17. Sulphuric acid

SULPHURIC ACID (H₂SO₄)

Manufacture :

Sulphuric acid is manufactured by the contact process which involves three steps :

(i) burning of sulphur or sulphide ores in air to generate SO₂

(ii) conversion of SO₂ to SO₃ by the reaction with oxygen in the presence of a catalyst (V₂O₅), and

(iii) absorption of SO₂ in H₂SO₄ to give Oleum (H₂S₂O₇)

The SO₂ produced is profiled by removing dust and other impurities such as arsenic compounds.

The key step in the manufacture of H₂SO₄ is the catalytic oxidation of SO₂ with O₂ to give SO₃ in the presence of V₂O₅ (catalyst).

2SO₂(g) +  O₂(g)  2SO₃(g) DrH = – 196.6 kJ mol^–1.

The reaction is exothermic reversible and the forward reaction leads to a decrease in volume. Therefore, low temperature and high pressure are the favourable conditions for maximum yield. But the temperature should not be very low other wise rate of reaction will become slow.

In practice the plant is operated at a pressure of 2 bar and a temperature of 720 K. The SO₃ gas from the catalytic converter is absorbed in concentrated H₂SO₄ to produce oleum. Dilution of oleum with water gives H₂SO₄ of the desired concentration. In the industry two steps are carried out simultaneously to make the process a continuous one and also to reduce the cost.

SO₃ +  H₂SO₄  ®    H₂S₂O₇ 

                                   (Oleum)

The sulphuric acid obtained by Contact process is 96-98% pure.

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 larger quantity of heat. The chemical reaction of sulphuric acid are as a result of the following characteristics : (a) low volatility (b) strong acidic character (c) strong affinity for water and (d) ability to act as an oxidising agent in aqueous solution,

(i) Sulphuric acid ionises in two steps.

H₂SO₄(aq)  + H₂O(l) ®  H₃O⁺ (aq) + HSO₄^– (aq)

Ka₁ = very larger (Ka₁ > 10)

H₂SO₄^– (aq)  + H2O(l) ® H₃O⁺ (aq) + SO₄^2– (aq)

Ka₂ = 1.2 × 10^–2 

The larger value of Ka₁ (Ka₁ > 10) means that H₂SO₄ is largely dissociated into H+ and HSO₄^–. Greater the value of dissociation constant (K_a) the stronger is the acid.

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

(b) Decomposes carbonates and bicarbonates in to C.

Na₂CO₃ +  H₂SO₄  ®  Na₂SO₄ +  H₂O +  CO₂

NaHCO₃ + H₂SO₄ ®  NaHSO₄ +  H₂O + CO₂

(c) Sulphuric acid, because of its low volatility can be used to manufacture more volatile acid from their corresponding salts.

2MX +  H₂SO₄ ® 2HX +  M₂SO₄ (X = F, Cl, NO₃

NaCl + H₂SO₄ ®  NaHSO₄ + HCl

                                 (M = Metal)

(ii) Concentrated sulphuric acid is a strong dehydrating agent. Many wet gases can be dried by passing them through sulphuric acid, provided the gases do not react with the acid. Sulphuric acid removes water from organic compound; it is evident by its charring action on carbohydrates.

C₁₂H₂₂O₁₁  12C + 11H₂O

H₂C₂O₄CO + CO₂

(iii) Hot concentrated sulphuric acid is moderately strong oxidising agent. In this respect it is intermediate between phosphoric and nitric acids. Both metals and non-metals are oxidised by concentrated sulphuric acid, which is reduced to SO₂.

Cu + 2H₂SO₄ (concentrated)  ® CuSO₄ + 2H₂O

3S  + 2H₂SO₄ (concentrated)  ® 3SO₂ + 2H₂O

C  + 2H₂SO₄ (concentrated)  ® CO₂ + 2SO₂ + 2H₂O

Uses :

Sulphuric acid is a very important industrial chemical. A nation’s industrial strength can be judged by the quantity of sulphuric acid it produces and consumes .It is needed for the manufacture of hundreds of other compounds also in many industrial processes .The bulk of sulphuric acid produced is used in the manufacture of fertilisers (e.g., ammonium sulphate, superphosphate). Other uses are in : (i) petroleum refining (ii) manufacture of pigment, paints and dyestuff intermediates (iii) detergent industry (iv) metallurgical applications (e.g., cleansing metal before enameling, electroplating and galvanising) (v) storage batteries (vi) in the manufacture of nitrocellulose products and (vii) as a laboratory reagent.