What happens when acid reacts with bleaching powder?

Answer 1

A potentially dangerous evolution of chlorine gas.

The mechanism is complex. Bleaching powder is a hypochlorite salt containing OCl- ion.

In acid solution the weak acid HOCl hypochlorous acid is formed.

The following reaction is slow but effective:-

2HOCl -> H+(aq) + Cl- + HClO2 (formation of chlorous acid)

HOCl + ClO2- -> ClO3- +H+ + Cl-

This creation of Cl- is key

There is an equilibrium HOCl + OH3+ + Cl- <-> Cl2 + 2H2O (this occurs when Cl2 dissolves in water) Adding acid to hypochlorite sets this up as shown above.

Excess acid (low pH) drives this equilibrium to the right (Le Chateliers principle) and chlorine gas is evolved first traces are dissolved in water but rapidly it comes out of solution further displacing the equilibrium.