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8.CHEMICAL EQUILIBRIUM eLearn.Punjab

pKa = -logKa

pKb = -logKb

Larger the pKa, weaker is the acid and vice versa. Similarly, larger the pKb, weaker is the base.
If the difference of pKa values of the two acids is one, then acid with smaller pKa is ten times stron-
ger than the other. If the difference is two, then one is hundred times stronger than the other.

8.6.0 LOWRY BRONSTED ACID AND BASE CONCEPT

According to this concept, acids are those species which donate the proton or have a tenden-

cy to donate and bases are those species which accept the proton or have a tendency to accept the

proton.

Whenever, a weak acid or a weak base is dissolved in water, the conjugate acid base pair is

produced. There is a close relationship between Ka of the acid, Kb of the conjugate base and Kw of
water. Let us have an acid HA, and it gives protons to water in a reversible manner. H3O+ gives pro-
ton to A- and is an acid, but A- accepts H+ from H3O+ and act as a conjugate base of HA.

HA + H2O  H3O+ + A-

acid base conjugate acid conjugate base

of H2O of HA

Now,

Kc = [H3O+ ][A- ] or Ka = [H+ ][A- ]
[H2O][HA] [HA]

In case A- is dissolved in water, the equation for hydrolysis of conjugate base A- will be,

A- + H2O  HA + OH-

base acid acid base

So, its

[HA][OH - ]
[A- ]
Kb =

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