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.

More videos on Constructing Born-Haber Cycles:

Summary of Lattice Enthalpy

Lattice Enthalpy: Atomisation and Bond Dissociation

Lattice Enthalpy: Electron Affinity

Summary of Born Haber Cycles

Exothermic and Endothermic Electron Affinity

Alternative Born-Haber Cycles (article)

Labelling Enthalpy Changes (article)

Perfect Ionic Model: Covalent Character

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Up Learn – A Level Chemistry (AQA)

Constructing Born-Haber Cycles

1. Introduction to Lattice Enthalpies
2. Enthalpies Recap
3. Lattice Enthalpy
4. Predicting Lattice Enthalpies
5. Finding Lattice Enthalpy Experimentally
6. Finding Lattice Enthalpy Using a Hess Cycle
7. Separation
8. Atomisation
9. Atomisation and Bond Dissociation
10. Removing Electrons
11. Adding Electrons
12. Electron Affinity
13. Representing Different Enthalpy Changes
14. Summary of the Alternate Route
1. The Aftermath of WWI
2. Exo or Endo?
3. Exothermic and Endothermic Electron Affinity
4. Constructing Born Haber Cycles: First Steps
5. Constructing Born Haber Cycles: Adding Atomisation
6. Constructing Born Haber Cycles: Removing Electrons
7. Constructing Born Haber Cycles: Adding Electrons
8. Alternative Born-Haber Cycles
9. Labelling Enthalpy Changes
10. Labelling Born-Haber Cycles
11. Constructing Born-Haber Cycles
12. Finding Lattice Enthalpy using Born-Haber Cycles
13. Calculating Lattice Enthalpies using Born-Haber Cycles
14. Constructing Born Haber Cycles: Adding More Than One Electron
15. Calculating Lattice Enthalpy Using More Complex Born-Haber Cycles
16. Calculating Enthalpies Other Than Lattice Enthalpy Using Born-Haber Cycles
1. Theoretical vs Experimental Lattice Enthalpies – Introduction
2. Calculating Lattice Enthalpies Quickly
3. Theoretical vs Experimental
4. Perfect Ionic Model: Covalent Character
5. Polarisation
6. Polarising Power
7. Polarisability
8. Comparing Covalent Character
9. The Curious Case of the Silver Halides
1. An Alternative Route For Lattice Enthalpy
2. Dissolving Salts In Water
3. The Enthalpy of Solution
4. Enthalpy Changes and The Enthalpy of Solution
5. Can I Feel The Enthalpy of Solution?
6. Gaseous Ion Hydration
7. Enthalpy of Hydration
8. Factors Affecting the Enthalpy of Hydration
9. Constructing an Alternative Route for Lattice Enthalpy
10. Calculating Lattice Enthalpy
11. Energy Level Diagrams, Hydration and Solution
12. Energy Level Diagrams and Exothermic Enthalpies of Solution
13. Converting SHL Cycles into Energy Level Diagrams

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.