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?
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,
MANUFACTURE of SULFURIC ACID
by the CONTACT PROCESS
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
Sulfur is burned in air to form sulphur
In the reaction the
sulphur is oxidised (O gain)
Sulfur dioxide can
also be indirectly obtained from the process of extracting copper
from copper sulphide ores e.g. in a copper smelter:
Note: Sulphur dioxide
itself is a useful chemical in its own right:
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
In the reactor, the sulphur dioxide is
mixed with sufficient air (to give the required SO2:O2 2:1
The mixture is passed over a catalyst of vanadium(V) oxide V205
at a high temperature (about 450°C) and at a pressure of between one and
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)
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).
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.
GCSE notes on reversible reactions and equilibrium rules.
trioxide is dissolved in concentrated sulphuric acid to form fuming
sulphuric acid (oleum).
Water is then carefully added to the oleum to
produce concentrated sulphuric acid (98%).
+ 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
+ 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
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.
concentrated sulfuric acid
can be used in the laboratory as a dehydrating agent.
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.
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.
are heated with conc. sulphuric acid, they are dehydrated to
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.
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).
Concentrated 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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