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SULPHURIC ACID

THE CONTACT PROCESS Manufacture of Sulphuric Acid

A brief summary of the Contact Process The Contact Process:  makes sulphur dioxide;  convers the sulphur dioxide into sulphur trioxide (the reversible reaction at the heart of the process);  converts the sulphur trioxide into concentrated sulphuric acid.

Preparation of Sulphur Making the sulphur dioxide  This

can either be made by burning sulphur in an excess of air: S(s)+O2(g) SO2(g)  Or by heating sulphide ores like pyrite in an excess of air: 4FeS2(s)+11O2(g) 2Fe2O3(s)+8SO2(g)  An excess of air is used so that the sulphur dioxide produced is already mixed with oxygen.

Purification  Purification

of air and SO2 (using an electrostatic precipitator) is necessary to avoid catalyst poisoning (i.e. removing catalytic activities). The gas is then washed with water and dried by sulphuric acid.  To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.  Sulphur dioxide and oxygen then react in the manner as follows, to produce sulphur trioxide:  2 SO2(g) + O2(g) ⇌ 2 SO3(g) : ΔH = −197 kJ mol−1

Converting the sulphur trioxide into sulphuric acid  This

can't be done by simply adding water to the sulphur trioxide - the reaction is uncontrollable and creates a fog of sulphuric acid. Instead, the sulphur trioxide is first dissolved in concentrated sulphuric acid: H2SO4(l)+SO3(g) H2S2O7(l)

 The

product is known as fuming sulphuric acid or oleum.

Converting the sulphur trioxide into sulphuric acid (Cont’d) 

This can then be reacted safely with water to produce twice as much concentrated sulphuric acid as originally used to make the fuming sulphuric acid. H2S2O7(l) + H2O(l) → 2 H2SO4(l)



The average percentage yield of this reaction is around 30%.

Equilibrium considerations relating to Temperature  The

position of the equilibrium is shifted as far as possible to the right in order to produce the maximum possible amount of sulphur trioxide in the equilibrium mixture.  The forward reaction (the production of sulphur trioxide) is exothermic.  2 SO2(g) + O2(g) ⇌ 2 SO3(g) : ΔH = −197 kJ mol−1

Equilibrium considerations relating to Temperature

 According

to Le Chatelier's Principle, this will be favoured if you lower the temperature. The system will respond by moving the position of equilibrium to counteract this - in other words by producing more heat.  In order to get as much sulphur trioxide as possible in the equilibrium mixture, you need as low a temperature as possible, but lower temperature means slower reaction.

Equilibrium considerations relating to Temperature  Therefore

400 - 450°C is a compromise temperature producing a fairly high proportion of sulphur trioxide in the equilibrium mixture, but in a very short time.

Equilibrium considerations relating to Pressure 2

SO2(g) + O2(g) ⇌ 2 SO3(g) : ΔH = −197 kJ mol−1  There are 3 molecules on the left-hand side of the equation, but only 2 on the right.  According to Le Chatelier's Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules.

Equilibrium considerations relating to Pressure  That

will cause the pressure to fall again.  In order to get as much sulphur trioxide as possible in the equilibrium mixture, you need as high a pressure as possible. High pressures also increase the rate of the reaction, however, the reaction is done at pressures close to atmospheric pressure.

Equilibrium considerations relating to Pressure  As

a compromise a gas pressure between 100 and 200 kPa is used and increases the collision frequency between the reacting gases. They increase the yield by driving the reaction to the right (1.5 moles to 1 mole of gas). These pressures are also sufficiently low to avoid expensive apparatus and reduce energy usage.

Equilibrium considerations relating to the Catalyst  The

catalyst has no effect whatsoever on the position of the equilibrium. Adding a catalyst doesn't produce any greater percentage of sulphur trioxide in the equilibrium mixture.

 In

the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time.

Equilibrium considerations relating to the Catalyst A

catalyst is used to increase the rate of reaction, compensating for the lower temperature. V2O5 supported on a silica bed is used as the catalyst.

 The

catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor.

Flow diagram

INDUSTRIAL IMPORTANCE OF COMPOUNDS OF SULPHUR

Importance of compounds of sulphur in industry

 SO2

is used as a food preservative in juices and jams. It creates an acidic and oxidising medium which prevents the growth of any microbes.

 Sulphuric

acid is used in the manufacture of fertilisers, manufacture of detergents, production of phosphoric acid, used to help manufacture fabrics e.g. nylon, in petroleum refining, to manufacture dyes and as the battery acid in car batteries.

Importance of compounds of sulphur in industry

 Vulcanisation

of rubber  Improves elasticity and textile strengths  Compound in fumigants and fungicides  Used in metallurgy  Helps to treat air i.e. removing exhaust fumes etc.

IMPACTS OF THE SULPHURIC ACID INDUSTRY

Impact on the environment  Emissions

of SO2 causes drastic changes in the atmosphere. It plays a major role in the formation of acid rain via the reaction: SO2 + H2O H2SO3 or even 2SO2 + O2 + 2H2O 2H2SO4  Acid rain damages or destroys buildings and vegetation and also is not healthy for the aquatic life as it reduces the pH of the water decreasing their quality of life. It also can cause the mobilisation of toxic ions in the soil which results in vegetation’s growth being affected.

Impact on the environment  Sulphur

oxides combine with other substances in the air to produce a haze that reduces visibility.

 SO2

emissions can cause respiratory problems as it is irritating and causes wheezing and shortness of breath. The gas irritates the throat and lungs and, if there are fine dust particles in the air, can damage a person's respiratory system.

Impact on the environment 

Sulphuric acid is a highly corrosive compound that mixes with water, and it causes problems for the environment as it kills plant life. In waterways it readily mixes with the water. Ingestion of this water by wildlife results in illness and sometimes death. The acid typically gets into water because of improper disposal, but it also can be airborne from factories. This airborne sulphuric acid can be inhaled by animals and humans.

Impact on the environment  Sulphuric

acid is soluble in both water and ethyl alcohol and it can cause fires with other combustible materials.  The industry moderates sulphur dioxide emissions by using the large amounts of SO2 produced from these processes for the manufacture of sulphuric acid in the contact process while any emissions of sulphur dioxide from the contact process itself is minimal.

Impact on the environment  Sulphuric

acid production is a net producer of energy. Heat evolved in the process is used to turn water into steam which is used to generate electricity.

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