Ionic Product of Water:

Pure water is a very weak electrolyte and ionises according to the equation

H2O ↔  H+ + OH-

Applying law of mass action at equilibrium, the value of dissociation constant, K comes to

         K = [H+] [OH-]/[H2O]

        or   [H+][OH-] = K[H20]

Since dissociation takes place to a very small extent, the concentration of undissociated water molecules, [H20], may be regarded as constant. Thus, the product #[H20] gives another constant which is designated as Kw. So,

[H+][OH-] = Kw

The constant, Kw, is termed as ionic product of water.

The product of concentrations of H1 and OH ions in water at a particular temperature is known as ionic product of water. The value of Kw increases with the increase of temperature, i.e., the concentration of H+ and OH- ions increases with increase in temperature.

Temperature (°C)  Value of K

0                0.11 x 10-14

10              0.31 x 10-14

 25             1.00 x 10-14

 100           7.50 x 10-14

 The value of Kw at 25°C is 1 x 10-14. Since pure water is neutral in nature, H+ ion concentration must be equal to OH- ion concentration.

 

[H+] = [OH˜] = x

or    [H+][OH-]=x2= 1 x 10-14

or    x = 1 x 10-7 M

or    [H+] = [OH-] = 1 × 10-7 mol litre-1

This shows that at 25°C, in 1 litre only 10-7 mole of water is in ionic form out of a total of approximately 55.5 moles.

When an acid or a base is added to water, the ionic concentration product, [H+][OH-], remains constant, i.e., equal to Kw but concentrations of H+ and OH- ions do not remain equal. The addition of acid increases the hydrogen ion concen­tration while that of hydroxyl ion concentration decreases, i.e.,

[H+] > [OH-];        (Acidic solution)

Similarly, when a base is added, the OH- ion concentration increases while H+ ion concentration decreases,

i.e.,                          [OH-] > [H+]; (Alkaline or basic solution)

In neutral solution,      [H+] = [OH-] = 1 x 10-7 M

In acidic solution,        [H+] > [OH-]

or                             [H+] > 1 x 10-7 M

and                            [OH-] < 1 x 10-7 M

In alkaline solution,      [OH-] > [H+]

 or                             [OH-] > 1 × 10-7 M

and                         [H+] < 1 x 10-7 M

 

Thus, if the hydrogen ion concentration is more than 1 x 10-7 M, the solution will be acidic in nature and if less than 1 x 10-7 M, the solution will be alkaline.

[H+]  = 10-0  10-1 10-2 10-3  10-4 10-5 10˜6            (Acidic)

[H+]  =         10-7                                              (Neutral)

[H+]  = 10-14 10-13 10-12 10-11 10-10 10-9 10-8        (Alkaline)

 

We shall have the following table if OH- ion concentration is taken into account.

[OH-] = 10-14   10-13 10-12 10-11 10-10  10-9 10-8    (Acidic)

[OH-] =            10-7                                         (Neutral)

[OH-] = 10-0 10-1 10-2 10-3 10-4 10-5 10-6            (Alkaline)

 

It is, thus, concluded that every aqueous solution, whether acidic, neutral or alkaline contains both H+ and OH- ions. The product of their concentrations is always constant, equal to 1 × 10-14 at 25°C. If one increases, the other decrease accordingly so that the product remains 1×10-14 at 25C.

If [H+] = 10-2 M, then [OH-] = 10-12 M; the product, [H+][OH-] = 10-2 × 10-12 = 10-14; the solution is acidic.

If [H+] = 10-10 M, then [OH-] = 10-4 M; the product, [H+][OH-] = 10-10 × 10-4 = 10-14; the solution is alkaline.

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