Self ionization of water or auto-ionization

 Self ionization of water or auto-ionization of Water

The in which two water molecules produce ions is called as self ionization or auto-ionization of water. This reaction  can be written as follows.

         H2O   ----------> H+,      +      OH-

 Explanation:

When molecules colloid with each other with enough energies, a protons is transferred from one molecule to an other molecule.

A water molecule that donates or loses proton becomes negatively charged and other water molecule which gains or accepts proton becomes positively charged hydronium ion H3O+.

Equilibrium constant expression for self ionization of water can be written as follows. 

          K_c =    [H+][OH-] / [H2O]

Since, H2O is a weak electrolyte, so concentration of [H2O] remains constant. 

         K_c  [H2O] = [H+][OH-] 

         K_w =  [H+][OH-]    •••••••••••    1

     Where K_w = [H⁺][OH^-]  is called ionization constant of water. It is also called ionic product of water. For example, 

At 25oc, experimentally determined concentrations H+ ions and OH- ions are equal and given as follows. 

[H+] = [OH,] = 1 × 10^-7

Equation 1 can be written as follows 

   K_w =  (1 × 10^-7) (1 × 10^-7 )      ^{at 25 oC.}

   K_w =  1   × 10^-14     

^{PH: pH may be defined as the negative logarithm of the concentration of H+ ions in aqueous solutions.}

         ^{Mathmatically,} 

                        PH= -log [H⁺]   

For pure water at 25oC 

              [H⁺] = [OH^-]  = 1 × 10^-7  

                  PH= -log (1 × 10^-7)   

                    PH= 7

Thus pH of water is 7.

All aqueous solutions with pH =7 at 25oC are neutral. If pH is less than 7  the solution becomes acidic, as [H+] increases and [OH-] decreases.

^{Question: What is the importance of KW?} 

^Answer.

^{KW is temperature dependent. In any aqueous solution at 25 o C,  no matter what does it contain,  the product of H+ concentration  and OH- ion  concentration is always equal to 1× 10-14.}

 This means that if H ion increases and OH- must decreases so that the product of two is always 1 × 10-14.  

When  

              [H+] = [OH-] = 1 × 10-7, solution is a neutral 

              [H+] >  1 × 10-7, solution is an acidic 

               [H+] < 1 × 10-7, solution is a basic

         If pH is greater than 7  the solution is a basic. As solution becomes basic, [OH-] increases and [H+] decreases.

If pH is lesser than 7 the solution is a acidic. As solution becomes acidic, [H+] increases and [OH-] decreases.

Example: classifying a solution as a neutral acidic or basic

1. Unrefined hydrochloric acid is used to clean stone building and swimming pools. If the [H+] in the solution of HCL is 1×10-6 M.  Is solution acidic, basic or neutral?

2. Sodium hydroxide is commonly used as a drain cleaner. if the concentration of OH-is 1.0 ×10-5 M. Is the solution acidic, basic or neutral?