_{Up Learn – A Level Chemistry (aqa) – Constructing Born-Haber Cycles}

_{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:

Lattice Enthalpy: Atomisation and Bond Dissociation

Lattice Enthalpy: Electron Affinity

Exothermic and Endothermic Electron Affinity

Alternative Born-Haber Cycles (article)

Labelling Enthalpy Changes (article)

Perfect Ionic Model: Covalent Character

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## Constructing Born-Haber Cycles

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

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

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

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.