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Self Ionization of Water

Self Ionization of Water

The reaction in which two water molecules produce ions is called self-ionization or auto ionization of water. and derivation of the value of water ionization constant.

Process of Self ionization of water

Water undergoes self-ionization to a very small extent. Itself ionization of water two water molecules produce hydronium ions (H3O+) and hydroxyl ion (OH¯) by a transfer of a proton.

H2O + H2O  ⇌ H3O  + OH (ions)

Self-ionization of water

It has been found that at 25°C Water molecules are much more stable and only one molecule of H2O ionizes into (H+) and (OH¯) ions. Equilibrium constant expression for the above reaction can be written as

Kc = [H] [OH] / [H2O]  or Kc . [H2O] = [H] [OH]   Where [H] [OH] = Kw

Kc. [H2O] = Kw  (Where Kw is called water ionization constant.)

Water Ionization Constant

In Water, the product of concentration of [H] and  [OH] ions is always 1 x 10-14 mol-dm-3 at 25°C and is called water ionization constant (Kw). It expressed as 

Kw = [H] [OH] = 1 x 10-14 mol-dm-3

Derivation of Value of Water ionization constant(Kw)

As we know one molecule of water produces one [H] and one [OH] ions a dissociation therefore we can say that.

         [H] [OH] 

        [H]  = [OH] 

 or      [H] [H]  =  1 x 10-14

          [H]² = 1 x 10-14

Taking square root on both side 

[H]²   =   √ 1 x 10-14

  [H]     =  1 x 10-7

and [OH] = 1 x 10-7

In water at 25°C [H] = 1 x 10-7and [OH] =1 x 10-7

So Kw = [H] [OH]

Kw = [1 x 10-7] [1 x 10-7]

Kw = 1 x 10-14

pH and pOH Scale

In 1909 Sorenson proposed a scale for the measurement of strength of acid and bases called pH and pOH scale. Where p stand for potenz (potential to be), it range from zero to 14. The acidity or basicity of an aqueous solution depend on the relative number of hydrogen ions and hydroxide ions present in it.  

pH:- The negative logarithm of hydrogen ions, concentration is called pH mathematically it can be written as pH = -log[H] ions. According to this scale pH of water can be calculated as pH = -log[H]

Putting the value of [H] ion

pH = -log[1 x 10-7]

pH = -(-7)log10     where log10=1

pH = 7.0

pOH:- The negative logarithm of hydroxyl ions, concentration is called pOH mathematically it can be written as pOH = -log[OH¯] ions. According to this scale pOH of water can be calculated as pOH = -log[OH]

Putting the value of [OH¯] ions

pOH = -log[1 x 10-7]

pOH = -(-7)log10     where log10=1

pOH = 7.0

For neutral solution the pH and pOH is equal to 14 

pH = 7 and pOH = 7

pH + pOH = 14

Difference between Neutral, Acidic and Basic Solution

Neutral Solution:- When the concentration of hydrogen ions and hydroxyl ions is equal as in pure water then solution is called neutral solution. In neutral solution the concentration of Hydrogen ions and hydroxide ions is equal to (1 x 10-7) each at 25°C [H] [OH] = [1 x 10-7].

Acidic Solution:- The solution in which the concentration of hydrogen ions is greater then the concentration of hydroxide ions is called acidic solution. In acidic solution the hydrogen ions is greater than 1 x 10-7 

Difference between Neutral, Acidic and Basic Solution


Basic Solution:- The solution in which the concentration of hydroxide ions is greater than the concentration of hydrogen ions is called a basic solution. In basic solution, the hydroxide ions are greater than 1 x 10-7

Neutralization Reaction.

The chemical reaction in which acids and bases react to form salt and water is called a neutralization reaction.

Example for neutralization reaction.

HCl + NaOH  ⟶ NaCl + H2O

H2SO4 + KOH  ⟶ K2SO4 + H2O.

>Acids, Bases and Salt class notes, Type, Properties, and Concept 

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