides and nitrates  *  2. Enzymes and Biotechnology  *  3. Contact Process, the importance of sulphuric acid  *  4. How can metals be made more useful? (alloys of Al, Fe, steel etc.) * 5. The importance of titanium  *  6. Instrumental Methods of Chemical Analysis * 7. Chemical & Pharmaceutical Industry Economics & Sustainability * 8. Products of the Chemical & Pharmaceutical Industries & impact on us * 9. The Principles & Practice of Chemical Production - Synthesising Molecules  and other web pages of industrial chemistry notes: Ammonia synthesis/uses/fertilisers * Oil Products * Extraction of MetalsHalogens - sodium chloride Electrolysis * Transition Metals * Extra Electrochemistry

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3. Why is sulphuric acid such a useful material? How is it made? What is the Contact Process?

Because sulfuric acid has so many uses the industrial development of a country is sometimes measured by the amount of sulphuric acid that is used each year. How is sulfuric acid made? What is the Contact Process? Sulphuric acid is made starting from the element sulphur which is found in the Earth's crust. What are the uses of sulfuric acid?


  • Sulphuric acid is used as the lead-acid battery, the manufacture of detergents, dyes (dyestuffs), explosives, artificial  fibres, pigments like white titanium dioxide and is used to make fertilisers, 


  • Sulphuric acid is manufactured from the raw materials sulfur, air and water and involves the production of sulphur trioxide in the Contact Process. Because of its usefulness

  • (1) Sulfur is burned in air to form sulphur dioxide (exothermic).

    • In the reaction the sulphur is oxidised (O gain)

      • (1a) S(s) + O2(g) ====> SO2(g)

    • Sulfur dioxide can also be indirectly obtained from the process of extracting copper from copper sulphide ores e.g. in a copper smelter:

      • (1b) Cu2S(s) + O2(g) ====> 2Cu(l) + SO2(g)

    • Note: Sulphur dioxide itself is a useful chemical in its own right:

    • It is used as a bleach in the manufacture of wood pulp for paper manufacture

    • and its toxic nature makes it useful as a food preservative by killing bacteria.

  • (2) In the reactor, the sulphur dioxide is mixed with sufficient air (to give the required SO2:O2 2:1 ratio).

    • The mixture is passed over a catalyst of vanadium(V) oxide V205 (vanadium pentoxide) at a high temperature (about 450C) and at a pressure of between one and two atmospheres.

    • It is a 2nd exothermic oxidation reaction and is known as the Contact Process.

    • In the reactor the sulphur dioxide is oxidised in the reversible exothermic reaction ...

    •  (2) 2SO2(g) + O2(g) (c) doc b 2SO3(g)

    • Top of page - sub-index and linksThe reaction forms sulphur trioxide and the equilibrium is very much to the right hand side ...

    • So, despite the reaction being exothermic (95 kJ released per mole of SO3), a relatively high temperature is used to ensure a reasonable rate of reaction (despite the fact that it favours reverse reaction R to L, from the energy change equilibrium rule, inc. T. favours endothermic direction).

    • The reaction is favoured by high pressure (pressure equilibrium rule, 3 => 2 gas molecules, LHS ==> RHS), but only a small increase in pressure is used to give high yields of sulphur trioxide, because the formation of SO3 on the right hand side is so energetically favourable (approx. 99% yield, i.e. only about 1% SO2 unreacted).

      • Using higher pressures might seem favourable, but it raises engineering costs and increases the rate of corrosion of piping because sulphur trioxide is a very corrosive gas!

    • The use of the V2O5 catalyst ensures a fast reaction without having to use too a higher temperature which would favour the left hand side and reduce the yield BUT it does not change the % of sulphur trioxide formed, you simply get there faster.

    • More GCSE notes on reversible reactions and equilibrium rules.

  • (3) The sulphur trioxide is dissolved in concentrated sulphuric acid to form fuming sulphuric acid (oleum).

    • (3) SO3(g) + H2SO4(l) ====> H2S2O7(l)

  • (4) Water is then carefully added to the oleum to produce concentrated sulphuric acid (98%).

    • (4) H2S2O7(l) + H2O(l) ====>  2H2SO4(l)

    • If the sulphur trioxide is added directly to water an acid mist forms which is difficult to contain because the reaction to form sulphuric acid solution is very exothermic!

    • If you 'add' equations (3) + (4) you get

      • (5) SO3(g) + H2O(l) ====>  H2SO4(l)

      • which is how it is usually written in GCSE textbooks, so learn equations (1a), (2) and (5) for the manufacture of sulfuric acid from sulfur.

  • Good anti-pollution measures need to be in place since the sulphur oxides are harmful and would cause local acid rain! To help this situation AND help the economics of the process, any unreacted sulphur dioxide is recycled through the reactor.

  • EXPERIMENTS with concentrated sulfuric acid

  • Concentrated sulphuric acid can be used in the laboratory as a dehydrating agent.

    • Dehydration is the removal of water or the elements of water from a compound and can be described as an elimination reaction. Usually and adjacent H and OH in a molecule are removed to form the water.

    • When added to some organic compounds containing hydrogen and oxygen, e.g. sugar, concentrated sulphuric acid removes the elements of water from the compound leaving a 'spongy' black carbon residue.

    • If alcohols are heated with conc. sulphuric acid, they are dehydrated to alkenes e.g.

      • ethanol ====> ethene + water

      • CH3CH2OH ====> CH2=CH2 + H2O

    • When added to blue copper sulphate crystals concentrated sulphuric acid removes the water of crystallisation leaving white anhydrous copper sulphate. In this case the water already exists BUT not in a mixture and so the following reaction is classified as a chemical change.

      • CuSO4.5H2O(s) (c) doc b CuSO4(s) + 5H2O(absorbed into the H2SO4 which it reacts with)

    • Conc. H2SO4 catalyses the reaction between an alcohol and carboxylic acid to form an pleasant smelling ester liquid but it isn't considered a dehydration reaction (H comes from one molecule and OH from the other).

      • e.g. the esterification ethanoic acid + ethanol ==> ethyl ethanoate + water

      • CH3COOH + CH3CH2OH ====> CH3COOCH2CH3 + H2O

    • a dreshchel/dreschler bottle (c) doc bConcentrated sulphuric acid can be used as a drying agent e.g. in the preparation of gases.

      • The prepared gas is bubbled through a dreschel/dreschler bottle (illustrated on the right), containing the concentrated sulphuric acid. In this case the water vapour is just a component in a gaseous mixture. Most gases can be dried in this way except the alkaline gas ammonia which will exothermically react to form a solid salt. In this case the water vapour is just a component in a gaseous mixture.

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