Chemistry, Third edition

the endothermic steps cause the total enthalpy to increase (i.e. to move up the

figure), whereas the exothermic steps cause the total enthalpy to decrease (i.e. to

move down the figure). The stages are:

1.The conversion of solid sodium to gaseous sodium (atomization):

Na(s) Na(g) H—^1 109kJmol^1

(i.e. the standard enthalpy of atomization of sodium).

2.The ionization of sodium atoms in the vapour:

Na(g) Na(g)e H—^2 494kJmol^1

(i.e. the standard first ionization energy of sodium).

3.The breaking of the Cl–Cl bond to produce isolated chlorine atoms:

(^1) ⁄
2 Cl 2 (g)Cl

• (g) H—
  3 121kJmol

 1

i.e. half the standard bond dissociation enthalpy of the Cl–Cl bond. The enthalpy

change for this process is halfthe energy required to break one Cl–Cl bond because

we only require one Cl•atom to react with each Na. The standard bond dissocia-

tion enthalpy of Cl–Cl is 242 kJ mol^1 , so that H^ — 3 242/2121 kJ mol^1.

4.The gain of one electron by the chlorine atom:

Cl•(g)eCl(g) H—^4  364 kJ mol^1

i.e. the standard enthalpy of electron gain of the chlorine atom. The Cl•atom is

reaching a more stable configuration in gaining an electron.

5.The attraction of the sodium and chloride ions to make crystalline sodium chloride:

Na(g)Cl(g) Na,Cl(s) H— 5   H^ —L

Note that addition of the equations involving the H^ — 1 ,H^ — 2 ,H^ — 3 ,H— 4   and

H^ — 5 terms gives equation (13.16). According to Hess’s law,

H—^ fH^ — 1 H—^2 H— 3     H 4 —^ H^ — 5 (13.17)

The stages shown in Fig. 13.8 are not meant to indicate the way that sodium and

chlorine actually react, i.e. they do not represent the mechanismof the reaction.

Similar energy cycles may be set up for other ionic compounds, such as

Ca^2 ,O^2 . The importance of equation (13.17) is that we can calculate one of the

enthalpy terms knowing all the others. The unknown is usually the lattice enthalpy

(which equalsH^ — 5 ), and which cannot be measured directly by experiment.

Rearrangement of equation (13.17) gives

H— 5    H—^ fH—^1 H— 2    H 3 —^ H^ — 4

For example, for the sodium and chlorine cycle,

H—^5   411 (109)(494)(121)(364) 771 kJ mol^1

LATTICE ENTHALPY 235

Enthalpy changes

(i)What is the standard enthalpy change for the process Na(s) Na(g)e?

(ii)Estimate the amount of energy required for the following (hypothetical) process under

standard conditions:

Na,Cl(s) Na(g)^1 ⁄ 2 Cl 2 (g)

Exercise 13M