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Enthalpy
2017 Paper One
Question One
Part of a series looking at past AQA A-level Chemistry questions with a focus on better performance in examinations.
If you haven't gone through the question/paper, click the direct link below, and have a go...
AQA A-level CHEMISTRY Paper 1 Inorganic and Physical Chemistry
Tuesday 13 June 2017
An 8-mark, 4-part question
One of those things you just have to learn by rote:
It's the change in enthalpy/heat when 1 mole of a solid ionic lattice is formed from its gaseous ions.
In your definition, you need to state a solid is formed from gaseous ions.
The heat energy used to break up 1 mole of the lattice is the lattice dissociation enthalpy. The heat energy released when new bonds are made between the ions and water is the hydration enthalpy of the ion. The hydration enthalpy is the enthalpy change when 1 mole of gaseous ions dissolve and is always negative.
lattice dissociation energy = (112 + 464 + 293 ) = + 869 (kJ/mol)
lattice formation energy = − 869 (kJ/mol)
Your calculation will show the lattice dissociation energy but the question wants formation energy - reverse the sign.
Silver iodide contains some covalent character therefore the forces/bonds (holding the lattice together) are stronger
Covalent character = stronger bonding
This is the test for halide ions - silver nitrate solution would give a pale yellow precipitate.
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