4.3.4Unit 4. Rates, Equilibria and Further Organic Chemistry

Enthalpy of Solution – Key Terms — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 4 parts Lesson 26 of 56 Unit quiz

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Part 1 of 4The Three Definitions

Each term describes one stage in turning a solid ionic compound into aqueous ions.

Lattice energy

The enthalpy change when one mole of an ionic solid forms from its gaseous ions under standard conditions, so it is always exothermic.

Enthalpy of hydration

The enthalpy change when one mole of a gaseous ion dissolves in water to form one mole of aqueous ions,
- which is always exothermic.

Enthalpy of solution

The enthalpy change when one mole of an ionic solid dissolves in enough water that further dilution gives no extra heat change.

Standard conditions

A pressure of 100 kPa, a stated temperature usually 298 K, and each substance in its standard state.

Dissolving can be imagined as two steps even though it happens at once.

Energy must be supplied to pull the ions of the lattice apart into separate gaseous ions, which is the reverse of lattice formation.

The separated gaseous ions are then surrounded by water molecules, and ion–dipole attractions release energy.

Water molecules orient their δ⁻ oxygen toward cations and their δ⁺ hydrogen atoms toward anions.

The overall enthalpy of solution is the sum of the energy put in to break the lattice and the energy released on hydration.

If hydration releases more energy than breaking the lattice costs, the dissolving is exothermic; otherwise it is endothermic.

Comparing the Terms

Sign of lattice energy

As defined by formation of the lattice it is negative;
- the breaking process has the same size with the opposite sign.

Sign of hydration

Forming bonds between water and gaseous ions always releases energy, so hydration enthalpies are negative.

Sign of solution

It can be positive or negative, depending on which of the two opposing energy terms is larger.

Gaseous ions

Lattice energy and hydration both start or finish with separated gaseous ions, which links them in a cycle.

Term	Starts from	Ends with	Sign
Lattice energy	Gaseous ions	Ionic solid	Negative
Hydration	Gaseous ion	Aqueous ion	Negative
Solution	Ionic solid	Aqueous ions	Either

Lattice energy (formation) Enthalpy change when one mole of an ionic solid forms from its gaseous ions.
Enthalpy of hydration Enthalpy change when one mole of gaseous ions becomes aqueous ions.
Enthalpy of solution Enthalpy change when one mole of solid dissolves in water to give aqueous ions.
Ion–dipole attraction Attraction between an ion and the partial charge on a water molecule.

Forgetting that the definition of lattice energy specifies gaseous ions as the starting point.
Giving hydration enthalpy a positive sign; bond formation releases energy.
Missing the phrase one mole in a definition.
Believing a compound that dissolves must have a negative enthalpy of solution;
- entropy also drives dissolving.

Last updated: 28 October 2026