Mada za sehemu hiiNon Metals And Their CompoundsMada 10
Sulphuric acid is one of the most essential chemicals to mankind. It is an important reagent in laboratories and industries.
Sulphuric acid is manufactured by the contact process. There are four stages in the process.
Production of sulphur dioxide
Sulphur dioxide is produced by burning or roasting a sulphide ore in an air.
The sulphur dioxide used in the Contact Process can be obtained from different sources. These include:
(a) Burning sulphur in air:
This is the most convenient method of producing sulphur dioxide.
(b) Burning sulphide ores such as iron pyrite () and zinc blend (ZnS):
Sulphur dioxide gas is produced as a by-product.
Purification of sulphur dioxide
A mixture of sulphur dioxide and air is passed through a purifier. This removes impurities such as dust and arsenic compounds which may poison the catalyst in the later stages. The mixture is then passed through a tower containing concentrated sulphuric acid in order to dry.
Catalytic conversion of sulphur dioxide to sulphur trioxide
The dry mixture of sulphur dioxide and air is passed through a converter containing a catalyst vanadium pentoxide, at a temperature of about 450 °C. sulphur trioxide is formed.
Conversion of sulphur trioxide to sulphuric acid
The sulphur trioxide is passed through an absorption tower and dissolve in a concentrated sulphuric acid to form an oily solution, oleum
Oleum is then carefully diluted with the correct amount of water to give ordinary concentrated sulphuric acid:
Sulphur dioxide cannot be dissolved directly in water to form sulphuric acid. The reaction is so highly exothermic that the heat produced vapourizes the sulphuric acid formed. This makes it difficult to collect the gas because the acid vapour (mist) produced is very stable and cannot be condensed.
The flow diagram for the manufacture of sulphuric acid is shown in the figure below
Dilute sulphuric acid reacts with metals, metal oxides, metal hydroxides and metal carbonates and hydrogen carbonates to produce salts.
Reaction with metals
Dilute sulphuric acid reacts with many metals above hydrogen in the activity series to form sulphates and hydrogen gas, e.g.
Reaction with metal oxides
Reactions of metal oxides with dilute sulphuric acid are neutralization reactions. Metal oxides react with dilute sulphuric acid to form a salt (sulphate) and water, e.g.
Reaction with metals hydroxides
The reaction between dilute sulphuric and a metal hydroxide is a neutralization reaction. Metal hydroxides react with dilute sulphuric acid to form a sulphate and water, e.g.
Reaction with metal carbonates and hydrogen carbonates
Dilute sulphuric acid reacts with metal carbonates and hydrogen carbonates to give metal sulphates, carbon dioxide and water, e.g.
Concentrated sulphuric acid as a dehydrating agent
As a dehydrating agent, it will remove the elements of water (hydrogen and oxygen) from a compound to form a new compound. It will dehydrate sugar, paper and wood. These are all made of carbon, hydrogen and oxygen. The acid removes the hydrogen and oxygen as water, leaving carbon behind.
When concentrated sulphuric acid is added to sugar, a vigorous reaction occurs, causing the reaction mixture to rise and fill the beaker.
The colour of the sugar changes to brown and finally black. Concentrated sulphuric acid dehydrates sugar (glucose) by taking away the elements of water (hydrogen and oxygen) from the sugar, leaving carbon.
The acid will also dehydrate sucrose to form carbon.
The final product is a black mass of carbon. The reaction is highly exothermic. The heat produced evaporates the water formed from the reaction.
The acid also dehydrates some hydrated salts. When concentrated sulphuric acid is added to hydrated blue copper (II) sulphate crystals, the colour changes from blue to white. The acid dehydrates the hydrated copper (II) sulphate crystals to anhydrous copper (II) sulphate powder:
Concentrated sulphuric acid also dehydrates methanoic acid and ethanol to give carbon monoxide and ethene gases respectively:
Concentrated sulphuric acid as a drying agent
As a drying agent, concentrated sulphuric acid absorbs traces of water from substances. Because of its ability to absorb water, it is used for drying most gases prepared in the laboratory that it would not react with. It cannot be used for drying ammonia, carbon dioxide, hydrogen sulphide or any gas with which it reacts.
Sulphuric acid as an oxidizing agent
Hot concentrated sulphuric acid is a strong oxidizing agent. It oxidizes both metals and non-metals while it is reduced to sulphur dioxide.
Concentrated sulphuric acid oxidizes charcoal (carbon) to carbon dioxide, sulphur to sulphur dioxide and copper to copper (II) sulphate.
With sulphur and copper, the orange colour of the dichromate (VI) paper changes to green. This confirms the presence of sulphur dioxide gas.
With carbon, a white precipitate is formed on the glass rod when the rod dipped in lime water (calcium hydroxide) is placed in the mouth of the test tube. This confirms the presence of carbon dioxide gas which reacts with the calcium hydroxide on the glass rod to produce a white precipitate of calcium carbonate.
- Dilute sulphuric acid is used to remove rust and scales from metals this is known as pickling.
- It is used in the manufacture of paints and pigments
- It is used as a catalyst in refining petroleum
- It is used in the manufacture of soaps and detergents
- It is used in lead-acid car batteries (accumulators)
- It is used in the manufacture of aluminium sulphate which is used in water treatment plants
- It is used in the manufacture of fertilizer such as ammonium sulphate
- It is used in the purification of gold
- It is used as a dehydrating agent in the laboratory
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