Chapter 9 looks at forces that keep atoms together. Ionic molecules and the Born-Haber cycle

The formation of an ionic solid can be described by the following equation. M(s) + 1/2X2(g) MX(s) The energetics or enthalpy associated with this process cannot be determined directly. You have been recently presented with the processes of ionization (loss of an electron from a gaseous atom), electron affinity (gain of an electron by a gaseous atom) and lattice energy (change in energy when two gaseous ions form an ionic solid). These can be used, according to Hesss Law to calculate the energetics of formation of an ionic solid. Note that the starting materials are in their elemental state.

Lets use the formation of NaF(s) as an example. Na(s) + F2(g) NaF(s) The following steps must occur: Process Energy change

Na(s) Na(g) sublimation 109 kJ/mol

Na(g) Na+(g) ionization 495 kJ/mol

F2(g) 2F(g) bond breaking 154 kJ/mol

F(g) F-(g) electron affinity -328 kJ/mol

Na+(g) + F

-(g) NaF(s) lattice energy -923 kJ/mol

When these equations and the energetics are added, the net result is the formation of the ionic solid with an

enthalpy of formation Hf of 570 kJ

Na(s) Na(g) sublimation 109 kJ/mol

Na(g) Na+(g) + e

- ionization 495 kJ/mol

F2(g) F(g) bond breaking (154 kJ/mol)

F(g) + e- F

-(g) electron affinity -328 kJ/mol

Na+(g) + F

-(g) NaF(s) lattice energy -923 kJ/mol

Na(s) + F2(g) NaF(s) -570 kJ/mol A second example: What is the lattice energy associated with Ca

2+(g) and Cl

-(g) coming together to form CaCl2?

Ca2+

(g) + 2Cl-(g) CaCl2(s)

sublimation energy of calcium +178 kJ/mol first ionization of calcium +590 kJ/mol second ionization of calcium +1150 kJ/mol bond dissociation of Cl2(g) +244 kJ/mol electron affinity of Cl

- -349 kJ/mol

enthalpy of reaction -326 kJ

Ca(s) Ca(g) 178 kJ

Ca(g) Ca+(g) + e

- 590 kJ

Ca+(g) Ca

2+(g) + e

- 1150 kJ

Cl2(g) 2Cl(g) 244 kJ

2Cl(g) 2Cl-(g) 2(-349 kJ)

Ca2+

(g) + 2Cl-(g) CaCl2(s) x

Ca(s) + Cl2(g) CaCl2(s) -326 kJ

The role of the lattice energy in stabilizing ions Consider the formation of CaO:

Ca(s) + O2(g) CaO(s) The following step are required:

Ca(s) Ca(g) sublimation +178 kJ/mol

Ca(g) Ca+(g) + e

- first ionication +589 kJ/mol

Ca+(g) Ca

2+(g) + e

- second ionization +1040 kJ/mol

O2(g) O(g) bond energy (+247 kJ/mol)

O(g) + e- O

-(g) electron affinity of O -141 kJ/mol

O- + e

- O

2-(g) electron affinity of O

- +878 kJ/mol

O2-

(g) + Ca2+

(g) CaO(s) lattice energy -3,100 kJ/mol

Ca(s) + O2(g) CaO(s) enthalpy of reaction -309 kJ Note that the formation of O

2- is endothermic (-141 kJ/mol + 878 kJ/mol = 737 kJ/mol) and the gaseous O

2- ion

is not stable. However, it can be formed, if it is stabilized by association with Ca2+

. Note the large exothermic lattice energy. If the lattice energy increases with charge and the association of oppositely charged ions is a stabilizing force, can NaF form, with Na

+2 and F

-2?

Na2+

(g) + F2-

(g) NaF(s)

Na(s) Na(g) sublimation 109 kJ/mol

Na(g) Na+(g) + e

- first ionization 495 kJ/mol

Na+(g) Na

2+ + e

- second ionization 4560 kJ/mol

F2(g) F(g) bond energy 77 kJ/mol

F(g) + e- F

-(g) electron affinity -328 kJ/mol

F-(g) + e

- F

2-(g) electron affinity +1250 kJ/mol

Na2+

(g) + F2-

(g) NaF lattice energy -3692 kJ/mol

Na2+

(g) + F2-

(g) NaF(s) enthalpy of reaction +2471 kJ

Ca2+

(g) + 2Cl-(g) CaCl2(s)

sublimation energy of calcium +178 kJ/mol first ionization of calcium +590 kJ/mol second ionization of calcium +1150 kJ/mol bond dissociation of Cl2(g) +244 kJ/mol electron affinity of Cl

- -349 kJ/mol

enthalpy of reaction -326 kJ

Ca(s) Ca(g) 178 kJ

Ca(g) Ca+(g) + e

- 590 kJ

Ca+(g) Ca

2+(g) + e

- 1150 kJ

Cl2(g) 2Cl(g) 244 kJ

2Cl(g) 2Cl-(g) 2(-349 kJ)

Ca2+

(g) + 2Cl-(g) CaCl2(s) x

Ca(s) + Cl2(g) CaCl2(s) -326 kJ

Ca(s) + O2(g) CaO(s) The following step are required:

Ca(s) Ca(g) sublimation +178 kJ/mol

Ca(g) Ca+(g) + e

- first ionication +589 kJ/mol

Ca+(g) Ca

2+(g) + e

- second ionization +1040 kJ/mol

O2(g) O(g) bond energy (+247 kJ/mol)

O(g) + e- O

-(g) electron affinity of O -141 kJ/mol

O- + e

- O

2-(g) electron affinity of O

- +878 kJ/mol

O2-

(g) + Ca2+

(g) CaO(s) lattice energy -3,100 kJ/mol

Ca(s) + O2(g) CaO(s) enthalpy of reaction -309 kJ

Na2+

(g) + F2-

(g) NaF(s)

Na(s) Na(g) sublimation 109 kJ/mol

Na(g) Na+(g) + e

- first ionization 495 kJ/mol

Na+(g) Na

2+ + e

- second ionization 4560 kJ/mol

F2(g) F(g) bond energy 77 kJ/mol

F(g) + e- F

-(g) electron affinity -328 kJ/mol

F-(g) + e

- F

2-(g) electron affinity +1250 kJ/mol

Na2+

(g) + F2-

(g) NaF lattice energy -3692 kJ/mol

Na2+

(g) + F2-

(g) NaF(s) enthalpy of reaction +2471 kJ