Define hydration and solution enthalpy
Distinguish hydration of gaseous ions from dissolution of an ionic solid and connect both to an enthalpy cycle.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 23. Use the syllabus sign convention for lattice enthalpy (gaseous ions forming the ionic solid); state phases, signs, units and temperature. Written practice is self-checked
Before you begin
- Use AS enthalpy changes and Hess's law, formulae for ions, balanced equations, and temperature in kelvins.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which process is described by hydration enthalpy?
What you will learn
- Define hydration enthalpy for gaseous ions forming aqueous ions
- Define solution enthalpy for one mole of solute
- Relate solution, lattice and hydration steps in a Hess cycle
Hydration transfers gaseous ions into aqueous solution
Hydration enthalpy, ΔHhyd, is the enthalpy change when one mole of gaseous ions becomes aqueous ions. Hydration of an ion is usually exothermic because ion–dipole attractions form between the ion and water molecules.
The sign and amount depend on the ion's charge and size. Keep the ion charge and physical state explicit: Mg²⁺(g) → Mg²⁺(aq) is a different step from dissolving solid MgCl₂.
Solution enthalpy includes lattice separation and hydration
For one mole of an ionic solid, ΔHsol describes forming aqueous ions from the solid. The Hess route first separates the solid into gaseous ions, which is the reverse of lattice formation, then hydrates each ion.
With lattice enthalpy defined for gaseous ions → solid, ΔHsol = −ΔHₗₐₜ + ΣΔHhyd. The minus sign reverses the lattice-formation step. Check the arrows before using the formula; a book that defines lattice dissociation enthalpy uses a different written relation.
Pause and explain
If ΔHₗₐₜ is defined for gaseous ions forming a solid, what sign appears before it in the dissolution cycle?
Worked example
Write the dissolution equation for NaCl(s) and express its solution enthalpy using lattice-formation and hydration enthalpies.
Show the worked solution
- Dissolution forms aqueous Na⁺ and Cl⁻ from one mole of solid.
- Reverse the lattice-formation step to separate gaseous ions.
- Add hydration of both ions.
Answer NaCl(s) → Na⁺(aq) + Cl⁻(aq); ΔHsol = −ΔHₗₐₜ + ΔHhyd(Na⁺) + ΔHhyd(Cl⁻).
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define hydration enthalpy for Ca²⁺(g).
Give me a hint
State the amount, starting ion state and aqueous product state.
Compare my reasoning
- Use one mole of gaseous Ca²⁺ ions.
- Water molecules surround and stabilise the ion.
- The product is aqueous Ca²⁺ ions.
ΔHhyd(Ca²⁺) is the enthalpy change when one mole of Ca²⁺(g) becomes Ca²⁺(aq).
Look for these in your work
- One mole stated
- Gaseous ion used
- Aqueous ion product stated
Describe the Hess steps for dissolving one mole of MgCl₂(s).
Give me a hint
Separate the solid into its gaseous ions, then hydrate each ion in the formula ratio.
Compare my reasoning
- Reverse lattice formation to make Mg²⁺(g) + 2Cl⁻(g).
- Hydrate one mole of Mg²⁺ and two moles of Cl⁻.
- Sum all signed steps.
MgCl₂(s) first separates to Mg²⁺(g) + 2Cl⁻(g), then the ions hydrate to Mg²⁺(aq) + 2Cl⁻(aq). Thus ΔHsol = −ΔHₗₐₜ + ΔHhyd(Mg²⁺) + 2ΔHhyd(Cl⁻).
Look for these in your work
- Lattice reversed
- Ion ratio retained
- Both hydration terms included
A student writes ΔHsol = ΔHₗₐₜ + ΔHhyd(cation) + ΔHhyd(anion), using ΔHₗₐₜ for gas ions → solid. Identify the correction.
Give me a hint
Compare the sign of the lattice arrow with the first step in dissolution.
Compare my reasoning
- Dissolution must break the lattice.
- The lattice-formation arrow is the reverse direction.
- Reverse its sign before adding hydration terms.
Use ΔHsol = −ΔHₗₐₜ + ΔHhyd(cation) + ΔHhyd(anion), with stoichiometric coefficients as needed. Dissolution separates the solid, so the lattice-formation term must be reversed.
Look for these in your work
- Process direction checked
- Lattice sign corrected
- Hydration terms retained
Common mistakes
- Calling gaseous-ion hydration the same process as salt dissolution
- Using lattice formation without reversing its sign
- Forgetting to hydrate both ions
- Leaving the salt or ions without state symbols
What can you explain now?
Which process is described by hydration enthalpy?
Compare with the explanation
Gaseous ions become aqueous ions
Hydration surrounds gaseous ions with water molecules.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Why does a dissolution cycle reverse lattice formation before adding the hydration enthalpies?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.