Enthalpies of solution and hydration
Cambridge A-Level Chemistry (9701) · Unit 23: Chemical energetics · 11 flashcards
Enthalpies of solution and hydration is topic 23.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 23 — Chemical energetics , alongside Lattice energy and Born-Haber cycles. In one line: ΔHhyd is the enthalpy change when one mole of gaseous ions dissolves in water to form an infinitely dilute solution.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 11 flashcards — 4 definitions, 6 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 4 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Enthalpy change of hydration (ΔHhyd)
ΔHhyd is the enthalpy change when one mole of gaseous ions dissolves in water to form an infinitely dilute solution.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- define and use the term enthalpy change with reference to hydration, ΔHhyd, and solution, ΔHsol
- construct and use an energy cycle involving enthalpy change of solution, lattice energy and enthalpy change of hydration
- carry out calculations involving the energy cycles in 23.2.2
- explain, in qualitative terms, the effect of ionic charge and of ionic radius on the numerical magnitude of an enthalpy change of hydration
- define the term entropy, S, as the number of possible arrangements of the particles and their energy in a given system
- predict and explain the sign of the entropy changes that occur: (a) during a change in state, e.g. melting, boiling and dissolving (and their reverse) (b) during a temperature change (c) during a reaction in which there is a change in the number of gaseous molecules
- calculate the entropy change for a reaction, ΔS, given the standard entropies, S ⦵, of the reactants and products, ΔS ⦵ = ΣS ⦵ (products) – ΣS ⦵ (reactants) (use of ΔS ⦵ = ΔSsurr + ΔSsys is not required)
- state and use the Gibbs equation ΔG ⦵ = ΔH ⦵ – TΔS ⦵
- perform calculations using the equation ΔG ⦵ = ΔH ⦵ – TΔS ⦵
- state whether a reaction or process will be feasible by using the sign of ΔG
- predict the effect of temperature change on the feasibility of a reaction, given standard enthalpy and entropy changes
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Enthalpies of solution and hydration
- › Remember that enthalpy of formation is zero for elements, but atomisation requires finite energy to break covalent bonds in diatomic molecules.
- › Do not use a Data Booklet unless specifically instructed; use the values provided in the question paper for calculations.
- › Always ensure ΔS is converted from J K⁻¹ mol⁻¹ to kJ K⁻¹ mol⁻¹ to match the units of ΔH in the Gibbs equation.
- › Always calculate moles for both reactants to identify the limiting reagent before proceeding to gas volume calculations using the balanced equation.
- › Ensure definitions include specific molar quantities; neutralisation is the enthalpy change when one mole of water is formed from an acid-alkali reaction.
Define enthalpy change of hydration (ΔHhyd).
ΔHhyd is the enthalpy change when one mole of gaseous ions dissolves in water to form an infinitely dilute solution.
Define enthalpy change of solution (ΔHsol).
ΔHsol is the enthalpy change when one mole of a substance dissolves in water to form an infinitely dilute solution.
Draw an energy cycle that relates enthalpy change of solution, lattice energy, and enthalpy change of hydration for NaCl.
Cycle: NaCl(s) + H₂O(l) -> Na+(aq) + Cl-(aq) (ΔHsol). Alternative route: NaCl(s) -> Na+(g) + Cl-(g) (ΔHlattice) then Na+(g) + H₂O(l) -> Na+(aq) (ΔHhyd(Na+)) AND Cl-(g) + H₂O(l) -> Cl-(aq) (ΔHhyd(Cl-)). ΔHsol = ΔHlattice + ΔHhyd(Na+) + ΔHhyd(Cl-).
How does ionic charge affect the magnitude of the enthalpy change of hydration?
Higher ionic charge leads to a more negative (larger magnitude) enthalpy change of hydration. This is because more highly charged ions attract water molecules more strongly, releasing more energy.
How does ionic radius affect the magnitude of the enthalpy change of hydration?
Smaller ionic radius leads to a more negative (larger magnitude) enthalpy change of hydration. Smaller ions have a higher charge density, leading to stronger attraction to water molecules and greater energy release.
Define entropy (S).
Entropy (S) is a measure of the number of possible arrangements of particles and their energy in a system. Higher entropy corresponds to greater disorder or randomness.
Predict the sign of the entropy change (ΔS) for the boiling of water.
ΔS is positive. Boiling increases the disorder as liquid water becomes gaseous water, increasing the number of possible arrangements and energy distributions of the water molecules.
Calculate the entropy change for a reaction given standard entropies of reactants and products.
ΔS ⦵ = ΣS ⦵ (products) – ΣS ⦵ (reactants). Sum the standard entropies of all products and subtract the sum of standard entropies of all reactants, considering stoichiometric coefficients.
State the Gibbs equation.
The Gibbs equation is ΔG ⦵ = ΔH ⦵ – TΔS ⦵, where ΔG ⦵ is the Gibbs free energy change, ΔH ⦵ is the enthalpy change, T is the temperature in Kelvin, and ΔS ⦵ is the entropy change.
How does the sign of ΔG predict the feasibility of a reaction?
If ΔG is negative, the reaction is feasible (spontaneous). If ΔG is positive, the reaction is non-feasible (non-spontaneous). If ΔG is zero, the reaction is at equilibrium.
Predict the effect of increasing temperature on the feasibility of a reaction if ΔH is positive and ΔS is positive.
Increasing temperature will make the reaction more feasible (more likely to be spontaneous). Since ΔH is positive and ΔS is positive, as T increases, the -TΔS term becomes more negative, eventually making ΔG negative.
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Enthalpies of solution and hydration sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
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