Chemguide: Support for CIE A level Chemistry

23.1: Lattice energy and Born-Haber cycles

Learning outcomes 23.1.3 and 23.1.4

Before you go on, find and read this statement in your syllabus.

You will find Born-Haber cycles explained on the page you will already have visited about lattice energies.

It wouldn't hurt to re-read the bits you have met before and then continue into the long section about Born-Haber cycles.

You can ignore the second version of the Born-Haber cycle for NaCl using lattice dissociation energy - CIE don't use that version.

You can ignore most of the section about magnesium chloride (unless you are interested), but you must look at the cycle for MgCl₂. Make sure you understand that you have to use the first and second ionisation energies of magnesium, and that you have to multiply the atomisation enthalpy and first electron affinity of chlorine by 2.

You could also be asked to construct a Born-Haber cycle for something like Na₂O where you are forming a 2- ion.

This time you would need:

• 2 x atomisation enthalpy for sodium;

• 2 x first ionisation energy for sodium;

• 1 x atomisation enthalpy for oxygen;

• 1 x first electron affinity for oxygen (which is negative);

• 1 x second electron affinity for oxygen (which is positive, so that the diagram moves upwards again).

Plus, of course, the enthalpy change of formation and the unknown lattice energy.

This is shown in detail in my calculations book.

You must think very carefully when you are constructing one of these diagrams to be sure that you understand why you sometimes multiply things by 2 (as with the sodium above), and sometimes have to use 1st + 2nd ionisation energies or electron affinities as with oxygen above or magnesium on the page you have read.

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