# Lattice Energy

1. Jul 7, 2014

My A level Chemistry textbook defines Lattice Energy as "the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions"; a definition which I can't fully grasp because of the "standard conditions" part. How can gaseous ions exist under standard conditions?

2. Jul 8, 2014

### DrDu

That's a hypothetical point of reference. More precisely you extrapolate the enthalpy of a gas of the ionic compound from it's ideal behaviour at very small pressure to the value at 1 bar. So effectively you calculate with a gaseous phase at infinitely small pressure, but (more so for entropy than enthalpy) you have to fix units.

3. Jul 8, 2014

What about temperature? How can gaseous ions exist at room temperature? Also, just to be clear, does this apply for ionisation energy and electron affinity as well?

4. Jul 8, 2014

### DrDu

Before discussing this any further, I want to point out that this question is probably not too relevant for what you are about (I suppose Born Haber cycles), as the ionization enthalpy does depend only to a minor extent on temperature and pressure.
This having been said, let's go on:
In principle there is always a gaseous phase at equilibrium with a solid. At low temperatures it will behave like an ideal gas as pressure is very low. Hence it is an easy exercise to extrapolate the ideal gas law to any pressure and temperature you like.

Ionisation energy and electron affinity aren't thermodynamic quantities, so they are independent of temperature and pressure.

5. Jul 8, 2014

So is my textbook wrong in defining (the first) electron affinity as "the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions under standard conditions."?

6. Jul 8, 2014

### Staff: Mentor

No, that's a correct definition. As DrDu stated several times, we EXTRAPOLATE to standard conditions. Otherwise we would have numbers that are not comparable.

7. Jul 8, 2014