Is lattice energy exothermic or endothermic?

Exothermic (negative), as ionic attractions form.

Equation for the lattice energy of NaCl

Na⁺(g) + Cl⁻(g) → NaCl(s)

Define the enthalpy change of atomisation

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

Define the first electron affinity

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

Why is the second electron affinity of oxygen endothermic?

The incoming electron is repelled by the negative O⁻ ion.

Steps in order in a Born–Haber cycle

Atomisation, ionisation, electron affinity, lattice energy, with formation as the direct route.

Why can the lattice energy not be measured directly?

The solid cannot be formed directly from separated gaseous ions in a lab.

What changes when the metal forms a 2+ ion?

Both the first and second ionisation energies are included.

Lattice energy and Born-Haber cyclesEdexcel A-Level Chemistry: Flashcards

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Define lattice energy

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Define lattice energy: The energy change when one mole of an ionic solid is formed from its gaseous ions.
Is lattice energy exothermic or endothermic?: Exothermic (negative), as ionic attractions form.
Equation for the lattice energy of NaCl: Na⁺(g) + Cl⁻(g) → NaCl(s)
Define the enthalpy change of atomisation: The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state.
Atomisation equation for chlorine: ½Cl₂(g) → Cl(g)
Define the first electron affinity: The enthalpy change when one mole of gaseous atoms gains one electron each to form one mole of gaseous 1− ions.
Equation for the first electron affinity of chlorine: Cl(g) + e⁻ → Cl⁻(g)
Why is the second electron affinity of oxygen endothermic?: The incoming electron is repelled by the negative O⁻ ion.
Which law is used in a Born–Haber cycle?: Hess's law
Steps in order in a Born–Haber cycle: Atomisation, ionisation, electron affinity, lattice energy, with formation as the direct route.
Lattice energy of NaCl from the data: −787 kJ mol⁻¹
Why can the lattice energy not be measured directly?: The solid cannot be formed directly from separated gaseous ions in a lab.
What changes when the metal forms a 2+ ion?: Both the first and second ionisation energies are included.

Exam questions on Lattice energy and Born-Haber cycles

A student is studying the formation of sodium chloride, NaCl, from its elements using a Born–Haber cycle, which is an application of Hess's law.
Define the enthalpy change of atomisation of chlorine and write an equation, with state symbols, to represent it.2 marks
A student is constructing a Born–Haber cycle for sodium chloride with these data: standard enthalpy change of formation of NaCl(s) = −411 kJ mol⁻¹; enthalpy change of atomisation of sodium = +107 kJ mol⁻¹; enthalpy change of atomisation of chlorine, ½Cl₂(g) → Cl(g) = +122 kJ mol⁻¹; first ionisation energy of sodium = +496 kJ mol⁻¹; first electron affinity of chlorine = −349 kJ mol⁻¹.
Write an equation, with state symbols, for the lattice energy of sodium chloride and explain why this enthalpy change is exothermic.2 marks
A student is using a Born–Haber cycle to find the lattice energy of magnesium chloride, MgCl₂. The data are: standard enthalpy change of formation of MgCl₂(s) = −641 kJ mol⁻¹; enthalpy change of atomisation of magnesium = +148 kJ mol⁻¹; first ionisation energy of magnesium = +738 kJ mol⁻¹; second ionisation energy of magnesium = +1451 kJ mol⁻¹; enthalpy change of atomisation of chlorine, ½Cl₂(g) → Cl(g) = +122 kJ mol⁻¹; first electron affinity of chlorine = −349 kJ mol⁻¹.
Calculate the lattice energy of magnesium chloride.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).