Sulphuric Acid

Inorganic Compound of Class 12

Lead chamber process

In this process sulphuric acid is obtained by the oxidation of SO2 by nitrogen dioxide, NO2. The simplest explanation of the changes taking place is due to Berzelius. According to him sulphur dioxide is first oxidized by nitrogen dioxide to sulphur trioxide which reacts with steam forming sulphuric acid. Nitrogen dioxide itself is reduced to nitric oxide, NO which is subsequently oxidized back to nitrogen dioxide by atmospheric oxygen. The various reactions are represented as follows:

SO2 + NO2 SO3 + NO

SO3 + H2O(steam) H2SO4

2NO + O2(from air) 2NO2

Thus NO and NO2 act as oxygen carriers i.e. they transfer oxygen from the air to sulphur dioxide.

It is, however, observed that if the chambers are insufficiently supplied with steam, lead chamber crystals are formed which consist of nitroso−sulphuric acid, HSO4.NO. The simple theory given by Berzelius as outlined above does not explain the formation of HSO4.NO. Hence it has been suggested by Davy and Lunge that the nitroso acid is formed as an intermediate product during the formation of sulphuric acid as follows:

NO + NO2 N2O3

2SO2 + N2O3 + O2 + H2O 2HSO4.NO

2HSO4.NO + H2O 2H2SO4 + NO + NO2

Here it must be noted that the fact that nitroso−sulphuric acid is formed if insufficient steam is present, does not prove that it is formed as an intermediate product under normal working conditions.

Properties of H2SO4

Sulphuric acid is a colourless syrupy liquid of specific gravity is 1.84 at 15°C. It fumes strongly in moist air and is highly corrosive. It is highly soluble in water. It dissolves in water liberating heat. Great care should be taken when the acid is to be diluted. Sulphuric acid should be added in a thin stream to water and never water to sulphuric acid. H2SO4 is strong dibasic acid and hence gives two types of salts which are called bisulphates (HSO₄^-) and sulphates . These salts are obtained by treating H2SO4 with alkalies.

H2SO acts as a strong oxidizing agent, because it can supply an atom of oxygen.

H2SO4 H2O + SO2 + O

Thus H2SO4 oxidises S, C, P, HI and HBr to SO2, H3PO3, I2 and Br2 respectively. H2SO4 has great affinity for H2O and hence is used as a strong dehydrating agent. Thus when oxalic acid (H2C2O4) is heated with H2SO4, the former loses H2O. Some of the metals (e.g. Cu, Ag, Hg etc) react with H2SO4 in presence of air to form the sulphates while other metals (e.g. Zn, Al, Mg etc) react with the acid directly to give sulphates. Noble metals like Au and Pt do not react with the acid. The acid dissolves sulphur trioxide forming fuming sulphuric acid, also called oleum or pyrosulpuric acid, H2S2O7 which is much more active than sulphuric acid and forms the salts pyrosulphates which are extensively used in the manufacture of aniline dyes and explosives.

Uses of H2SO4

(i) In the manufacture of chemical manures like ammonium sulphate and calcium superphosphate.

(ii) In the manufacture of other acids like nitric acid, hydrochlorid acid, phosphoric
acid etc.

(iii) In refining petroleum.

(iv) In the manufacture of coal−tar dyes and a number of pharmaceuticals.

(v) In the manufacture of explosives.

(vi) In the textile, paper and dyeing industries.

(vii) For cleansing metals before enamelling, electroplating, galvanic−sing etc.(pickling)

(viii) In storage batteries.

(ix) In the manufacture of sulphates, bisulphates and sodium carbonate.

(x) In the laboratory as an important reagent and as a drying and dehydrating agent.

Structure of H2SO4 molecule and ion

The structure of H2SO4 molecule is as shown below:

This structure is confirmed by the following facts:

(i) Sulphuric acid (H2SO4) contains two hydroxyl (OH) groups because it reacts with PCl5 to give sulphuryl chloride (SO2Cl2) which in turn is hydrolysed by water to sulphuric acid. The structure of sulpuryl chloride is Cl2SO2 which is confirmed by the fact that SO2Cl2 can be obtained by the direct combination of SO2 and Cl2.

(HO)2SO2 + 2PCl5 Cl2SO2 + 2POCl3+ 2HCl

Sulphuric Sulphuryl

acid chloride

Cl2SO2 + 2HOH (OH)2SO2 + 2HCl

SO2+ Cl2 Cl2SO2

(ii) The two hydroxyl groups are symmetrically arranged in H2SO4 molecule because we get only one type of organic derivative like diethyl sulphone, C2H5SO2 −C2H5. Both OH groups are directly linked to S−atom as is evident from the formation of diethyl sulphone, (C2H5)2SO2 by different routs:

(C2H5)2S (C2H5)2SO2 Cl2SO2 H2SO4

Diethyl

sulphone

(iii) When we consider the S−O bond lengths in H2SO4 molecule, we are suggested to believe that O−atoms are attached with S−atom by double bond.

X−ray analysis of the ionic crystals containing ions has shown that ion has tetrahedral shape. This shape results from sp3 hybridisation of S−atom (central atom). Both S−O π bonds are d(S)−p(O) π bonds.