Dissociation Constant and Ionic Product of Water

Pure water is poor conductor of electricity.

Water is a weak electrolyte i.e. it is ionized to a very small extent as:

H₂O  H⁺ + OH‾

H₂O + H₂O  H₃O⁺ + OH‾

This ionization is called self ionization of water.

where K_eq is the dissociation constant of water.

K_w is the ionic product of water.

Ionic product of water may be defined as the product of the molar concentration of H⁺ ions and OH‾ ions.

As H⁺ ions in water exist as H₃O⁺ ions, therefore, ionic product may also be defined as the product of molar concentration of H₃O⁺ ions and OH‾ ions, i.e.

OR

 

Ionic product of water is constant only at constant temperature.

K_w at 298 K

Kw = 1.00 × 10^-14

Dissociation or ionization of constant of water is different from ionic product of water.

K_eq = K_w / 55.55

K_w = K_eq × 55.55

K_eq = 10^-14 /55.55 = 1.8 × 10^-16

In pure water

Effect of temperature on K_w

The ionic product of water increases with increase of temperature.

With increase of temperature ,the degree of ionization of water increases. More of water molecule dissociate into H⁺ ions and OH‾ ions.The concentration of H⁺ and OH‾ ions increases and hence the ionic product also increases.

H₃O⁺ ion and OH‾ ion concentration in aqueous solution of acid and bases

If some acid is added to pure water, then ( H₃O⁺ ) > 10^-7M

 = K_w / 

If some base is added to pure water, then   = 10^-7 M

 = K_w / 

The increase or decrease of the H₃O⁺ ion concentration in an aqueous solution of an acid or a base may be explained qualitatively on the basis of Le Chatelier’s principle.

2H₂O H₃O⁺ + OH‾

If some acid is added to pure water, H₃O⁺ ion concentration increases , therefore the equilibrium shifts in the backward direction.Thus OH‾ ion concentration decreases.

If a base added, OH‾ concentration increases.Again the equilibrium shifts in the backward direction and hence the H₃O⁺  ion concentration decreases.