4.3.6Unit 4. Rates, Equilibria and Further Organic Chemistry
Enthalpy of Solution – Ionic Charge and Radius — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 28 of 56 Unit quiz
Section 1 of 4Effect on Lattice Energy
Lattice energy depends on the strength of attraction between oppositely charged ions.
Higher charge
- Ions with larger charges attract each other much more strongly, so the lattice energy becomes much more exothermic.
Smaller radius
- Small ions can approach each other more closely, which strengthens the attraction and makes the lattice energy more exothermic.
Charge density
- Both effects combine in charge density, the charge divided by the size of the ion, which controls the strength of attraction.
Example
- MgO has a far more exothermic lattice energy than NaCl because both ions carry a double charge and are small.
Section 2 of 4Effect on Hydration Enthalpy
Water molecules are attracted to ions in the same way, through ion–dipole forces.
Effect on Hydration Enthalpy
Charge
- A highly charged ion attracts water dipoles more strongly, so its hydration enthalpy is more exothermic.
Radius
- A small ion allows water molecules to get closer, so the hydration enthalpy is more exothermic than for a large ion.
Down Group 2
- Hydration enthalpy of the cations becomes less exothermic from Mg²⁺ to Ba²⁺ as the ions get larger.
Down Group 1
- The same trend applies, with Li⁺ having the most exothermic hydration enthalpy of the alkali metal ions.
Section 3 of 4Predicting the Net Effect
Enthalpy of solution depends on the balance between the two changes.
1
Both lattice energy and hydration enthalpy become more exothermic as charge density rises, so the effects partly cancel.
2
The overall enthalpy of solution depends on which of the two terms changes more quickly with ion size.
3
For Group 2 sulfates the anion is large, so the lattice energy changes little but the cation hydration enthalpy changes a lot.
4
The sulfates therefore become less soluble down the group because the enthalpy of solution becomes less favourable.
5
For Group 2 hydroxides the small anion makes the lattice energy fall faster, so solubility increases down the group.
Section 4 of 4Exam Checklist & Tips
4.3.6 – Key Points for the Exam
- Charge density Charge on an ion divided by its radius or volume.
- Ionic radius The size of an ion, which determines how closely ions can approach.
- Ion–dipole force Attraction between an ion and a polar water molecule.
- Group 2 trend Systematic change in properties of Mg to Ba compounds.
Common Mistakes
- Saying lattice energy depends on charge only; radius matters as well.
- Claiming smaller ions have less exothermic hydration enthalpy.
- Forgetting that the larger of the two ions in a compound controls how much the lattice energy changes.
- Quoting a solubility trend without linking it to enthalpy of solution.
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