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The relative strength of acids and bases
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It depends upon their degree of dissociation which in turn is given by a = Ö(Ka/C). For two acids of equimolar concentrations,
(Strength of acid HA1 )/( Strength of acid HA2) = Ö(Ka1/Ka2)
similarly,
(Strength of base (BOH)1) /(Strength of (BOH)2) = Ö(Kb1/Kb2)
(1) From Bronsted-Lowry concept, it can be proved that a strong acid has a weak conjugate base and vice versa.
(2) A few common conjugate acid-base pairs arranged in order of strength are given below
HClO4 > H2 SO4 > HI > HBr > HCl > HNO3 > H3 O+ > HSO- 4 > H3 PO4 > HNO2 > CH3 COOH > H2 CO3 > H2 S > NH+4 > HCN > H2O > NH3
Conjugate base –
Cl-4 < HSO-4 < I- < Br- < Cl- < NO-3 < H2O < SO2-4 < H2PO-4 < NO-2 < CH3 COO-
< HCO-3 < HS- < NH3 < CN- < NH-2
Note : q The strength of acid depends upon the nature of solvent for example,
HCl, HNO3 , H2 SO4 and HClO4have same strength in water. It is due to levelling effect of water. In acetic acid solvent the strength of above acids follow the order : HClO4 > H2 So4 > HNO3
q Acetic acid behaves as a strong acid in ammonia and as a base in anhydrous HF.
q All Bronsted bases are also lewis bases but all bronsted acids are not lewis acids.
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