5.4.5Physical Chemistry and Transition Elements
The Effect of Ionic Charge and Radius on Enthalpy
5 min read 4 parts Lesson 25 of 40 Virtual lab
Part 1 of 4Electrostatic Attraction
Both lattice enthalpy and hydration enthalpy come from attraction between opposite charges.
Higher charge
- Greater charges on the ions give a stronger attraction, so lattice and hydration enthalpies are more exothermic.
Smaller radius
- Smaller ions allow their centres to approach closely, so the force between them is greater and enthalpy changes are more exothermic.
Charge density
- Charge divided by size indicates the strength of the field around an ion, which explains both effects.
Distance matters
- The force falls rapidly as separation increases, so radius of both cation and anion matters in the lattice.
Part 2 of 4Comparing Compounds
Use the same reasoning pattern for any comparison question.
1
Identify the ions in each compound and compare their charges first, since charge usually has the biggest effect.
2
Where the charges are equal, compare the ionic radii; a smaller sum of radii gives a more exothermic lattice enthalpy.
3
MgO has a much more exothermic lattice enthalpy than NaCl, because both ions have double the charge and Mg²⁺ is smaller than Na⁺.
4
Down a group, for example NaF, NaCl, NaBr, NaI, the anion grows larger, so the lattice enthalpy becomes less exothermic.
5
State the conclusion in terms of the strength of attraction between ions, not in terms of 'bonds being stronger' without explanation.
Part 3 of 4Hydration of Ions
Hydration of Ions
Mg²⁺ and Na⁺
- Mg²⁺ has a higher charge and smaller radius than Na⁺, so its hydration enthalpy is much more exothermic.
Group 1 cations
- Hydration enthalpy becomes less exothermic from Li⁺ to Cs⁺ as the ions get larger and charge density falls.
Halide anions
- F⁻ is hydrated more strongly than I⁻ because it is smaller and has a greater charge density.
Overall effect
- Solubility depends on the balance between the change in lattice enthalpy and the change in hydration enthalpy.
| Property | Increased charge | Increased radius |
|---|---|---|
| Lattice enthalpy | More exothermic | Less exothermic |
| Hydration enthalpy | More exothermic | Less exothermic |
Part 4 of 4Exam Checklist & Tips
5.4.5 – Key Points for the Exam
- Charge density The ratio of an ion's charge to its size.
- Ionic radius The size of an ion, which is smaller for a cation than its atom and larger for an anion.
- Lattice enthalpy The enthalpy change when one mole of an ionic lattice forms from gaseous ions.
- Hydration enthalpy The enthalpy change when gaseous ions become aqueous ions.
Common Mistakes
- Explaining a change in lattice enthalpy using the strength of 'covalent bonds' instead of ionic attraction.
- Comparing only the radius of one ion and ignoring that of the other ion in the lattice.
- Ignoring charge when comparing, for example claiming that NaF and MgO are similar because both contain small ions.
- Writing 'more exothermic' as 'higher' without saying whether the value is more negative.