How do Born-Haber cycles and enthalpies of solution let us quantify the energetics of ionic compounds?
Lattice enthalpy, Born-Haber cycles, the enthalpy changes of formation, atomisation, ionisation, electron affinity and lattice formation, and enthalpies of solution and hydration.
An Eduqas A-Level Chemistry PI4.1 answer on lattice enthalpy, Born-Haber cycles, the component enthalpy changes, and enthalpies of solution and hydration.
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What this topic is asking
Eduqas topic PI4.1 covers the energetics of ionic compounds: lattice enthalpy and its calculation through Born-Haber cycles, the component enthalpy changes (formation, atomisation, ionisation, electron affinity and lattice formation), and the enthalpies of solution and hydration that explain whether an ionic solid dissolves. It extends the Hess's-law reasoning of topic C2.2 to ionic systems.
Lattice enthalpy
The Born-Haber cycle
Lattice enthalpy cannot be measured directly, so it is found from a Born-Haber cycle, a Hess's-law cycle linking it to measurable enthalpy changes.
Enthalpies of solution and hydration
Hydration enthalpy is more exothermic for smaller, more highly charged ions, because water is attracted more strongly to them.
Examples in context
Example 1. Why MgO is refractory. Its very large (exothermic) lattice enthalpy from doubly charged, small ions means an enormous energy input is needed to break the lattice, so magnesium oxide has a very high melting point and lines furnaces.
Example 2. Instant cold packs. Ammonium nitrate has a positive (endothermic) enthalpy of solution because lattice breaking costs more than hydration releases; dissolving it absorbs heat, cooling a sports injury pack.
Try this
Q1. State two factors that make a lattice enthalpy more exothermic. [2 marks]
- Cue. Smaller ionic radius and greater ionic charge, both of which increase the electrostatic attraction between the ions.
Q2. Explain why the enthalpy of hydration of is more exothermic than that of . [2 marks]
- Cue. is smaller and more highly charged, so it attracts the polar water molecules more strongly, releasing more energy on hydration.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20195 marksUse the following data () to calculate the lattice enthalpy of sodium chloride: enthalpy of formation ; enthalpy of atomisation of Na ; first ionisation energy of Na ; enthalpy of atomisation of Cl ; electron affinity of Cl .Show worked answer →
By Hess's law around the Born-Haber cycle, the lattice formation enthalpy is the enthalpy of formation minus the sum of the other steps:
(2).
(3).
Markers reward the Born-Haber relationship, correct substitution and the final value (a large negative number for lattice formation).
Eduqas 20214 marks(a) Define the term lattice enthalpy (lattice formation). (b) Explain why the lattice enthalpy of magnesium oxide is more exothermic than that of sodium chloride.Show worked answer →
(a) The lattice (formation) enthalpy is the enthalpy change when one mole of an ionic solid is formed from its constituent gaseous ions under standard conditions (1).
(b) Magnesium oxide has and ions, whereas sodium chloride has and ions (1). The greater ionic charges (and smaller ionic radii) give stronger electrostatic attraction between the ions (1), so more energy is released when the lattice forms, making the lattice enthalpy more exothermic (1).
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Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)