CAIE A-Level Chemistry 23.1.5 Ionic Charge, Radius and Lattice Energy

Practise comparing lattice-energy magnitudes from ionic charge and radius and explaining electrostatic attraction.

Exam points

predict a more exothermic formation lattice energy when the product of ionic charges is larger
predict a less exothermic value as ionic radii and interionic distance increase
compare compounds by holding one factor constant before attributing the difference to charge or size

[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

The ionic radius of $\mathrm{Pb}^{2+}$ is 0.120 nm compared to 0.133 nm for $\mathrm{K}^{+}$ .

Suggest how the $\Delta H_{\text {latt }}^{\ominus}$ of $\mathrm{PbI}_{2}(\mathrm{~s})$ differs from $\Delta H_{\text {latt }}^{\ominus}$ of KI(s).
Explain your answer.

$\mathrm{Pb}^{2+}$ has a greater charge

- $\mathrm{Pb}^{2+}$ is smaller (than $\mathrm{K}^{+}$ ) $\mathbf{OR}\left(\mathrm{Pb}^{2+}\right)$ smaller ionic radius

- greater attraction between $\mathrm{Pb}^{2+}$ and $\mathrm{I}^{-}$

OR ionic bond between $\mathrm{Pb}^{2+}$ and $\mathrm{I}^{-}$ is stronger

OR lattice energy is more exothermic / more negative

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