Chemistry, Third edition

Bond dissociation enthalpies of polyatomic

molecules

The standard enthalpy change for the reaction:

H–O–H(g) H(g)OH(g) H—^ 499 kJ mol^1

shows that the bond dissociation enthalpy of the HO–H bond, symbolized H—HO^ – H, is

499 kJ per mol of O–H bonds. However, if we now break the O–H bond in OH(g), only

428 kJ of energy per mol of O–H is required for complete dissociation:

O–H(g) O(g)H(g) H—^ H—O–H^ 428 kJ mol^1

Themean bond enthalpyfor the stepwise dissociation of both O–H bonds in water is

(499428)/2463.5 kJ mol^1.

This data does not mean that the O–H bonds in water are different. On the

contrary, they are identical in every respect. The different values of bond enthalpies

for the O–H bonds arise because they apply to very different species, H 2 O and

OH.

Mean bond enthalpies

The bond dissociation enthalpies of bonds such as C–H, C–Cl, C=O, N=N and O–H

are approximately the same in different molecules. If the values of bond dissociation

enthalpy for a bond between two atoms (A and B) in several different molecules are

averaged, the resulting value is called the mean bond enthalpy(Table 13.4). Mean

bond enthalpies are useful in estimating enthalpy changes for reactions for which

standard enthalpies of formation are unavailable, but the fact that such values may

have been obtained by averaging bond dissociation enthalpies from different types

of molecule may lead to substantial errors in the calculated value of H^ —.

ENERGETICS OF BOND BREAKING AND BOND MAKING 237

Table 13.4Standard

bond enthalpies

Bond H—A–B^ (298K)

/kJ mol^1

Bond dissociation enthalpies

H–H 436

NN 945

O=O 497

F–F 158

Cl–Cl 242

H–F 565

H–Cl 431

H–Br 366

H–I 299

OC=O 531

H 3 C–Cl 339

H 2 C=CH 2 699

Mean bond enthalpies

C–H 412

C–C 348

C=C 612

C=O 743

O–H 463

N–N 163

N–H 388

C–Cl 338

The atoms on either side of the

bond are displayed in bold.

Bond enthalpies 1 (use of data in Table 13.4)

(i) Table 13.4 shows that HC^ —–H412kJmol^1. Estimate the enthalpy change for the

following reaction under standard conditions:

CH 3 (g) H(g) CH 4 (g)

(ii)The bond lengths (in pm) for the hydrogen halides are H–F(92), H–Cl(128), H–Br(141) and

H–I(160). Do these figures support the idea that long bonds are weak bonds?

(iii)Does the bond enthalpy data in Table 13.4 suggest that double and triple bonds are

stronger than single bonds?

Exercise 13P

Mean bond enthalpy of C=O in CO 2

The carbon dioxide molecule (O=C=O) contains two identical C=O bonds. If the bonds in CO 2

are broken in sequence it is found that HOC^ —=O531 kJ mol^1 andH^ —C=O1075 kJ mol^1.

What is the mean bond enthalpy of the C=O bond in CO 2?

Exercise 13O