Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.17 Energy cycles for ΔH_solution-Study Notes - New Syllabus

12.17 Enthalpy Change of Solution using Energy Cycles

The enthalpy change of solution (\( \mathrm{\Delta_{sol}H} \)) can be calculated using an energy cycle that relates lattice energy and hydration enthalpies. This is another application of Hess’s Law, where the total enthalpy change is independent of the pathway taken.

Key Idea (Energy Cycle)

Dissolving an ionic solid can be broken into two steps:

• Breaking the ionic lattice into gaseous ions (endothermic).
• Hydrating the gaseous ions to form aqueous ions (exothermic).

Energy Cycle Relationship

\( \mathrm{\Delta_{sol}H = \Delta_{latt}H (dissociation) + \sum \Delta_{hyd}H} \)

Note:

• Lattice energy is often given as formation (exothermic), so must be reversed (sign changed) when used for dissociation.
• Hydration enthalpies are always negative.

Energy Level Diagram (Concept)

• Start with solid ionic compound at a low energy level.
• Move up (endothermic) to gaseous ions (lattice dissociation).
• Move down (exothermic) to aqueous ions (hydration).
• Overall change gives \( \mathrm{\Delta_{sol}H} \).

Key Features

• Based on Hess’s Law.
• Requires careful handling of signs.
• Balance between lattice energy and hydration enthalpy determines overall value.
• Can be visualised using an energy cycle or diagram.