Lattice Energy Diagrams They look scary at first – but they’re actually very prescriptive

Lattice Energy Diagrams

They look scary at first – but they’re actually very prescriptive

Lattice energy diagrams (often called Born–Haber cycles) are one of those A-Level Chemistry topics that students expect to be difficult. Lots of arrows, lots of enthalpy changes, and plenty of opportunities to panic.

The reality?
👉 They are highly structured and almost algorithmic.
If you follow the steps, the diagram practically builds itself.

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What is lattice energy (in plain English)?

Lattice energy is the energy change when one mole of an ionic solid is formed from its gaseous ions.

• Usually exothermic (energy released)

• Stronger ionic attractions → more negative lattice energy

• Depends mainly on:

  • Ionic charge

  • Ionic radius

But lattice energy itself can’t be measured directly – so we use a Hess’ Law cycle to calculate it.

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Why do we use a lattice energy diagram?

Because it links experimental data (like enthalpy of formation) with theoretical steps (like ionisation energy).

Every lattice energy diagram uses the same building blocks:

• Enthalpy of formation

• Atomisation

• Ionisation energy

• Electron affinity

• Lattice energy

Once students realise this, the fear disappears.

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The key idea students miss

🔑 You are not inventing the diagram – you are following a recipe.

There is:

• A fixed start point

• A fixed end point

• A fixed set of steps in between

Change the compound, and the numbers change –
but the structure stays the same.

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Step-by-step structure (the “recipe”)

Let’s take a typical ionic compound like sodium chloride.

1️⃣ Start with the elements in their standard states

This links directly to enthalpy of formation.

Na(s) + ½Cl₂(g)

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2️⃣ Convert elements to gaseous atoms (atomisation)

Solids and molecules → gaseous atoms.

• Na(s) → Na(g)

• ½Cl₂(g) → Cl(g)

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3️⃣ Form gaseous ions

This is where many marks live.

• Ionisation energy
  Na(g) → Na⁺(g) + e⁻

• Electron affinity
  Cl(g) + e⁻ → Cl⁻(g)

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4️⃣ Bring the gaseous ions together

This final step is lattice energy:

Na⁺(g) + Cl⁻(g) → NaCl(s)

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Using Hess’ Law (the exam-winning bit)

Once the cycle is drawn:

• Go around the cycle

• Apply Hess’ Law

• Rearrange to calculate lattice energy

Most exam errors come from:

• Missing a step

• Wrong sign (+/–)

• Forgetting coefficients (½Cl₂!)

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Why examiners love lattice energy questions

Because they test:

• Understanding of bonding

• Use of enthalpy data

• Ability to apply Hess’ Law logically

They are not testing creativity – they are testing method.

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How I teach students to master them

At Hemel Private Tuition, I get students to:

✅ Memorise the order of steps
✅ Practise drawing the diagram before adding numbers
✅ Colour-code different enthalpy changes
✅ Write the algebra symbolically first
✅ Only substitute numbers at the end

After 2–3 examples, most students say:

“Oh… it’s the same every time.”

And they’re absolutely right.