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Lattice Energy

Discussion Questions

Lattice Energy

The Lattice energy, U, is the amount of energy requried to separate a mole of the solid (s) into a gas (g) of its ions. MaLb(s) ® a Mb+(g) + b Xa- (g)     U kJ/mol This quantity cannot be experimentally determined directly, but it can be estimated using Hess Law in the form of Born-Haber cycle. It can also be calculated from the electrostatic consideration of its crystal structure.

As defined, the lattice energy is positive, because energy is always required to separate the ions. For the reverse process, the energy released is called energy of crystallization, Ecryst.

a Mb+(g) + b Xa- (g) ® MaLb(s)     Ecryst kJ/mol Therefore, U = - Ecryst

Values of lattice energies for various solids have been given in literature, especially for some common solids. Some are given here.

Comparison of Lattice Energies (U in kJ/mol) of Some Salts
SolidUSolidUSolidU SolidU
LiF 1036LiCl 853LiBr 807LiI757
NaF 923NaCl 786NaBr747NaI704
KF 821KCl 715KBr682KI649
--
MgF22957MgCl22526 MgBr2 2440 MgI22327
The following trends are obvious at a glance of the data above:

How is lattice energy estimated using Born-Haber cycle?

Estimating lattice energy using the Born-Haber cycle has been discussed in Ionic Solids. For a quick review, the following is an example that illustrate the estimate of the energy of crystallization of NaCl. Hsub of Na = 108 kJ/mol (Heat of sublimation)
D of Cl2 = 244 (Bond dissociation energy)
IP of Na(g) = 496 (Ionization potential or energy)
EA of Cl(g) = -349 (Electron affinity of Cl)
Hf of NaCl = -411 (Enthalpy of formation)
 The Born-Haber cycle to evaluate Elattice is shown below: 
     -----------Na+ + Cl(g)--------
         ­                       |
         |                       |-349
         |496+244/2              ¯
         |                 Na+(g) + Cl-(g)
         |                       |
   Na(g) + 0.5Cl2(g)             |
         ­                       |
         |108                    |
         |                       |Ecryst= -788 
   Na(s) + 0.5Cl2(l)             |
         |                       |
         |-411                   |
         ¯                       ¯
     -------------- NaCl(s) --------------
Ecryst = -411-(108+496+244/2)-(-349) kJ/mol
    = -788 kJ/mol.

Discussion
The value calculated for U depends on the data used. Data from various sources differ slightly, and so is the result. The lattice energies for NaCl most often quoted in other texts is about 765 kJ/mol.

Compare with the method shown below

Na(s) + 0.5 Cl2(l) ® NaCl(s) - 411 Hf
Na(g) ® Na(s)
 - 108 -Hsub
Na+(g) + e ® Na(g) - 496 -IP
Cl(g) ® 0.5 Cl2(g) - 0.5 * 244 -0.5*D
Cl-(g) ® Cl(g) + 2 e 349 -EA
Add all the above equations leading to
Na+(g) + Cl-(g) ® NaCl(s) -788 kJ/mol = Ecryst

How is lattice energy related to crystal structure?

There are many other factors to be considered such as covalent character and electron-electron interactions in ionic solids. But for simplicity, let us consider the ionic solids as a collection of positive and negative ions. In this simple view, appropriate number of cations and anions come together to form a solid. The positive ions experience both attraction and repulson from ions of opposit charge and ions of the same charge.

As an example, let us consider the the NaCl crystal. In the following discussion, assume r be the distance between Na+ and Cl- ions. The nearest neighbors of Na+ are 6 Cl- ions at a distance Ö1r, 12 Na+ ions at a distance Ö2r, 8 Cl- at Ö3r, 6 Na+ at Ö4r, 24 Na+ at Ö5r, and so on. Thus, the energy due to one ion is 

      z2e2    6   12    8    6   24
E = - ---- [ -- - -- + -- - -- + -- ...]
      4peor   Ö1       Ö2         Ö3          Ö4          Ö5
where z is the number of charges of the ions, (=1 for NaCl);
e is the charge of an electron (= 1.6022x10-19 C);
4peo = 1.11265x10-10 C2/(J m)
and the series in the [ ] is called the Madelung constant, M. The above discussion is valid only for the sodium chloride (also called rock salt) structure type. This is a geometrical factor, depending on the arrangement of ions in the solid. The Madelung constant depends on the structure type, and its values for several structural types are given below:
Solid M A : CType
NaCl 1.747558 6 : 6Rock salt
CsCl 1.747558 8 : 8CsCl type
CaF22.519398 : 4Fluorite
TiO22.4086 : 3Rutile
Al2O34.17196 : 4Corundum
A is the number of anions coordinated to cation and C is the numbers of cations coordinated to anion.
Madelung constants for a few more types of crystal structures are available from the Handbook Menu. Madelung Energy discuss further the lattice energy of ionic crystals.

There are other factors to consider for the evaluation of energy of crystallization, and the treatment by M. Born led to the formula for the evaluation of crystallization energy Ecryst, for a mole of crystalline solid: 

          N z2e2        1
Ecryst = - ------ ( 1 - ---)
          4peor         n
where N is the Avogadro's number (=6.022x10-23), and n is a number related to the electronic configurations of the ions involved. The n values and the electronic configurations (e.c.) of the corresponding inert gases are given below:
n =5 7 9 10 12
e.c.He Ne Ar Kr Xe

The following values of n have been suggested for some common solids:

n =5.9 8.0 8.7 9.1 9.5
e.c.LiF LiCl LiBr NaCl NaBr

Example 1

Estimate the energy of crystallization for NaCl.

Solution
Using the values giving in the discussion above, the estimation is given by 

          6.022x1023 /mol (1.6022-19)2 * 1.747558
Ecryst = - -------------------------------------- ( 1 - 1/9.1)
             4p * 8.854x10-12 C2/m * 282x10-12 m

   = - 766376 J/mol
   = - 766 kJ/mol

Discussion
Much more should be considered in order to evaluate the lattice energy accurately, but the above calculation leads you to a good start.

When methods to evaluate the energy of crystallization or lattice energy lead to reliable values, these values can be used in the Born-Hable cycle to evaluate other chemical properties, for example the electron affinity, which is really difficult to determine directly by experiment.

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