   Chapter 16, Problem 31PS

Chapter
Section
Textbook Problem

Which is the stronger of the following two acids? (a) benzoic acid, C6H5CO2H, pKa = 4.20 (b) 2-chlorobenzoic acid, ClC6H4CO2H, pKa = 2.90

Interpretation Introduction

Interpretation:  The comparison of acidic strength of the given acids has to be done by using their given pKa values.

Concept introduction: In aqueous solution an acid undergoes ionization. The ionization of an acid is can be expressed in terms of equilibrium constant. The quantitative measurement tells about the strength of the acid. Higher the value of Ka stronger will be the acid. The acid dissocition can be represented as following equilibrium,

HA(aq)+  H2O(l) H3O+(aq)+ A1(aq)

The dissociation constant for the acid is Ka,

Ka=[H3O+][A][HA]

For simplifications, pKa value is used to find the acidic strength of acid which is calculated by taking negative logarithm of Ka.

pKa=log(Ka)

The lower value of pKa of an acid, stronger will be the acid. Thus on comparing the pKa value it can be determined which acid is stronger one among them.

Explanation

On comparing the pKa values of both acid it is found that 2-Chlorobenzoic acid (ClC6H4CO2H) has lower value of pKa than Benzoic acid(C6H5CO2H). Thus, 2-Chlorobenzoic acid (ClC6H4CO2H) is stronger acid than Benzoic acid(C6H5CO2H).

Given: The pKa values of both acids are given for Benzoic acid(C6H5CO2H)pKa value is 4.20 and for 2-Chlorobenzoic acid(ClC6H4CO2H)pKa value is 2.90. The Ka values for the acids are calculated as,

pKa=log(Ka)Ka=10(pKa)

pKa value for Benzoic acid(C6H5CO2H) is 4

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