The discussion centers around the concept of lattice energy, particularly how gaseous ions can exist under standard conditions. Participants explore the definitions and implications of lattice energy, ionization energy, and electron affinity, while questioning the relevance of temperature and pressure in these contexts.
Participants generally agree on the definitions of lattice energy and electron affinity, but there is disagreement regarding the relevance of temperature and pressure to these concepts, as well as the classification of certain quantities as thermodynamic.
Participants note that the relevance of temperature and pressure may be minor for certain thermodynamic quantities, and there is an ongoing exploration of the definitions and implications of these concepts without reaching a consensus on all points.
DrDu said: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.
MohammedRady97 said: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?
DrDu said: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.
Borek said: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.
DrDu said: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.