brnst-lowry theory of acids and base - Shailendra Kumar Chemistry

BRNST-LOWRY THEORY OF ACIDS AND BASEAcids and Bases are substance which upset the k wbalance of [H + ] and [OH – ] in water.k w= [H 3O] [OH – ] = [H + ] [OH – ]Acids will increses the amount of H + present, while bases increses the amount of OH – present. As one ofthe two values [H + ] and [OH – ] changes in one directio, the other will change in the opposite direction, so asto presnrve the value of the constant k w.This theory definds an acid is a substance that will donate a proton tp another substance and Base is asubstance that will accept a proton from other substance Hence Acids is proton Donar and Base is protonacceptor.HA + H 2O H 3O + + A –Note:- That in the forward direction of this reaction HA is an acid because it donates a proton to H 2O, andH 2O behaves a Base because it accept proton.In the reverce direction A – behaver as Base and H 3O + behaves as acid H 3O + is called acid of H 2O, whileA – is called conjugate Base of HAB + H 2O HB + + OH –In forward direction B behaver are base and H 2O behaves are acid.In reverse direction HB + behaver are base. HB + is called conjugate base of B, while OH – is calledconjugate base of H 2O.Treatment of week acid and week Base in Bronsted-Lowery theoryThe formula for acetic acid is CH 3COOH represented as HAC.When HAC ionises AC – and H +formed.H 2O + CH 3COOH CH 3COO – + H 3O +HAC AC –An equilibrium constant can be written for this reaction, as followsKc =[H 3O + ] [AC – ][HAC] [H 2O]Since the value for [H 2O] is constant, move it to left side of the equation, since Kc [H 2O] is it self aconstant, we rename the new constant as K a.K a=[H 3O + ] [AC – ][HAC]When ammonia NH 3ionizes in water, the following equation cna be writtenNH 3+ H 2O NH + 4+ OH –PHYSICAL CHEMISTRY Page No.: 1By: Shailendra Kumar

Kc =[NHa + ] [OH – ][NH 3] [H 2O]Since the value for [H 2O] is a constant, move it to left side of the equation, Since Kc [H 2O] .It self constant we rename the new constant as K b.K b=[NH 4+] [OH – ][NH 3]PHYSICAL CHEMISTRY Page No.: 2By: Shailendra Kumar