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Sulphuric acid, which was known as Oil of Vitriol in ancient times, is made up of the elements hydrogen, oxygen, and sulphur. It is a mineral acid with the molecular formula of H2SO4.
It is without odour and does not have any colour. It is also soluble in water. Pure sulphuric acid is hygroscopic in nature and readily absorbs water vapour when exposed to air. This is the reason why sulphuric acid is not found naturally on earth.
Sulphuric acid is an extremely critical industrial chemical globally. A nation’s industrial strength is often measured by the production of sulphuric acid in the country.
The Properties Of Sulphuric Acid
H2SO4 is a viscous liquid with a pungent odour. It is either colourless or has a slightly yellow colour.
Physical properties:
Its density is 1.84 g/mL
It has a boiling point of 337 °C, or 611 K, and its melting point is 10 °C. The high boiling point is due to strong hydrogen bonding.
Around 98% or concentrated sulphuric acid is the most stable form of the acid
Chemical properties:
Sulphuric acid is a strong diprotic acid.
It is hygroscopic in nature and readily absorbs moisture present in the air.
It is a strong oxidising and dehydrating agent. This makes it extremely corrosive towards many materials.
When we add water to concentrated (99.8%) sulphuric acid, a very high amount of heat is generated since it is an exothermic reaction. This can often lead to explosions.
How Is Sulphuric Acid Manufactured?
In nature, sulphuric acid is formed by the oxidation of sulphide minerals in rocks. Sulphur dioxide, released in the atmosphere due to the burning of fuels, undergoes oxidation when exposed to moisture, forming dilute sulphuric acid, also known as ‘acid rain’.
Sulphuric acid is commercially manufactured by the Contact Process. We can control several conditions in this process in order to maximise the sulphuric acid yield during manufacturing. The Contact Process involves the following steps:
Production of Sulphur Dioxide
This is done by burning sulphur or sulphide ores, such as iron pyrites in an abundance of air.
S (Sulphur) + O2 (Oxygen) + Δ (Heating) → SO2 (Sulphur dioxide)
4FeS (Iron pyrites) + 7O2 (Oxygen) + Δ (heating) → 2Fe2O3 (Ferric Oxide) + 4SO2(Sulphur dioxide)
Preparing Sulphur Trioxide
Sulphur dioxide is oxidised to sulphur trioxide with oxygen (present in the atmosphere) in the presence of Vanadium Pentoxide (V2O5) as a catalyst.
2SO2 (Sulphur Dioxide) + O2 (Oxygen) + [V2O5 (Catalyst)] → SO3 (Sulphur Trioxide)
Converting Sulphur Trioxide to Sulphuric Acid
The sulphur trioxide is absorbed in concentrated (98%) sulphuric acid to give oleum or pyrosulphuric acid. This oleum is then mixed with water to obtain sulphuric acid of the required concentration.
SO3 (Sulphur Trioxide) + H2SO4 (Sulphuric Acid-98%) → H2S2O7 (Pyrosulphuric acid – Oleum)
H2S2O7 (Pyrosulphuric acid – Oleum) + H2O (Dilution) → 2H2SO4(Sulphuric acid)
How Do We Increase The Sulphuric Acid Yield?
When a chemical reaction occurs, the amount of product formed is called yield. Thus, the sulphuric acid yield is the amount of acid manufactured by the reactions taking place in the contact process.
When manufacturing Sulphuric Acid by the Contact Process, the most critical step is the catalytic oxidation of SO2 to give SO3 in the presence of catalyst Vanadium Pentoxide V2O5.
2SO2 (Sulphur Dioxide) + O2 (Oxygen) + [V2O5 (Catalyst)] → SO3 (Sulphur Trioxide)
This reaction, the generation of SO3 is an exothermic reaction. It is reversible in nature and the forward reaction leads to a reduction in volume since two molecules of SO3 are produced for every three molecules of reactants.
Le Chatelier’s principles help us to understand the conditions required to maximise the production of Sulphur Trioxide and, hence, sulphuric acid yield. These are as below:
Concentration
A high concentration of the reactants helps the forward reaction.
Temperature
Since the forward reaction is exothermic, hence a low temperature helps in promoting the oxidation process. However, an optimum temperature of around 720K must be maintained to maximise the yield.
Pressure
In general, a high pressure promotes oxidation since the volume of the gaseous products is lesser than that of the reactants. However, excessive pressure can lead to corrosion of the vessel in which the oxidation is taking place. Hence, a pressure between 2-3 bar (or atmosphere) is optimal for maximising the production of Sulphur Trioxide.
Catalyst
We know that a catalyst increases the pace of a chemical reaction. Earlier platinised asbestos was used as a catalyst in this process. However, it was not only costly but also lost its efficacy when exposed to the impurities present in the reactant gases. Hence, we now use Vanadium Pentoxide (V2O5), which overcomes these limitations.
Purity of Gases
Gases must be made free of any impurities like dust and poisonous gases like Arsenic Oxide, before they are subjected to the oxidation process.
Uses Of Sulphuric Acid
Sulphuric acid is used in a wide variety of applications. Due to this, it is also referred to as the King of Chemicals.
In several concentrations, sulphuric acid is used in the manufacture of fertilisers, drugs, pigments, and even explosives (like TNT). For instance, lead battery acid is 29-32% concentration, chamber acid is 62-70%, and tower acid is 78-80%.
Some common uses of sulphuric acid are:
It is used in petroleum refining
In preparation of fertilisers like superphosphate and ammonium sulphate
Dyes, shades, and paints
It is used in the formation of many other acids (for example, nitric acid, hydrochloric acid, phosphoric acid) and chemicals (like sodium carbonate).
As pickling agent
Oxidising and dehydrating agents in laboratories
It is used in several metallurgical processes
Conclusion
In this article, we learned about sulphuric acid and its properties such as strong diprotic acid , hygroscopic in nature, strong oxidising and dehydrating agent.We studied the Contact Process used to manufacture sulphuric acid commercially .The essential Conditions for Obtaining maximum Sulphuric Acid yield by contact process are high concentration of reactants ,low temperature ,high pressure etc. The presence of catalysts such as V2O5 is also helpful in maximising yield of sulphuric acid as it accelerates the reaction. Sulphuric acid is used in dyes, shades and acts as oxidising and dehydrating agents.
