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A-Level Enthalpy, entropy and the feasibility of reactions: worked solution
2 marks. Full working, one step per line.
Question
When HCl is neutralised by NaOH the NaCl produced has a standard enthalpy of formation of −411 kJ mol⁻¹, and the enthalpy change of atomisation of sodium metal is measured as +108 kJ mol⁻¹. Using this salt as your reference, define the term lattice enthalpy and include an equation in your definition.
Worked answer
Lattice enthalpy is the enthalpy change when one mole of a solid ionic compound is formed from its constituent gaseous ions (under standard conditions). It is an exothermic quantity. For sodium chloride: Na+(g) + Cl-(g) -> NaCl(s) (State symbols essential; the given values of formation and atomisation enthalpies are not needed for the definition itself.)
Practise this topic
This question is part of A-Level Enthalpy, entropy and the feasibility of reactions, in A-Level H2 Chemistry.
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