Up Learn – A Level Chemistry (aqa) – Constructing Born-Haber Cycles
Perfect Ionic Model: Covalent Character
Theoretical lattice enthalpies assume bonding is 100% ionic.
Experimental lattice enthalpies, on the other hand, also account for covalent character.
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Constructing Born-Haber Cycles
Last time we saw that the perfect ionic model isn’t so…well, perfect
Namely, it seems to work well for some ionic compounds and not so well for others.
And now, we can investigate why…
Now, like any theoretical model, the perfect ionic model makes some assumptions…
And the two major ones are that ions are perfectly spherical, and, as a result of that, the bonding is perfectly ionic.
However, in reality, neither of these assumptions are actually true…
…and, as a result, the values that this equation – which is based on the perfect ionic model – spits out won’t always be accurate.
Now, it’s very rare for compounds to bond purely ionically or purely covalently
In fact, as we’ve seen before, bonding is more of a spectrum.
This means the majority of ionic compounds aren’t purely ionic, and that they have some covalent character too.
This covalent character actually strengthens the bonding in the salt…
And, as a result, it makes the lattice enthalpy more exothermic.
Now this extra covalent character is accounted for in experimental lattice enthalpies, because that’s based on actual experimental data
But, it isn’t accounted for in the theoretical lattice enthalpies because that value is based on the assumption that bonding is 100% ionic
And so, because theoretical lattice enthalpies ignore covalent character, they’re less exothermic than their experimental counterparts
In other words, the discrepancy between theoretical and experimental lattice enthalpies is thanks to the covalent character of ionic compounds.
And so, if there’s a large difference between the two like there is here, that means the salt has a lot of covalent character.
And if there’s a small difference between the two, like there is here, that means the salt has a very small, almost insignificant amount of covalent character
So, based on this data, which of these salts has more covalent character…
Silver iodide has a larger difference between its theoretical and experimental lattice enthalpies, so this suggests it has the most covalent character.
And, based on this data, which of these salts has the least covalent character…
Caesium fluoride has the smallest difference between its theoretical and experimental lattice enthalpies, so this suggests it has the least covalent character
So, to sum up…
Theoretical lattice enthalpy calculations are based on the perfect ionic model, which assumes that the bonding in salts is 100% ionic.
However, that’s wrong. Most ionic compounds have some covalent character too, which increases the strength of the bonding in the compound.
This covalent character is only accounted for in experimental lattice enthalpies.
…and that’s why experimental lattice enthalpies are more exothermic than theoretical ones.