Sulphur and Sulphur
dioxide
Where is it found?
Sulphur is a non-metal. It is quite a common element in the Earth’s crust.
Found as an element in underground beds.
Occurs as a compound in many metal ores. E.g Galena ore (lead( ii) sulfide ),PbS
Fossil fuels: Coal, Petroleum (crude oil) and natural gas.
Extracting the Sulfur
From oil and gas
Most Sulphur is obtained from Sulphur compounds these compounds are mainly pollutants and are
removed to reduce air pollution.
For instance natural gas is mainly methane. But it can have as much as much as 30% hydrogen
sulfide. This is separated from the methane. Then it is reacted with oxygen, with the help of a
catalyst to give Sulphur.
Sulfuric acid is synthesized using the contact process
Step 1
Oxidation of sulfur
S + O2 SO2
Step 2
The main stage is the oxidation of sulfur dioxide to sulfur trioxide using a V2O5 catalyst
2SO2 + O2 2SO3
Conditions needed for the main stage of sulfur trioxide in stage 2
Temperature of 450 and pressure
Step 3
Once stage 2 is completed, the sulfur trioxide is absorbed in 98% sulfuric acid to produce a thick
liquid called oleum.
SO3 + H2SO4 H2S2O7
It is not absorbed into water because a fine mist of sulfuric acid would be produced and this would
be difficult to condense and is also highly dangerous
Oleum is added to water to form concentrated sulfuric acid
H2O + H2S2O7 2H2SO4
Properties of sulfuric acid
Sulfuric acid is a strong diabasic acid as two of its hydrogen atoms can be replaced by a metal
Mg + H2SO4 MgSO4 + H2
It reacts in a similar way to other acids with metal carbonates, oxides, hydroxides (and ammonia) and metals
e.g:
ZnO + H2SO4 ZnSO4 + H2O
Na2CO3 + H2SO4 Na2SO4 + CO2 +H2O
Concentrated sulfuric acid is corrosive and a powerful oxidizing agent
Concentrated sulfuric acid is also a very powerful dehydrating agent and is very good at removing
water from other substances
For example, if mixed with sugar (C6H12O6), concentrated H2SO4 will remove water molecules and
leave behind carbon in a spectacular looking tower.
Reaction conditions
The reaction of sulfur to sulfur dioxide is an equilibrium reaction. The yield of sulfur increases with an
increase in pressure and decreased volume. But atmospheric pressure is used. There is no need to increase
pressure because the yield of sulfur trioxide is already very high at atmospheric pressure.
The reaction is exothermic. So the yield of sulfur trioxide decreases with increasing g temperature. This is
because an increase in temperature favors endothermic reaction which is the reverse reaction. But if we use
a temperature that is too low the rate of reaction is slow.
A compromise temperature of 450 degree celsius gives quite a good yield of sulfur trioxide and a fast enough
rate of reaction.
The catalyst has no effect on equilibrium, it speeds up the rate of the forward and reverse reactions equally.
