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Lattice Energy Trends

Learn and apply Lattice Energy Trends in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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H1 Lattice Energy and Ionic Trends: Orientation

H1 lattice-energy questions test definition and electrostatic comparison. Compare ionic charge and ionic radius systematically, while keeping the negative lattice-formation convention separate from numerical magnitude.

H1 8873 scope
  • 8873 requires the lattice-energy definition and qualitative charge/radius effects.
  • Constructing or calculating a Born–Haber cycle is not required.

Definitions (Must Know)

  • Lattice energy is the enthalpy change when one mole of an ionic solid forms from its separated gaseous ions under standard conditions.
  • Under the 8873 formation convention, lattice energy is negative because lattice formation is exothermic.
  • Lattice-energy magnitude is the absolute size of the value; a more negative lattice energy has a larger magnitude.
  • Ionic radius describes the size of an ion and therefore affects the separation between opposite charges.

Detailed Explanations

A. Electrostatic model

Oppositely charged gaseous ions release energy when they assemble into an ionic lattice. Attraction strengthens as the product of the ionic charges increases and the distance between their centres decreases.

B. A reliable comparison sentence

“Both solids contain ions with the same charges, but ion X is smaller. The oppositely charged ions are therefore closer together, electrostatic attraction is stronger and the lattice energy has a larger magnitude.”

C. Signed value versus magnitude

-3000 kJ mol⁻¹ is numerically lower than -800 kJ mol⁻¹, but has a larger magnitude. Avoid saying only “greater lattice energy”; state either “more negative” or “larger magnitude”.

D. Keep this at H1 depth

Ionisation energy, electron affinity and atomisation are required in H2 Born–Haber cycles, but H1 does not construct those cycles. Do not import them into a qualitative trend answer.

Worked Examples

Modelled example 1

Charge dominates

Core

Problem

Compare the lattice-energy magnitudes of NaCl and MgO.
Study the worked solution

  1. Compare charge products

    Method

    Identify the ionic charges in each solid.

    Reason

    Greater charge produces stronger electrostatic attraction at comparable separation.

    Working

    Na⁺/Cl⁻ gives a charge-product magnitude of 1; Mg²⁺/O²⁻ gives 4.

  2. State the energy comparison

    Method

    Select magnesium oxide.

    Reason

    Stronger attraction releases more energy when its gaseous ions form the lattice.

    Working

    MgO has the more negative lattice energy and larger lattice-energy magnitude.

Guided practice 2

Radius decides

About 6 min

Problem

Compare the lattice-energy magnitudes of LiF and LiI.

Try this before viewing the solution

Hints

Hint 1: control charge
Both solids contain ions with charges 1 + and 1-.
Hint 2: compare separation
F⁻ is smaller than I⁻, so decide which lattice places opposite charges closer.
View solution step by step

  1. Hold charge constant

    Method

    State that both ion pairs have the same charges.

    Reason

    Charge cannot distinguish the two attractions.

    Working

    Both contain Li⁺ with a singly charged halide ion.

  2. Compare ionic radius

    Method

    Identify fluoride as the smaller anion.

    Reason

    Smaller ions allow a shorter centre-to-centre separation.

    Working

    r(F⁻) < r(I⁻).

  3. Complete the causal chain

    Method

    Select lithium fluoride.

    Reason

    Shorter separation strengthens attraction and increases the energy released on lattice formation.

    Working

    LiF has the larger lattice-energy magnitude.

Common misconception 3

Use the ionic electrostatic model

Find and correct the mistake

Learner claim

A learner says NaCl has a larger lattice-energy magnitude than NaF because chlorine has more electrons and therefore makes the stronger bond. Identify and correct the error.

Choose the controlling factor

With charges equal, compare

View solution step by step

  1. Control the charges

    Method

    Recognise that both lattices contain 1 + and 1- ions.

    Reason

    The ionic charge product is identical.

    Working

    Charge does not distinguish NaF from NaCl.

  2. Compare radius

    Method

    Use the smaller radius of F⁻.

    Reason

    Closer opposite charges attract more strongly.

    Working

    NaF, not NaCl, has the larger lattice-energy magnitude.

Examiner practice 4

Define and apply lattice energy

4 marks

Problem

Define lattice energy using the formation convention, then explain why MgO has a larger lattice-energy magnitude than CaO. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Define the change

    1 mark

    Method

    State formation of one mole of ionic solid.

    Reason

    The molar amount distinguishes the defined enthalpy change.

    Working

    Formation of one mole of the ionic solid from its ions.

  2. State gaseous ions and conditions

    1 mark

    Method

    Specify separated gaseous ions under standard conditions.

    Reason

    The physical states and reference conditions are part of the reviewed definition.

    Working

    Separated gaseous ions form the solid under standard conditions.

  3. Compare radius

    1 mark

    Method

    State that Mg²⁺ is smaller than Ca²⁺ while charges are equal.

    Reason

    Smaller cation radius reduces separation between opposite charges.

    Working

    r(Mg²⁺) < r(Ca²⁺).

  4. Complete the attraction link

    1 mark

    Method

    Link shorter separation to stronger attraction and magnitude.

    Reason

    Stronger attraction releases more energy during lattice formation.

    Working

    MgO has the more negative value and larger lattice-energy magnitude.

Challenge 5

Infer ionic radius from lattice-energy data

Minimal support

Problem

Two ionic solids, MX and MY, contain the same cation and singly charged anions. Their lattice energies of formation are -860 and -710 kJ mol⁻¹ respectively. Deduce which anion is smaller.

Try this before viewing the solution

Hints

Hint 1: compare magnitude
Decide which negative value represents more energy released on lattice formation.
Hint 2: reverse the chain
With charges and cation fixed, stronger attraction implies shorter separation and a smaller anion.
View solution step by step

  1. Compare signed values and magnitudes

    Method

    Identify -860 kJ mol⁻¹ as more negative and larger in magnitude.

    Reason

    Its lattice formation releases more energy.

    Working

    |Δ Hₗₐₜₜ(MX)| > |Δ Hₗₐₜₜ(MY)|.

  2. Infer the structural cause

    Method

    Deduce that X⁻ is smaller than Y⁻.

    Reason

    Equal charges and the same cation leave anion radius as the differing factor; shorter separation gives stronger attraction.

    Working

    r(X⁻) < r(Y⁻).

Mind Stretchers

Attempt each unfamiliar application before opening the hint, then compare your reasoning chain with the solution.

Mind stretcher 1: Two factors changeExtension

Question. Predict whether MgF₂ or NaF has the larger lattice-energy magnitude.

Show Hint

The anion is unchanged; compare the cation charge and size.

Show Answer

Mg²⁺ has greater charge and is smaller than Na⁺. Both changes strengthen attraction, so MgF₂ has the larger lattice-energy magnitude.

Mind stretcher 2: Auditing imprecise languageExtension

Question. A student writes: “NaCl has a greater lattice energy than MgO because −787 is greater than −3795.” Rewrite this as a chemically useful comparison.

Show Hint

Distinguish numerical signed value from magnitude and explain the particle cause.

Show Answer

MgO has the more negative lattice energy and the larger lattice-energy magnitude. Its 2 + and 2- ions have a larger charge product than the 1 + and 1- ions in NaCl, so electrostatic attraction is stronger.