Stability of water, dissociation, electrolysis
Water displays a versatile and unique behaviour. It behaves as an acid, as a base, an oxidant, a reductant and as ligand to metals. But water is a very stable compound and does not dissociate easily.
Why water is so stable?
Water is very stable compound. It does not dissociate into its elements (to H and O) even at high temperatures. This is due to its high negative enthalpy of formation. (Hf = -285.9 kJmol-1). At about 1500 K (12270C), its decomposition to elements is only 2%.
How to decompose water into oxygen and hydrogen?
But, when an electric current is passed water decomposes.
Electrolysis of water
But it is occurred in very slowly. If in the presence of a small amount of an acid or alkali or salt, water decompose to hydrogen and oxygen in good efficiency.
Hydrogen gas is formed at cathode and oxygen is formed at anode.
Electrolysis of water is a redox reaction.
Redox reaction of water electrolysis
Redox reaction is a oxidation - reduction reaction. These oxidation, reduction reaction should be occurred in same time.
In water electrolysis,
- O2 is given by oxidizing reaction.
- H2 is given by reducing reaction