Question

How do you find the lattice energy of ionic compounds?

Answer 1

Lattice Energy - The strength of ionic compounds is measured with something called lattice energy. It is the energy released when one mole of an ionic compound is formed.

Lattice energy is calculated in following steps:

Step 1
Determine the energy of the metal and nonmetal in their elemental forms. (Elements in their natural state have an energy level of zero.) Subtract from this the heat of formation of the ionic solid that would be formed from combining these elements in the appropriate ration. This is the energy of the ionic solid, and will be used at the end of the process to determine the lattice energy.

Step 2
It is required that the elements involved in the reaction are in their gaseous forms. Add the changes in enthalpy to turn one of the elements into its gaseous state, and then do the same for the other element.

Step 3
Metals exist in nature as single atoms and thus no dissociation energy needs to be added for this element. However, many nonmetals will exist as polyatomic species. For example, Cl exists as Cl2 in its elemental state. The energy required to change Cl2 into 2Cl atoms must be added to the value obtained in Step 2.

Step 4
Both the metal and nonmetal now need to be changed into their ionic forms, as they would exist in the ionic solid. To do this, the ionization energy of the metal will be added to the value from Step 3. Next, the electron affinity of the nonmetal will be subtracted from the previous value. It is subtracted because it is a release of energy associated with the addition of an electron.

Step 5
Now the metal and nonmetal will be combined to form the ionic solid. This will cause a release of energy, which is called the lattice energy. The value for the lattice energy is the difference between the value from Step 1 and the value from Step 4.

Equation - The final equation to find lattice energy is given as:

Heat of formation = Heat of atomization + Dissociation energy + (sum of Ionization energies) + (sum of Electron affinities) + Lattice energy

Rearrangement to solve for lattice energy gives the equation:

Lattice energy = Heat of formation - Heat of atomization - Dissociation energy - (sum of Ionization energies) - (sum of Electron Affinities)