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.
Some cards in this deck are on other syllabus topics: 23.3 Entropy change, ΔS and 23.4 Gibbs free energy change, ΔG. Each card is labelled with its topic.
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
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define the enthalpy change of hydration, ΔHhyd, and the enthalpy change of solution, ΔHₛₒₗ, and use both terms.
- Build and use an energy cycle that links the enthalpy change of solution, the lattice energy and the enthalpy change of hydration.
- Perform calculations with energy cycles that connect enthalpy change of solution, lattice energy and enthalpy change of hydration.
- Explain qualitatively how the charge and the radius of an ion affect the size of its enthalpy change of hydration.
Also in this deck: 23.3 Entropy change, ΔS
- Define entropy, S, as how many different ways a system's particles, and the energy they have, can possibly be arranged.
- Predict whether the entropy change is positive or negative, and explain why, for: (a) changes of state such as melting, boiling and dissolving, and the reverse processes; (b) a change in temperature; (c) a reaction where the number of gaseous molecules changes.
- Calculate a reaction's entropy change, ΔS⦵, from the S⦵ values (standard entropies) of its products and reactants: ΔS⦵ = Σ S⦵ of products − Σ S⦵ of reactants. You do not need to use ΔS⦵ = ΔS⦵sys + ΔS⦵surr.
Also in this deck: 23.4 Gibbs free energy change, ΔG
- State and apply the Gibbs relationship: ΔG⦵ = ΔH⦵ − TΔS⦵.
- Carry out calculations with ΔG⦵ = ΔH⦵ − TΔS⦵.
- Use the sign of ΔG to decide, and state, whether a process or reaction is feasible.
- Predict how a temperature change affects whether a reaction is feasible, when given the standard enthalpy change and standard entropy change.
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 these topics
- › 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.
Topic 23.2 · Enthalpies of solution and hydration
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.
Topic 23.2 · Enthalpies of solution and hydration
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.
Topic 23.2 · Enthalpies of solution and hydration
Draw an energy cycle that relates enthalpy change of solution, lattice energy, and enthalpy change of hydration for NaCl.
Route 1: NaCl(s) + aq -> Na⁺(aq) + Cl⁻(aq) (ΔHsol). Route 2: NaCl(s) -> Na⁺(g) + Cl⁻(g), which is the reverse of lattice formation (−ΔHlatt), then Na⁺(g) + aq -> Na⁺(aq) (ΔHhyd(Na⁺)) and Cl⁻(g) + aq -> Cl⁻(aq) (ΔHhyd(Cl⁻)). By Hess's law: ΔHsol = −ΔHlatt + ΔHhyd(Na⁺) + ΔHhyd(Cl⁻).
Topic 23.2 · Enthalpies of solution and hydration
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.
Topic 23.2 · Enthalpies of solution and hydration
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.
Topic 23.3 · Entropy change, ΔS
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.
Topic 23.3 · Entropy change, ΔS
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.
Topic 23.3 · Entropy change, ΔS
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.
Topic 23.4 · Gibbs free energy change, ΔG
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.
Topic 23.4 · Gibbs free energy change, ΔG
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.
Topic 23.4 · Gibbs free energy change, ΔG
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.
More Chemistry flashcards
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All Chemistry FlashcardsMore topics in Unit 23 — Chemical energetics
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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