Chemistry, Third edition

236 13 · ENERGY CHANGES IN CHEMICAL REACTIONS

Lattice enthalpy of magnesium fluoride

Calculate the lattice enthalpy of magnesium fluoride given the following standard enthalpy

data:

enthalpy of atomization of magnesium, H^ —atom(Mg(s))148kJmol^1

first ionization energy of magnesium, H—ion^ (Mg(g))744kJmol^1

Second ionization energy of magnesium, H—ion^ (Mg(g))1456 kJ mol^1

bond dissociation enthalpy of F 2 (g),H—^ (F–F)158 kJ mol^1

enthalpy of electron gain of F(g), H—^ eg 339 kJ mol^1

enthalpy of formation of MgF 2 (s),H^ —f(MgF 2 (s)) 1121 kJ mol^1.

Exercise 13N

Since

H— 5     H—^ L(NaCl)

H—L^ (NaCl) H^ — 5  (771)771 kJ mol–1

Energetics of bond breaking and bond

making

Bond dissociation enthalpies

If a covalent bond exists between two atoms, it will take energy to pull those atoms

apart. For example, the dissociation of the A–B bond in the molecule AB may be

represented by the equation

A–B(g) A(g)B(g)

(Note that both reactants and products are gaseous.) The amount of energy required

to break a particular bond in a gaseous molecule under standard conditions is called

the(standard) bond dissociation enthalpyof that bond, symbolised HA—^ – B. For

example, the standard enthalpy change for the dissociation of chlorine molecules at

298 K,

Cl 2 (g) Cl•(g) Cl•(g)

is 242 kJ per mole of Cl–Cl bonds. Therefore, the bond dissociation enthalpy is

242 kJ mol^1 :

H—^ (Cl–Cl)242 kJ mol^1

Selected bond dissociation enthalpies are shown in Table 13.4. Covalent bonds

whereH—A^ – Bis greater than 400 kJ mol^1 are usually described as strong bonds.

When a reaction is reversed the energy changes are also reversed. We predict that

the reaction

Cl(g)Cl(g) Cl 2 (g)

will involve a standard enthalpy change at 298 K of242 kJ mol^1 , the released heat

energy reflecting the fact that in this reaction the two chlorine atoms are achieving a

stable electronic configuration through the formation of a covalent bond.

13.9