Energy of Formation for Ionic Compounds

Ionization Energy  Na right arrow Na+ + e-  DeltaH = +495 kJ/mol
Electron Affinity Energy  Cl + e- right arrow Cl-  DeltaH = -348 kJ/mol
Total   DeltaH = + 147 kJ/mol
  However, this value is for the free ions. If we allow them to come together by coulombic attraction into a crystal lattice a large additional amount of energy is released:   
Lattice Energy Na+(g) + Cl-(g) right arrow NaCl(s)  DeltaH = - 449 kJ/mol
  Overall   DeltaH = - 302 kJ/mol

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 Interaction Type
Example Average Strength, kcal/mol (kJ/mol) Range**
Charge-dipole -NH3+ Cl-   1/r2
Dipole-dipole ClCH3 ClCH3    1/r3
Dipole-induced dipole* CH4 ClCH3 0.1-0.2 (0.4-4)  1/r6
Dispersion*
(induced dipole-induced dipole)
CH4 CH4

0.1-0.2 (0.4-4)
1/r6
Hydrogen bond  two H-bonded water molecules 3-8 (12-30)  

 van der Waals repulsion
    1/r12

 

 

 

 

© R A Paselk

Last modified 17 April 2013