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A2 · Practice questions · Lattice energy and Born-Haber cycles

Close the cycle.

Six original Cambridge-style questions on lattice energy and Born-Haber cycles.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Gaseous ions in, solid lattice out.

01
Recall
[3 marks]

Define (a) lattice energy, (b) first electron affinity, (c) enthalpy change of atomisation. Give an equation for each, using sodium chloride, chlorine and sodium.

  • (a) The enthalpy change when 1 mol of an ionic compound is formed from its gaseous ions under standard conditions: Na+(g) + Cl−(g) → NaCl(s). ✓
  • (b) The enthalpy change when 1 mol of electrons is added to 1 mol of gaseous atoms to form 1 mol of gaseous 1− ions: Cl(g) + e− → Cl−(g). ✓
  • (c) The enthalpy change when 1 mol of gaseous atoms is formed from the element in its standard state: Na(s) → Na(g). ✓
02
Calculation
[3 marks]

Calculate the lattice energy of NaCl. Data (kJ mol−1): ΔHf(NaCl) −411; ΔHat(Na) +107; IE1(Na) +496; Cl–Cl bond energy +244; EA1(Cl) −349.

  • ΔHat(Cl) = ½ × 244 = +122. ✓
  • Sum of the other steps = 107 + 496 + 122 − 349 = +376. ✓
  • LE = −411 − 376 = −787 kJ mol−1. ✓
03
Calculation
[3 marks]

Calculate the lattice energy of MgCl2. Data (kJ mol−1): ΔHf(MgCl2) −641; ΔHat(Mg) +148; IE1(Mg) +736; IE2(Mg) +1450; ΔHat(Cl) +122; EA1(Cl) −349.

  • Two Cl atoms are needed: 2 × 122 = +244 and 2 × (−349) = −698. ✓
  • Sum of the other steps = 148 + 736 + 1450 + 244 − 698 = +1880. ✓
  • LE = −641 − 1880 = −2521 kJ mol−1. ✓
04
Explain
[2 marks]

Explain why the second electron affinity of oxygen is positive, even though the first is negative.

  • The first electron is attracted by the nucleus of the neutral O atom, so energy is released. ✓
  • The second electron is added to a negative O− ion and is repelled, so energy must be supplied (+798 kJ mol−1). ✓
05
Explain
[3 marks]

The lattice energies of NaCl and MgO are −787 and −3841 kJ mol−1. Explain the difference.

  • Mg2+ and O2− carry twice the charge of Na+ and Cl−. ✓
  • Mg2+ is smaller than Na+ and O2− is smaller than Cl−, so the ions are closer. ✓
  • Both give much stronger electrostatic attraction, so far more energy is released when the lattice forms. ✓
06
Analysis
[2 marks]

Place KCl, NaCl and NaBr in order of increasingly exothermic lattice energy and explain.

  • KCl (−718) < NaBr (−747) < NaCl (−787). ✓
  • All ions are singly charged, so the order follows ionic size: K+ is larger than Na+ and Br− larger than Cl−, and larger ions attract less strongly. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.