Define enthalpy of lattice dissociation.

The enthalpy change when one mole of an ionic compound is dissociated into its gaseous ions. It is endothermic.

Define enthalpy of atomisation.

The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state.

Define first ionisation energy.

The enthalpy change when one mole of gaseous atoms loses one mole of electrons to form one mole of gaseous 1+ ions.

Define first electron affinity.

The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1− ions.

Define enthalpy of formation.

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Define bond dissociation enthalpy.

The enthalpy change when one mole of a covalent bond is broken in gaseous molecules.

Why is the second electron affinity of oxygen endothermic?

The electron is added to a negative ion (O⁻), so there is repulsion that must be overcome.

Which law is used to construct a Born–Haber cycle?

Hess's law: the enthalpy change is independent of the route taken.

Write the Hess's law equation for NaCl in a Born–Haber cycle.

ΔHf = ΔHat(Na) + IE1(Na) + ΔHat(Cl) + EA(Cl) + ΔHlattice formation.

What changes in the Born–Haber cycle for MgCl₂ compared with NaCl?

Add the second ionisation energy of magnesium, and double the atomisation and electron affinity of chlorine.

What does the perfect ionic model assume?

Ions are perfect spheres with evenly spread charge, held together only by electrostatic attraction.

What does a Born–Haber value more exothermic than the perfect ionic model value show?

The compound has covalent character, because polarisation of the anion adds extra stabilisation.

Born-Haber cycles and lattice enthalpyAQA A-Level Chemistry: Flashcards

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Define enthalpy of lattice formation.

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Define enthalpy of lattice formation.: The enthalpy change when one mole of an ionic compound is formed from its gaseous ions. It is exothermic.
Define enthalpy of lattice dissociation.: The enthalpy change when one mole of an ionic compound is dissociated into its gaseous ions. It is endothermic.
Define enthalpy of atomisation.: The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state.
Define first ionisation energy.: The enthalpy change when one mole of gaseous atoms loses one mole of electrons to form one mole of gaseous 1+ ions.
Define first electron affinity.: The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1− ions.
Define enthalpy of formation.: The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
Define bond dissociation enthalpy.: The enthalpy change when one mole of a covalent bond is broken in gaseous molecules.
Why is the second electron affinity of oxygen endothermic?: The electron is added to a negative ion (O⁻), so there is repulsion that must be overcome.
Which law is used to construct a Born–Haber cycle?: Hess's law: the enthalpy change is independent of the route taken.
Write the Hess's law equation for NaCl in a Born–Haber cycle.: ΔHf = ΔHat(Na) + IE1(Na) + ΔHat(Cl) + EA(Cl) + ΔHlattice formation.
What changes in the Born–Haber cycle for MgCl₂ compared with NaCl?: Add the second ionisation energy of magnesium, and double the atomisation and electron affinity of chlorine.
What does the perfect ionic model assume?: Ions are perfect spheres with evenly spread charge, held together only by electrostatic attraction.
What does a Born–Haber value more exothermic than the perfect ionic model value show?: The compound has covalent character, because polarisation of the anion adds extra stabilisation.

Exam questions on Born-Haber cycles and lattice enthalpy

A student is constructing a Born–Haber cycle for magnesium oxide, MgO, starting from the elements in their standard states. The cycle contains the enthalpy of formation of MgO, the enthalpies of atomisation of magnesium and oxygen, the first and second ionisation energies of magnesium, the first and second electron affinities of oxygen, and the lattice enthalpy of MgO.
The second electron affinity of oxygen is endothermic. Explain why.2 marks
Born–Haber data are used to find the lattice enthalpy of magnesium chloride, MgCl₂. Standard enthalpy changes (kJ mol⁻¹): enthalpy of formation of MgCl₂ = −641; enthalpy of atomisation of Mg = +148; first ionisation energy of Mg = +738; second ionisation energy of Mg = +1451; enthalpy of atomisation of Cl = +122 (per mole of Cl atoms); first electron affinity of Cl = −349.
Write an equation, including state symbols, for the second ionisation energy of magnesium. Explain why it is larger than the first ionisation energy.2 marks
The enthalpy of formation of calcium oxide, CaO, is −635 kJ mol⁻¹. Other standard enthalpy changes (kJ mol⁻¹): atomisation of Ca = +178; first ionisation energy of Ca = +590; second ionisation energy of Ca = +1145; atomisation of oxygen, ½O₂(g) → O(g) = +249; first electron affinity of oxygen = −141. The enthalpy of lattice formation of CaO is −3401 kJ mol⁻¹.
Calculate the second electron affinity of oxygen. Show your working.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).