Chemistry 121 (Amar) -- Fall 2007
Ionic energy and Lattice Energy
(©2007, François G. Amar, All rights reserved)

The ionic energy is calculated according to the formula

Eionic=kQ1Q2/d

To use this fomula, Q1 and Q2 must be converted from elementary charge units to Coulombs, d must be given in units of meters, and k= 8.99x109 J-m/C2.

For example, for NaCl,

Q1 = +1 e =1.602x10-19 C and Q1 = -1 e = -1.602x10-19 C

d = 0.97 Å + 1.81 Å = 2.78 Å = 2.78x10-10 m

then Eionic for NaCl = -8.30x10-19 J for one single NaCl ionic bond.

The following table lists Eionic (J per bond) and lattice energies (kJ/mol of compound ) for a series of ionic compounds

Compound

Q1

Q2

d

Eionic

|E ionic |

Lattice energy

 

(e)

(e)

(Å)

(J)

(J)

(kJ/mol)

LiF

1

-1

2.01

-1.15E-18

1.15E-18

1030

LiCl

1

-1

2.49

-9.27E-19

9.27E-19

834

LiI

1

-1

2.88

-8.01E-19

8.01E-19

730

 

 

 

 

 

 

 

NaF

1

-1

2.3

-1.00E-18

1.00E-18

910

NaCl

1

-1

2.78

-8.30E-19

8.30E-19

788

Nabr

1

-1

2.93

-7.87E-19

7.87E-19

732

NaI

1

-1

3.17

-7.28E-19

7.28E-19

682

 

 

 

 

 

 

 

KF

1

-1

2.66

-8.67E-19

8.67E-19

808

KCl

1

-1

3.14

-7.35E-19

7.35E-19

701

KBr

1

-1

3.29

-7.01E-19

7.01E-19

671

 

 

 

 

 

 

 

CsCl

1

-1

3.5

-6.59E-19

6.59E-19

657

CsI

1

-1

3.89

-5.93E-19

5.93E-19

600

 

 

 

 

 

 

 

MgCl2

2

-1

2.47

-1.87E-18

1.87E-18

2326

SrCl2

2

-1

2.94

-1.57E-18

1.57E-18

2127

 

 

 

 

 

 

 

MgO

2

-2

2.06

-4.48E-18

4.48E-18

3795

CaO

2

-2

2.39

-3.86E-18

3.86E-18

3414

SrO

2

-2

2.53

-3.65E-18

3.65E-18

3217

 

 

 

 

 

 

 

ScN

3

-3

2.54

-8.18E-18

8.18E-18

7547

 

The lattice energies for the series of +1/-1 salts like NaCl are plotted vs |Eionic| in the graph below:

We see that there is an extremely good correlation between the two quantities. The ordering of Eionic can

predict the ordering of the lattice energies. In this graph, only the distance, d, between the two anions

is changing since all the salts are +1/-1 type.

In the graph below, all the data in the table is graphed showing the dramatic impact of changing the

charge on the ions:

 

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