JEE Main & Advanced Chemistry Equilibrium / साम्यावस्था Relative Strength Of Acids And Bases

In practice \[{{K}_{a}}\] is used to define the strength only of those acids that are weaker than \[{{H}_{3}}{{O}^{+}}\] and \[{{K}_{b}}\] is used to define the strength of only those bases that are weaker than \[O{{H}^{-}}\].  For two weak acids \[H{{A}_{1}}\] and  \[H{{A}_{2}}\] of ionisation constant \[{{K}_{{{a}_{1}}}}\] and \[{{K}_{{{a}_{2}}}}\] respectively at the same concentration \[C\], we have,

\[\frac{\text{Acid strength of }H{{A}_{\text{1}}}}{\text{Acid strength of }H{{A}_{2}}}=\sqrt{\frac{{{K}_{{{a}_{1}}}}}{{{K}_{{{a}_{2}}}}}}\]

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. i.e.,  

\[\frac{\text{Basic strength of }B\text{O}{{\text{H}}_{\text{1}}}}{\text{Basic strength of }B\text{O}{{H}_{2}}}=\sqrt{\frac{{{K}_{{{b}_{1}}}}}{{{K}_{{{b}_{2}}}}}}\]

(1) Relative strength of Inorganic acids

(i) Hydrides

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

\[H-F>H-OH>H-N{{H}_{2}}>H-C{{H}_{3}}\]  \[HCI>{{H}_{2}}S>P{{H}_{3}}>Si{{H}_{4}}\]

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

\[HF<HCl<HBr<HI\]; \[{{H}_{2}}O<{{H}_{2}}S<{{H}_{2}}Se<{{H}_{2}}Te\]

(ii) Oxyacids

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

\[HOI<HOBr<HOCl\];  \[HI{{O}_{4}}<HBr{{O}_{4}}<HCl{{O}_{4}}\]

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

\[\underset{+1}{\mathop{HOCl}}\,<\underset{+3}{\mathop{HCl{{O}_{2}}}}\,<\underset{+5}{\mathop{HCl{{O}_{3}}}}\,<\underset{+7}{\mathop{HCl{{O}_{4}}}}\,\];\[{{H}_{2}}S{{O}_{3}}<{{H}_{2}}S{{O}_{4}}\] \[HN{{O}_{2}}<HN{{O}_{3}}\]

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

\[{{H}_{4}}Si{{O}_{4}}<{{H}_{3}}P{{O}_{4}}<{{H}_{2}}S{{O}_{4}}<HCl{{O}_{4}}\]

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

\[HN{{O}_{3}}>HP{{O}_{3}}\] ; \[{{H}_{3}}P{{O}_{4}}>{{H}_{3}}As{{O}_{4}}\] \[HCl{{O}_{4}}>HBr{{O}_{4}}>HI{{O}_{4}}\]

(2) Relative strength of organic acids

(i) A compound is acidic in nature, if its conjugate base can stabilize through  resonance. Thus phenol is acidic while ethanol is neutral because the conjugate base of phenol \[({{C}_{6}}{{H}_{5}}{{O}^{-}})\] can be stabilized through resonance while that of alcohol \[({{C}_{2}}{{H}_{5}}{{O}^{-}})\] can not.

(ii) Hydrogen atom attached to sp-hybridized carbon is more acidic than that on \[s{{p}^{2}}\] hybridized carbon which in turn is more acidic than that on \[s{{p}^{3}}\] hybridized carbon.

Thus,  \[HC\equiv \underset{sp}{\mathop{CH}}\,>C{{H}_{2}}=\underset{s{{p}^{2}}}{\mathop{C{{H}_{2}}}}\,>C{{H}_{3}}-\underset{s{{p}^{3}}}{\mathop{C{{H}_{3}}}}\,\]

(3) Relative strength of Inorganic bases

(i) The basicity of a compound decreases with increase in electronegativity of the atom holding the electron pair, \[\overset{.\,\,\,.}{\mathop{N}}\,{{H}_{3}}>{{H}_{2}}\overset{.\,\,\,\,.}{\mathop{O}}\,:\ >\ H\underset{.\,\,\,\,.}{\overset{.\,\,\,\,.}{\mathop{F}}}\,:\]

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

\[{{F}^{-}}>C{{l}^{-}}>B{{r}^{-}}>{{I}^{-}}\];  \[{{O}^{2-}}>{{S}^{2-}}\]

(iii) Presence of negative charge on the atom holding the electron pair increases the basicity, while the presence of positive charge on the atom holding the electron pair decreases the basicity. \[O{{H}^{-}}>{{H}_{2}}O>{{H}_{3}}{{O}^{+}}\]

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

\[LiOH<NaOH<KOH<RbOH<CsOH\];

\[Be{{(OH)}_{2}}<Mg{{(OH)}_{2}}<Ca{{(OH)}_{2}}<Sr{{(OH)}_{2}}<Ba{{(OH)}_{2}}\]

\[CsOH\] is the strongest known base

(v) On going down the group; basic nature decreases with size of the central atom due to decrease in the ability to donate the lone pair.  \[N{{H}_{3}}>P{{H}_{3}}>As{{H}_{3}}>Sb{{H}_{3}}>Bi{{H}_{3}}\]

(4) Relative strength of Organic bases

(i) Higher the electron density on nitrogen, more is the basic character of amine.

(ii) A compound is basic in nature, if its conjugate acid can be stabilized through resonance. Thus guanidine \[(N{{H}_{3}}-\overset{N{{H}_{2}}}{\mathop{\overset{|\ \ \ \ \ }{\mathop{C=}}\,}}\,NH)\] is as strong alkali as metal hydroxides because its conjugate acid \[({{H}_{3}}{{N}^{+}}-\overset{N{{H}_{2}}}{\mathop{\overset{|\ \ \ \ \ }{\mathop{C=}}\,}}\,NH)\] is very much stabilised through resonance.