Chapter 21

Chapter 21
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The industrial production of sulfuric acid
 Factors affecting the production, including rate
and equilibrium position, catalysts, temperature,
pressure
 Waste management including generation,
treatment and waste reduction
 Health and safety
 Uses of Sulfuric acid
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Name the raw materials used in the
production of sulfuric acid
Describe the reaction steps by which sulfuric
acid is manufactured
Explain how the principles of equilibrium and
reaction rates play a significant role in
determing reaction conditions
Explain the reasons for choice of reaction
conditions, such as pressure, temprrature and
catalysts.
Describe waste management procedures
Describe health and safety issues involved in
the production of sulfuric acid
 Describe how the production of sulfuric acid
through the contact process realates to the
principles of green chemistry
 Recall the major uses of sulfuric acid
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Absorption tower
Contact process
Converter
Dehydrating agent
Diprotic acid
Double absorption
oleum
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Sulfuric acid is produced in greater quantities
than any other chemical in both Australia and
the world.
Annual worldwide production is estimated at
about 170 million tonnes and Australian
production at 4 million tonnes.
In future years it is anticipated that Australia
will become a major exporter of the chemical.
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Transport and storage of sulfuric acid are
hazardous.
A high proportion of the acid is used close to
the site of manufacture.
Most sulfuric acid plants are located near
smelting and refining industries that produce
waste sulfur dioxide, a raw material for the
production of sulfuric acid.
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It is also used in the
manufacturing of
paper, household
detergents, pigments,
dyes and drugs.
It is the electrolyte in
car batteries.
Many Australian soils are phosphorous deficient and it
must be added to the land.
 During superphosphate manufacture, insoluble
calcium phosphate contained in rock phosphate is
converted to a soluble form that plants can absorb.
 This reaction takes several weeks to occur:
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Ca3(PO4)2(s) + 2H2SO4(l) + 4H2O(l) → Ca(H2PO4)2(s) + 2CaSO4.H2O(l)
superphosphate
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The final mixture is superphosphate. It is
crushed into a powder and bagged for easy
distribution.
Although Queensland has deposits of
phosphate rock, it is not really used as a
reactant.
Instead we use rock phosphate that comes
from North Africa as it is cheap and readily
available.
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Pure sulfuric acid is a viscous liquid that
reacts with water in two steps.
H2SO4(l) + H2O(l) → HSO4 - (aq) + H3O +(aq)
Ka = 109 mol L-1
HSO4 - (aq) + H2O(l) <==> SO4 2- (aq) + H3O +(aq)
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Sulfuric acid is a diprotic acid.
The first step proceeds virtually to
completion.
The second step has a much smaller Ka value.
H2SO4(l) + H2O(l) → HSO4 - (aq) + H3O +(aq)
Ka = 109 mol L-1
HSO4 - (aq) + H2O(l) <==> SO4 2- (aq) + H3O +(aq)
Ka = 1.0 x 10-2 mol L-1
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It is used as a strong acid in the ‘pickling’ of
iron and steel. This is where the iron(III) oxide
is removed from the surface of the iron.
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A large amount of heat is evolved during this
process.
For this reason when preparing sulfuric acid,
you ALWAYS add the acid to water slowly
with continuous stirring.
Never add water to acid as this can cause the
water to boil and the acid to splatter.
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Concentrated sulfuric acid is a powerful
dehydrating agent.
Sugar is dehydrated:
C12H22O11(s)
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H2SO4(l)
12C(s) + 11H2O(l)
The dehydrating ability of sulfuric acid is often
utilised in laboratories to dry gas mixtures that
are being prepared or analysed.
It is not suitable for bases as they will react with
the acid
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Concentrated sulfuric acid is a strong oxidant,
especially when hot.
Sulfuric acid can be reduced to sulfur dioxide
(SO2), sulfur (S) or hydrogen sulfide (H2S),
depending on the temperature, the strength
of the reductant involved and the mole ratio
of the reactants.
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The following reactions can occur when zinc
is added to sulfuric acid:
Zn(s) + 2H2SO4(aq) → ZnSO4(aq) + 2H2O(l) + SO2(g)
3Zn(s) + 4H2SO4(aq) → 3ZnSO4(aq) + 4H2O(l) + S(s)
4Zn(s) + 5H2SO4(aq) → 4ZnSO4(aq) + 4H2O(l) + H2S(g)
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Like other strong acids, dilute sulfuric acid
reacts with zinc to produce hydrogen gas:
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
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What is the oxidant?? H+ (g)
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Sulfuric Acid is manufactured in stages from
sulfur dioxide.
These involve oxidation of sulfur dioxide to
sulfur trioxide.
Followed by conversion to the acid.
The process can be summarised:
SO2(from various sources) → SO3 → H2SO4
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The sulfur dioxide used to produce sulfuric
acid is obtained from two principal sources
 Combustion of sulfur recovered from natural gas
and crude oil
 Sulfur dioxide formed during the smelting of
sulfide ores of copper, zinc or lead.
 A third process can be used from mining of the
underground deposits of elemental sulfur but this
is not used in Australia due to the first two being
in high abundance.
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If sulfur is used as a raw material, the first
step is to spray molten sulfur under pressure
into a furnace up to 1000°C.
Here it burns in air to produce sulfur dioxide
gas. The sulfur dioxide gas is then cooled for
the next step
The high surface area of the sulfur spray
allows combustion to be rapid.
S(l) + O2(g) → SO2(g); ∆H = -297 kJ mol-1
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Sulfur dioxide gas is oxidised to sulfur trioxide
gas by oxygen, using Vanadium oxide as a
catalyst.
2SO2(g) + O2(g)
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2SO3(g); ∆H = -197 kJ mol-1
This step is performed in a reaction vessel called
a converter.
Sulfur dioxide is mixed with air and passed
through trays containing loosely packed porous
pellets of catalysts.
The converter contains several catalyst beds and
the gas mixture passes over each in succession.
 Because the reaction is exothermic it is necessary to
cool the gas mixture as it passes from one tray to
another to maintain the desired reaction
temperature.
 The temperature in the converter is maintained
between 400°C and 500°C and the pressure is close
to 1 atm.
 Nearly complete conversion of sulfur dioxide to
sulfur trioxide is achieved.
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Using Le Chatelier’s principal, the equilibrium
yield of sulfur trioxide will increase:
 As temperature decrease. Since the reaction is
exothermic a decrease in temperature will favour
the forward reaction.
 As pressure increases. Since there are more gas
particles on the reactants the forward reaction will
result in a decreased pressure.
 If excess reactants are added.
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The rate of reaction will be faster:
 As temperature increases
 As pressure increases
 If a catalyst is employed
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What compromises have been made to get
the fastest reaction with the best yields?
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Sulfur trioxide reacts with water to form
sulfuric acid:
SO3(g) + H2O(l) → H2SO4(aq); ∆H = -130 kJ mol-1
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However direct reaction with water is not used,
because so much heat evolves when sulfur trioxide is
added to water that a fine mist of acid is produced
which is difficult to collect.
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Instead, sulfur trioxide gas is passed into
concentrated sulfuric acid in an absorption tower.
This reaction occurs in two steps
The sulfur trioxide gas dissolves almost totally in
the acid to form a liquid known as oleum
SO3(g) + H2SO4(l) → H2S2O7(l)
2. Oleum obtained from the absorption tower is then
carefully mixed with water to produce sulfuric acid:
H2S2O7(l) + H2O(l) → H2SO4(l)
1.
1.
2.
3.
4.
Oxidation of S to SO2
Catalytic oxidation of SO2 to SO3
The absorption of the SO3 by previously
prepared sulfuric acid to produce oleum,
H2S2O7
The dilution of the oleum with water to
make sulfuric acid.
Sulfuric acid plants use sulfur or sulfur dioxide that is
a by-product from other industries.
 To maximise their conversion of sulfur dioxide to
sulfur trioxide most plants now use a double
absorption process.
 Any unreacted gas from the absorption tower is
passed over the catalytic beds again and re passed
through the absorption tower.
 This improves the percentage of sulfur dioxide
converted from 98% to better than 99.6%
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Emissions from the plant have to be
continuously monitored for sulfur dioxide as
this can cause acid rain.
The amount of sulfuric acid mist emitted
from the process is minimised by controlling
the operating temperature of the absorber,
gas flow rates and concentrations.
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Improvements in conversion have also been
made by adding small amounts of caesium to
the vanadium oxide catalyst to increase its
efficiency and allow it to operate at lower
temperatures
Caesium-doped catalysts are about 3x more
expensive than the usual vanadium oxide
catalyst.
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There is relatively little solid waste produced
from sulfuric acid manufacturing.
The catalyst is dumped in landfill after
recovering the mildly toxic vanadium.
The cooling water is recycled.
All three processes are exothermic, meaning
energy is produced. This energy is used to
generate its electricity or as a source to
produce other chemicals.
Sulfuric acid is highly corrosive and can burn skin
and eyes severely.
 It can cause blindness and third degree burns on
contact.
 Exposure to sulfuric acid mist can cause other health
problems.
 Workers in sulfuric acid plants can also be exposed
to the acid through breathing air contaminated with
emissions containing oxides of sulfur
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Strict safety procedures including adequate
methods to trap the fumes are required to minimise
the risks to workers and the environment in the case
of accidental release
 Work areas must be well ventilated and employees
wear protective clothing.
 Acid spills are contained using materials such as
earth, clay or sand and then slowly diluted with
water before being neutralised with a base such as
limestone or sodium carbonate
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