Sulphuric Acid (H₂SO₄)

Sulphuric acid is one of the most important industrial chemicals.

It is manufactured by lead chamber process or contact process.

Contact process

Contact process involves three steps:

(i) Burning of sulphur or sulphur ores in air to generate SO₂.

(ii) Conversion of SO₂ to SO₃ by the reaction with oxygen in the presence of a catalyst V₂O₅).

SO₂ (g) + O₂ (g)  2SO₃ (g), Δ_rH^o = -196.6 kJmol^-1

(iii) Absorption of SO₃ in H₂SO₄ to give oleum (H₂S₃O₇) which upon hydrolysis gives H₂SO₄.

Physical properties

Sulphuric acid is a colourless, dense, oily liquid with a specific gravity of 1.84 at 298 K. The acid freezes at 283 K and boils at 611 K.

It dissolves in water with the evolution of a large quantity of heat. Hence, care must be taken while preparing sulphuric acid solution from concentrated sulphuric acid. The concentrated acid must be added slowly into water with constant stirring.

Chemical properties

The chemical reactions of sulphuric acid are as a result of the following characteristics:

i. low volatility

ii. strong acidic character

iii. strong affinity for water and

iv. ability to act as an oxidising agent.

· In aqueous solution, sulphuric acid ionises in two steps.

H₂SO₄ (aq) + H₂O (l) → H₃O⁺ (aq) + HSO₄^– (aq); K_a1 = very large (>10)

HSO₄^– (aq) + H₂O (l) → H₃O⁺ (aq) + SO₄^2- (aq); K_a2 = 1.2 × 10^–2

The larger value of K_a1 means that H₂SO₄ is largely dissociated into H⁺ and HSO₄^–. Greater the value of dissociation constant (K_a), the stronger is the acid.

· Because of its low volatility, sulphuric acid can be used to manufacture more volatile acids from their corresponding salts.

· The acid forms two series of salts: normal sulphates (such as sodium sulphate and copper sulphate) and acid sulphates (e.g., sodium hydrogen sulphate).

MX + H₂SO₄ → HX + MHSO₄

2MX + H₂SO₄ → 2HX + M₂SO₄

· Concentrated H₂SO₄ is a strong dehydrating agent.

C₁₂H₂₂O₁₁     conc.  H2SO4     12C + 11H₂O

The burning sensation of concentrated H₂SO₄ on skin is due to the dehydrating reaction.

· Hot concentrated sulphuric acid is a moderately strong oxidising agent. It is intermediate between phosphoric acid and nitric acid.

Cu + 2H₂SO₄(conc.) → CuSO₄ + SO₂ + 2H₂O

S + 2H₂SO₄(conc.) → 3SO₂ + 2H₂O

C + 2H₂SO₄(conc.) → CO₂ + 2SO₂ + 2H₂O

Uses of sulphuric acid

Ø The bulk of sulphuric acid produced is used in the manufacture of fertilizers (e.g., ammonium sulphate, superphosphate).

Other uses,

i. petroleum refining

ii. manufacture of pigments, paints and dyestuff intermediates

iii. detergent industry

iv. metallurgical applications (e.g., cleansing metals before enameling, electroplating and galvanizing)

v. storage batteries

vi. in the manufacture of nitrocellulose products.

vii. as a laboratory reagent