Process of Self Ionization of Water and the pH and pOH Scale

The reaction in which two water molecules produce ions is called self-ionization or auto-ionization of water. and derivation of the value of the 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 produces hydronium ions (H3O+) and hydroxyl ions (OH¯) by a transfer of a proton.

H2O + H2O  ⇌ H3O  + OH (ions)

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. The 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 the water ionization constant.)

Water Ionization Constant

In Water, the product of the concentration of [H] and  [OH] ions is always 1 x 10-14 mol-dm-3 at 25°C and is called the 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] ion 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 sides 

√ [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 the strength of acids and bases called the pH and pOH scale. Where p stands for potenz (potential to be), it ranges from zero to 14. The acidity or basicity of an aqueous solution depends 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, the 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 a neutral solution the pH and pOH is equal to 14 

pH = 7 and pOH = 7

pH + pOH = 14

Neutral, Acidic, and Basic Solutions

Neutral Solution

When the concentration of hydrogen ions and hydroxyl ions is equal as in pure water then the solution is called a neutral solution. In a 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 than the concentration of hydroxide ions is called an acidic solution. In an acidic solution, the hydrogen ions are greater than 1 x 10-7 

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