CAIE A-Level Chemistry 23.2.4 Ionic Charge, Radius and Hydration Enthalpy

Practise comparing hydration enthalpies from ionic charge density and ion–dipole attraction to water.

Exam points

predict more exothermic hydration for higher ionic charge because attraction to water is stronger
predict less exothermic hydration for larger ions with lower charge density
compare Cl− and NO3− or related ions by linking smaller radius to stronger ion–dipole attraction

[Maximum number: 2]

Potassium iodide, KI, is used as a reagent in both inorganic and organic chemistry.

Table 1.1 gives some data about the halide ions, $\mathrm{Cl}^{-}, \mathrm{Br}^{-}$ and $\mathrm{I}^{-}$ , and their potassium salts.

Table 1.1

Explain the trend in the enthalpy change of hydration of the halide ions.

anionic charge density decreases (down the group / $\mathrm{Cl}^{-}$ to $\mathrm{I}^{-}$ )

(so hydration enthalpies become less negative / less exothermic because)

less attraction of ion to water / the dipole-ion force weakens

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23.2.4—Explain, in qualitative terms, the effect question 1