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 an important
chemical produced in large quantities in the chemical industry.
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
(supplies the element sulfur for plant growth),
MANUFACTURE of SULFURIC ACID
by the CONTACT PROCESS
raw materials and other factors requiring decisions in manufacturing
You need to know and find the raw materials that you can convert into a chemical feedstock that
becomes the starting point for the synthesis of the chemical compound - the
desired product, in this case sulfur trioxide and sulfuric acid.
The phrase chemical
feedstock means the actual reactant molecules that are fed into the
reactor chamber e.g. sulfur dioxide and oxygen.
The raw material usually
requires processing e.g. purifying or chemically modifying it to produce
a purer starting feedstock material - sometimes this involves molecules
than can poison a catalyst, reducing its efficiency.
There are several factors to be taken
into consideration before choosing the starting raw materials - that is
assuming there is a choice e.g.
The cost of extracting, separating
and purifying the starting chemicals of sulfur, oxygen and water from
which to make the product.
Is the process economically viable
and the product ultimately profitable - sulfuric acid is made on a huge
scale - reduces costs, especially if the process is continuous and not a
What are the energy costs like - can
you operate the chemical process efficiently using the minimum energy -
lower temperatures and pressure conditions use less energy and
engineering costs are lower too - it should be taken into account that
the oxides of sulfur are very corrosive gases.
The conditions chosen must be
carefully controlled to allow an efficient rate of product production
(economically acceptable rate) and maximise the yield of sulfur trioxide
e.g. by control of reactant concentrations, reactor pressure, reactor
temperature and appropriate catalyst.
Sulfuric acid is
manufactured from the raw materials sulfur, air and water and
involves the production of sulfur trioxide in the Contact Process.
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
two atmospheres - a high pressure isn't needed, which reduces
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 ...
2SO2(g) + O2(g)
forms sulfur trioxide and the equilibrium is very much to the right
hand side ...
The reaction is favoured by low
Because it is an exothermic reaction,
a lower temperature removes the heat energy and moves the equilibrium to
the right-hand side, SO3 formation.
BUT, 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
This is from the pressure equilibrium rule, 3 => 2
gas molecules, LHS ==> RHS, the equilibrium moves in the
direction of minimising the effect of increased pressure), see
In fact, only a small increase in pressure is
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).
Using higher pressures might
seem favourable, but it raises engineering costs and increases the rate
of corrosion of piping because sulfur trioxide is a very corrosive gas!
The use of
the V2O5 catalyst
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
- electrolysis and cells
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