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?
USES of SULFURIC ACID
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,
MANUFACTURE of SULFURIC ACID
by the CONTACT PROCESS
Sulfuric acid is
manufactured from the raw materials sulfur, air and water and
involves the production of sulfur trioxide in the Contact Process.
Because of its usefulness
Sulfur is burned in air to form sulfur
In the reaction the
sulfur is oxidised (O gain)
Sulfur dioxide can
also be indirectly obtained from the process of extracting copper
from copper sulfide ores e.g. in a copper smelter:
Note: Sulfur 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 sulfur 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 sulfur dioxide is
oxidised in the reversible exothermic reaction ...
(2) 2SO2(g) + O2(g)
forms sulfur 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 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).
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 sulfur trioxide formed, you
simply get there faster.
GCSE notes on reversible reactions and equilibrium rules.
trioxide is dissolved in concentrated sulfuric acid to form fuming
sulfuric acid (oleum).
Water is then carefully added to the oleum to
produce concentrated sulfuric acid (98%).
+ 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
+ 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 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.
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 sulfuric acid removes the elements of water from
the compound leaving a 'spongy' black carbon residue.
are heated with conc. sulfuric acid, they are dehydrated to
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.
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 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.
selected pages describing industrial processes:
- uses, thermal decomposition of carbonates, hydroxides and nitrates
Contact Process, the importance of sulfuric acid
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
Extraction of Metals
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