Relative strength of acids and Bases

In practice K_a is used to define the strength only of those acids that are weaker than H₃O⁺ and K_b  is used to define the strength of only those bases that are weaker than OH^-.  For two weak acids HA₁ and HA₂ of ionisation constant K_a1 and K_a2 respectively at the same concentration then we can say that

Similarly, relative strengths of any two weak bases at the same concentration are given by the ratio of the square-roots of their dissociation constants which means

(1) The relative strength of the Inorganic acids

(i) Hydrides

(a) The acidic strength increases with increase in the electronegativity of the element directly attached with hydrogen.

H-F>H-OH>H-NH₂>H-CH₃>HCl>H₂S>PH₃>SiH₄

(b) The acidic strength increases with increase in atomic size,

HF<HCl<HBr<HI;H₂O<H₂S<H₂Se<H₂Te

(ii) Oxyacids

(a) Among oxyacids of the same type formed by the different elements, acidic nature increases with the increasing electronegativity,

HOI < HOBr < HOCl; HIO₄ < HBrO₄ < HClO₄

    (b) In the oxyacids of same element, acidic nature increases with the increase in its oxidation number

(c) The strength of the oxyacids increases from left to right across the period

(d) For the identical oxidation state and configuration of the elements, acid strength decreases with the increase in size of atom.

(2) Relative strength of organic acids

(i) The compound is said to be acidic in nature, if the conjugate base of it can stabilize through the process of resonance. Therefore phenol is acidic while ethanol is neutral because the conjugate base of phenol (C₆H₅O^-) can be stabilized through the resonance while that of alcohol (C₆H₅O^-) cannot.

(ii) Hydrogen atom attached to sp-hybridized carbon is more acidic than that on sp² hybridized carbon which in turn is more acidic than that on sp³ hybridized carbon.

Thus, 

(3) The Relative strength of the Inorganic bases

(i) The basicity of a compound decreases with increase in electronegativity of the atom holding the electron pair, 

(ii) The larger size of the atom holding unshared electrons, the lesser will be the availability of electrons.

(iii) Presence of negative charge on the atom holding the electron pair increases basicity, while presence of positive charge on the atom holding the electron pair decreases the basicity. OH^- > H₂O > H₃O⁺

(iv) Among the alkali and alkaline earth hydroxides (oxides) basic nature increases with the electropositivity

CsOH is the strongest known base

(v) On moving down the group; the basic nature decreases with the size of the central atom due to decrease in ability to donate the lone pair. 

 

         (4) Relative strength of Organic bases

    (i) The higher electron density on nitrogen, more will be the basic character of amine.

(ii) A compound is basic in the nature, if the conjugate acid of it can be stabilized through the resonance. Hence guanidine   is as strong alkali as metal hydroxides because its conjugate acid   is very much stabilised through resonance.

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