Discuss physical and chemical properties of sulphuric acid.

Explanation

i) Physical properties : Sulphuric acid is a colourless, dense, oily liquid with a specific gravity of 1.84 at 298 K.
The freezing point is 283K and boiling point is 61 IK. It dissolves in water with evolution of large quantity of heat. Hence, while diluting an acid, the acid must be added to water and not water to an acid with a constant stirring.
(ii) Chemical properties : The chemical reactions of sulphuric acid are as a result of the following characteristics :
(a) Low volatility
(b) Strong acidic character
(c) Strong affinity for water
(d) Ability to act as an oxidising agent
In aqueous medium, sulphuric acid ionises in two steps.
`H_(2)SO_(4)(aq) + H_(2)O(l) to H_(3)O_(aq)^(+) + HSO_(4)^(-) (aq), Ka_(1)`=very large `(Ka_(1) gt 10)`
`HSO_(aq)^(-) + H_(2)O(l) to H_(3)O_(aq)^(+) + SO_(4)^(2-) (aq), Ka_(2) =1.2 xx 10^(-2)`
The larger value of `Ka_1(Ka_1 gt 10`) means that `H_2SO_4` is largely dissociated into `H^+` and `HSO_4`. Greater the value of dissociation constant `(K_a)` the stronger is the acid.
The acid forms two series of salts : Normal salts such as sodium sulphate, copper sulphate etc. and acid sulphates (e.g., sodium hydrogen sulphate).
Sulphuric acid, because of its low volatility can be used to manufacture more volatile acids from their corresponding salts.
`2MX + H_(2)SO_(4) to 2HX + M_(2)SO_(4) (X =F, Cl, NO_(3))`
(M = Metal)
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 compounds, it is evident by its charring action on carbohydrates.
`C_(12)H_(22)O_(11) overset(H_(2)SO_(4)) to 11H_(2)O + 12C`
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 oxidized by concentrated sulphuric acid, which is reduced to `SO_2`.
`Cu + underset("conc") (2H_(2)SO_(4)) to CuSO_(4) + SO_(2) + 2H_(2)O`
`C + underset("conc")(2H_(2)SO_(4)) to CO_(2) + 2SO_(2) + 2H_(2)O`