Note: Find out which of these versions your syllabus is likely to want you to know (even if they just call it "lattice enthalpy") and concentrate on that one, but be aware of the confusion! Lattice formation enthalpies are always negative. The lattice formation enthalpy is the enthalpy change when 1 mole of solid crystal is formed from its scattered gaseous ions. Lattice dissociation enthalpies are always positive. The lattice dissociation enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions. That immediately removes any possibility of confusion. You should talk about "lattice formation enthalpy" if you want to talk about the amount of energy released when a lattice is formed from its scattered gaseous ions.įor NaCl, the lattice formation enthalpy is -787 kJ mol -1. You should talk about "lattice dissociation enthalpy" if you want to talk about the amount of energy needed to split up a lattice into its scattered gaseous ions.įor NaCl, the lattice dissociation enthalpy is +787 kJ mol -1. I suggest that you never use the term "lattice enthalpy" without qualifying it. This is an absurdly confusing situation which is easily resolved. Unfortunately, both of these are often described as "lattice enthalpy". In the sodium chloride case, that would be +787 kJ mol -1.īoth refer to the same enthalpy diagram, but one looks at it from the point of view of making the lattice, and the other from the point of view of breaking it up. In other words, you are looking at an upward arrow on the diagram.
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