Define atomisation and lattice enthalpy
Describe the gaseous-atom step and the formation of an ionic solid from gaseous ions, keeping the direction and sign clear.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 23. Use the syllabus sign convention for lattice enthalpy (gaseous ions forming the ionic solid); state phases, signs, units and temperature. Written practice is self-checked
Before you begin
- Use AS enthalpy changes and Hess's law, formulae for ions, balanced equations, and temperature in kelvins.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which direction does lattice energy use in this syllabus?
What you will learn
- Define enthalpy change of atomisation
- Define lattice energy using gaseous ions and one mole of ionic solid
- Assign signs from the direction of each process
Atomisation produces gaseous atoms
The enthalpy change of atomisation, ΔHₐₜ, forms one mole of gaseous atoms from an element in its standard state. For chlorine, the step is ½Cl₂(g) → Cl(g); breaking the Cl–Cl bond requires energy, so this atomisation step is endothermic.
Write the amount and phases explicitly. For a diatomic element, forming one mole of atoms uses half a mole of molecules; it does not mean atomising one mole of Cl₂ molecules.
Lattice energy forms the solid from gaseous ions
In this syllabus, lattice energy is the enthalpy change when gaseous ions form one mole of an ionic solid. For NaCl, Na⁺(g) + Cl⁻(g) → NaCl(s). Electrostatic attraction releases energy, so the formation direction is exothermic and ΔHₗₐₜ is negative.
Some references define a lattice dissociation enthalpy in the reverse direction. Check the stated definition before using a sign. Here the taught cycle uses gas ions → solid, so reversing that arrow changes the sign.
Pause and explain
What sign is expected when gaseous oppositely charged ions form an ionic lattice?
Worked example
Write the atomisation step for chlorine and the lattice-formation step for magnesium oxide. State the expected sign of each.
Show the worked solution
- Atomisation forms one mole of gaseous atoms from the element's standard state.
- Use ½Cl₂(g) to form Cl(g).
- Lattice formation joins gaseous Mg²⁺ and O²⁻ ions into MgO(s).
Answer ½Cl₂(g) → Cl(g) is endothermic, so ΔHₐₜ is positive. Mg²⁺(g) + O²⁻(g) → MgO(s) is exothermic, so ΔHₗₐₜ is negative.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the atomisation equation for sodium and state the sign of ΔHₐₜ.
Give me a hint
Start with sodium in its standard state and form one mole of gaseous atoms.
Compare my reasoning
- Sodium's standard state is Na(s).
- Form Na(g).
- Energy is required to separate atoms from the metallic solid.
Na(s) → Na(g); ΔHₐₜ is positive.
Look for these in your work
- Correct standard state
- One mole of gaseous atoms formed
- Endothermic sign stated
Write the lattice-formation equation for calcium fluoride and state the sign.
Give me a hint
Use the formula unit CaF₂ and the gaseous ions with their correct charges.
Compare my reasoning
- One formula unit contains Ca²⁺ and two F⁻ ions.
- Use gaseous ions as reactants and solid CaF₂ as product.
- Formation of the lattice releases energy.
Ca²⁺(g) + 2F⁻(g) → CaF₂(s); ΔHₗₐₜ is negative.
Look for these in your work
- Ion charges and ratio correct
- Correct direction and states
- Exothermic sign stated
A table labels +790 kJ mol⁻¹ as the energy to form NaCl(s) from Na⁺(g) and Cl⁻(g). Explain the sign problem.
Give me a hint
Compare the arrow in the definition with the sign convention for lattice formation.
Compare my reasoning
- Gas ions → solid is the defined formation direction.
- That process is exothermic because attractions form.
- A positive value would fit the reverse dissociation direction, not the stated arrow.
For gas ions forming NaCl(s), the lattice enthalpy should be negative. +790 kJ mol⁻¹ has the sign for the reverse lattice-dissociation process, so the table's direction and sign disagree.
Look for these in your work
- Definition direction identified
- Formation sign justified
- Reverse-direction sign explained
Common mistakes
- Using one mole of Cl₂ instead of one-half mole for one mole of Cl atoms
- Omitting gaseous or solid state symbols
- Using lattice dissociation sign for lattice formation
- Calling every energy change in a cycle positive
What can you explain now?
Which direction does lattice energy use in this syllabus?
Compare with the explanation
Gaseous ions form one mole of ionic solid
The defined step is gas ions → solid lattice.
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Make it stick
How do the reactant and product phases tell you whether a quoted value is atomisation, lattice formation or lattice dissociation?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.