OxNotes > GCSE/IGCSE Revision > IGCSE Chemistry > Contact Process: Manufacture of Sulphuric Acid
Ammonia in the manufacture of nitric acid and fertilisers
Ammonia is used to make nitric acid and fertilisers as it contains nitrate ions, plants need nitrates to grow.
Raw materials used in the manufacture of sulfuric acid
The raw materials are:
- sulphur (sulphur is found in rocks and some natural gases)
- oxygen (from the air)
- water
Manufacture of sulfuric acid by the contact process
Essential conditions:
Stage 1: making sulfur dioxide
Either burn sulfur in air:
S(s) + O2(g) --> SO2(g)
or heat sulfide ores strongly in air:
4FeS2(s) + 11O2(g) à 2Fe2O3(s) + 8SO2(g)
(FeS2 is pyrite or iron pyrite)
Stage 2: Making sulfur trioxide
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 can cause dangerous pollution. (Sulfur dioxide gas can dissolve in water and form acid rain)
As the forwards reaction is exothermic (releases heat due to bonds being created), there would be a higher percentage conversion of sulfur dioxide into sulfur trioxide at a low temperature. However, at a low temperature, the rate of reaction would be 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. (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). The conversion is so good at low pressures already that it isn't economically worthwhile to use higher pressures. 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.
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 (minimum amount of energy needed for a reaction to take place). If the activation energy is lowered, more particles will have the required activation energy so there will be a greater number of the collisions.
Stage 3: Making the sulfuric acid
You can react sulfur trioxide with water to make sulfuric acid.
SO3(g) + H2O(l) --> 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) --> H2S2O7 (l)
This is converted into twice as much concentrated sulfuric acid by careful addition of water.
H2S2O7(l) + H2O(l) ---> 2H2SO4(l)
- Temperature of about 450°C
- Pressure of about 2 atmospheres
- A vanadium(V) oxide catalyst
Stage 1: making sulfur dioxide
Either burn sulfur in air:
S(s) + O2(g) --> SO2(g)
or heat sulfide ores strongly in air:
4FeS2(s) + 11O2(g) à 2Fe2O3(s) + 8SO2(g)
(FeS2 is pyrite or iron pyrite)
Stage 2: Making sulfur trioxide
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 can cause dangerous pollution. (Sulfur dioxide gas can dissolve in water and form acid rain)
As the forwards reaction is exothermic (releases heat due to bonds being created), there would be a higher percentage conversion of sulfur dioxide into sulfur trioxide at a low temperature. However, at a low temperature, the rate of reaction would be 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. (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). The conversion is so good at low pressures already that it isn't economically worthwhile to use higher pressures. 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.
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 (minimum amount of energy needed for a reaction to take place). If the activation energy is lowered, more particles will have the required activation energy so there will be a greater number of the collisions.
Stage 3: Making the sulfuric acid
You can react sulfur trioxide with water to make sulfuric acid.
SO3(g) + H2O(l) --> 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) --> H2S2O7 (l)
This is converted into twice as much concentrated sulfuric acid by careful addition of water.
H2S2O7(l) + H2O(l) ---> 2H2SO4(l)
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.
Many detergents (including shampoos) use sulfates.
In fertilisers, it is reacted to make phosphates soluble to plants.
In paints, it is reacted with titanium ore to make a main pigment in paint.
Many detergents (including shampoos) use sulfates.
In fertilisers, it is reacted to make phosphates soluble to plants.
In paints, it is reacted with titanium ore to make a main pigment in paint.