School chemistry notes: The Contact Process and chemistry of sulfuric acid

Scroll down, take time to study the content or follow links or [Use the website search box]

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

(Suitable for AQA, Edexcel and OCR GCSE chemistry students)

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 sulfuric acid and the many important uses of sulfuric acid e.g. in fertilisers and manufacturing processes. The uses of sulfuric acid include lead-acid batteries, manufacture of fibres, titanium dioxide, dyestuffs, explosives, acid baths to clean metal surfaces, fertilisers e.g. the salt ammonium sulfate. Once spelt as sulphuric acid

INDEX of selected industrial chemistry sections

ALL my GCSE Chemistry Notes

Find your GCSE science course for more help links to revision notes

Use your mobile phone or ipad etc. in 'landscape' mode

This is a BIG website, you need to take time to explore it [SEARCH BOX]

email doc brown

3. Why is sulfuric 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 sulfuric acid that is used each year. How is sulfuric acid made? What is the Contact Process? Sulfuric acid is made starting from the element sulfur which is found in the Earth's crust. What are the uses of sulfuric acid?


  • Sulfuric acid is an important chemical produced in large quantities in the chemical industry.

  • Sulfuric 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 (supplies the element sulfur for plant growth)


  • The raw materials and other factors requiring decisions in manufacturing sulfuric acid

    • You need to know and find the raw materials that you can convert into a chemical feedstock that becomes the starting point for the synthesis of the chemical compound - the desired product, in this case sulfur trioxide and sulfuric acid.

    • The phrase chemical feedstock means the actual reactant molecules that are fed into the reactor chamber e.g. sulfur dioxide and oxygen.

    • The raw material usually requires processing e.g. purifying or chemically modifying it to produce a purer starting feedstock material - sometimes this involves molecules than can poison a catalyst, reducing its efficiency.

    • There are several factors to be taken into consideration before choosing the starting raw materials - that is assuming there is a choice e.g.

    • The cost of extracting, separating and purifying the starting chemicals of sulfur, oxygen and water from  which to make the product.

    • Is the process economically viable and the product ultimately profitable - sulfuric acid is made on a huge scale - reduces costs, especially if the process is continuous and not a batch process.

    • What are the energy costs like - can you operate the chemical process efficiently using the minimum energy - lower temperatures and pressure conditions use less energy and engineering costs are lower too - it should be taken into account that the oxides of sulfur are very corrosive gases.

    • The conditions chosen must be carefully controlled to allow an efficient rate of product production (economically acceptable rate) and maximise the yield of sulfur trioxide e.g. by control of reactant concentrations, reactor pressure, reactor temperature and appropriate catalyst.

  • Sulfuric acid is manufactured from the raw materials sulfur, air and water and involves the production of sulfur trioxide in the Contact Process.

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

    • In the reaction the sulfur 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 sulfide ores e.g. in a copper smelter:

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

    • Note: Sulfur 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 sulfur 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 - a high pressure isn't needed, which reduces engineering costs.

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

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

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

    • The reaction forms sulfur trioxide and the equilibrium is very much to the right hand side ...

    • The reaction is favoured by low temperature

      • Because it is an exothermic reaction, a lower temperature removes the heat energy and moves the equilibrium to the right-hand side, SO3 formation.

      • BUT, 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

      • This is from the pressure equilibrium rule, 3 => 2 gas molecules, LHS ==> RHS, the equilibrium moves in the direction of minimising the effect of increased pressure), see Chatelier's Principle.

      • In fact, only a small increase in pressure is needed to give high yields of sulfur 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 sulfur trioxide is a very corrosive gas!

    • The use of the V2O5 catalyst

      • The use of vanadium pentoxide 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 sulfur trioxide formed, you simply get to equilibrium position of maximum yield faster.

    • More GCSE notes on reversible reactions and equilibrium rules.

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

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

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

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

    • If the sulfur trioxide is added directly to water an acid mist forms which is difficult to contain because the reaction to form sulfuric 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 sulfur oxides are harmful and would cause local acid rain! To help this situation AND help the economics of the process, any unreacted sulfur dioxide is recycled through the reactor.

  • EXPERIMENTS with concentrated sulfuric acid

  • Concentrated sulfuric 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 sulfuric acid removes the elements of water from the compound leaving a 'spongy' black carbon residue.

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

      • ethanol ====> ethene + water

      • CH3CH2OH ====> CH2=CH2 + H2O

    • When added to blue copper sulfate crystals concentrated sulfuric acid removes the water of crystallisation leaving white anhydrous copper sulfate. 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   (water 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 reaction:

        • ethanoic acid + ethanol ==> ethyl ethanoate + water

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

    • a dreshchel/dreschler bottle (c) doc bConcentrated sulfuric 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 sulfuric 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.

Where next?

Index of selected pages describing industrial processes:

Limestone, lime - uses, thermal decomposition of carbonates, hydroxides and nitrates

Enzymes and Biotechnology 

Contact Process, the importance of sulfuric acid 

How can metals be made more useful? (alloys of Al, Fe, steel etc.)

Instrumental Methods of Chemical Analysis

Chemical & Pharmaceutical Industry Economics & Sustainability and Life Cycle Assessment

Products of the Chemical & Pharmaceutical Industries & impact on us

The Principles & Practice of Chemical Production - Synthesising Molecules  

Ammonia synthesis/uses/fertilisers

Oil Products

Extraction of Metals

Halogens - sodium chloride Electrolysis

Transition Metals

Extra Electrochemistry - electrolysis and cells

Website content Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry