Contact Process-Manufacture of sulfuric acid

d) The industrial manufacture of chemicals

This part is for SINGLE SCIENCE.

5.25 recall the raw materials used in the manufacture of sulfuric acid

The raw materials are:

• sulphur
• air (oxygen)
• water

5.26 describe the manufacture of sulfuric acid by the contact process, including the

essential conditions:

i a temperature of about 450 °C

ii a pressure of about 2 atmospheres

iii a vanadium(V) oxide catalyst

Stage 1: making sulfur dioxide

You can either burn sulfur in air:

S(s) + O₂(g) à SO₂(g)

or heat sulfide ores strongly in air:

4FeS₂(s) + 11O₂(g) à 2Fe₂O₃(s) + 8SO₂(g)

(FeS₂ is pyrite or iron pyrite)

Iron pyrite crystals

Stage 2: Making sulfur trioxide

Now the sulfur dioxide is converted into sulfur trioxide using an excess of air from the previous processes. 

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)     ∆H= -196 kJ/mol 

An excess of oxygen is used in this reaction, because it is important that as much sulfur dioxide as possible is converted into sulfur trioxide. Having sulfur dioxide left over at the end of the reaction is wasteful, and could cause possibly dangerous pollution. (Remember sulfur dioxide can dissolve in water and form acid rain, this can kill plants and animals-by altering pH of water fish live in for example. It will corrode limestone which is basically calcium carbonate. It can also leach nutrients from the soil.)

As the forwards reaction is exothermic, there would be a higher percentage conversion of sulfur dioxide into sulfur trioxide at a low temperature. (Remember your equilibrium stuff, go to my equilibrium post if you want to revise that first.)

However, at a low temperature, the rate of reaction would be very slow. 450°C is a compromise. Even so, there is already about a 99.5% conversion. 

There are 3 gas molecules on the left-hand side of the equation, but only 2 on the right. Reactions in which number of gas molecules decrease are favoured by high pressures. (Remember Le Chatelier's principle where you're trying to remove the change, if you increase pressure, moving the equilibrium to the side with less gas molecules would decrease pressure.) In this case though, the conversion is so good at low pressures already that it isn't economically worthwhile to use higher ones. So a pressure of 2 atmospheres is sufficient.

The catalyst, vanadium (V) oxide, has no effect on the percentage conversion, but helps to speed up the reaction. Without the catalyst, the reaction would be extremely slow. 

Remember, catalysts remain chemically unchanged at the end of the reaction. They help to speed up the rate of reaction, by providing an alternative pathway with a lower activation energy. Activation energy is the minimum amount of energy needed for a reaction to take place. So if the activation energy is lowered, more particles will have the required activation energy so a greater number of the collisions will be effective. Effective collisions are ones where reactions actually take place. Sometimes particles collide without reacting because they don't have the minimum activation energy required.

Sulfur dioxide is converted into sulfur trioxide

Stage 3: Making the sulfuric acid

In principle, you can react sulfur trioxide with water to make sulfuric acid. 

SO₃(g) + H₂O(l) à H₂SO₄(aq)

In practice, this produces an uncontrollable fog of concentrated sulfuric acid. Instead, the sulfur trioxide is absorbed in concentrated sulfuric acid to give fuming sulfuric acid (also called oleum). 

H₂SO₄(l) +SO₃(g) à H₂S₂O₇ (l)

This is converted into twice as much concentrated sulfuric acid by careful addition of water. 

H₂S₂O₇(l) + H₂O(l) à 2H₂SO₄(l)

I'm not sure which equation you guys have learnt, so I've included both the principle and the real life one. :)

5.27 recall the use of sulfuric acid in the manufacture of detergents, fertilisers and paints

Sulfuric acid has a wide range of uses throughout the chemical industry. The highest single use is in making fertilisers (including ammonium sulfate and 'superphosphate'-essentially a mixture of calcium phosphate and calcium sulfate).

It is also used in the manufacture of detergents and paints. If you look at the list of ingredients on any industrial or domestic detergents (including shampoos and liquid 'hand-soaps) and find the words 'sulfate' or 'sulfonate', then sulfuric acid was used in the manufacturing process. Even those simply labelled as containing 'anionic surfactants' almost certainly contain these sorts of ingredients, even if they don't name them.

In paint manufacture, sulfuric acid is used in extracting the white pigment titanium oxide, TiO₂ from titanium ores.