9701 · 23.1
Lattice energy and Born-Haber cycles flashcards
Revision flashcards for Cambridge 9701 Lattice energy and Born-Haber cycles (syllabus 23.1). Flip, recall, then mark a real past-paper question.
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What is Standard Lattice Energy (ΔH°_latt)?
The standard enthalpy change when one mole of an ionic solid is formed from its constituent gaseous ions under standard conditions (298 K, 100 kPa). It is always an exothermic process.
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What is Standard Enthalpy of Formation (ΔH°_f)?
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
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What is Standard Enthalpy of Atomisation (ΔH°_at)?
The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state. It is always an endothermic process.
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What is First Ionisation Energy (IE₁)?
The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. It is always endothermic.
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What is First Electron Affinity (EA₁)?
The enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous 1- ions. It is usually exothermic for non-metals.
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Why is the Second Electron Affinity (e.g., for O⁻ → O²⁻) always endothermic?
Because you are adding a negative electron to an already negative ion (O⁻). Energy is required to overcome the electrostatic repulsion between the like charges.
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What is the fundamental principle behind a Born-Haber cycle?
Hess's Law, which states that the total enthalpy change for a reaction is independent of the pathway taken. The cycle equates the direct route (ΔH°_f) with the indirect route (sum of all other steps).
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How does ionic charge affect lattice energy?
Lattice energy becomes significantly more exothermic as the magnitude of the ionic charges increases. For example, MgO (Mg²⁺, O²⁻) has a much more exothermic lattice energy than NaCl (Na⁺, Cl⁻).
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How does ionic radius affect lattice energy?
Lattice energy becomes less exothermic as the ionic radius increases. Larger ions have a greater separation between their centres of charge, leading to weaker electrostatic attraction.
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What does a large difference between the experimental (Born-Haber) and theoretical lattice energy values indicate?
It indicates a significant degree of covalent character in the bonding. The theoretical value assumes a perfectly ionic model, so any deviation suggests the electron clouds are distorted and shared.
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Exam Trap: What is the enthalpy of atomisation of Chlorine, Cl₂(g)?
It is the enthalpy change for the reaction ½Cl₂(g) → Cl(g). It is equal to half the bond energy of the Cl-Cl bond. Be careful not to use the full bond energy unless you are forming 2 moles of Cl(g).
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In a Born-Haber cycle for MgCl₂, which enthalpy terms need to be doubled?
The enthalpy of atomisation of chlorine (to form 2Cl(g)), the electron affinity of chlorine (to form 2Cl⁻(g)), and the ionic radius of Cl⁻ is used. Note that for magnesium, you use both the first AND second ionisation energies (IE₁ + IE₂).