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

Relative Strength Of Acids And Bases

Category : JEE Main & Advanced

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.

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