Potassium iodide, KI, is used as a reagent in both inorganic and organic chemistry. Table 1.1 gives some data about the halide ions, $$\(\mathrm{Cl}^{-}, \mathrm{Br}^{-}\)$$and $$\(\mathrm{I}^{-}\)$$, and their potassium salts. (i) Explain the trend in the enthalpy change of hydration of the halide ions. ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ................................................................................................................................. (ii) The $$\(\Delta H_{\text {sol }}\)$$ values of these potassium halides are almost constant. Use the $$\(\Delta H_{\text {hyd }}\)$$ and $$\(\Delta H_{\text {latt }}\)$$ data in Table 1.1 to suggest why. ....................................................................................................................................... . ....................................................................................................................................... . ................................................................................................................................. (iii) The enthalpy change of solution of $$\(\mathrm{KI}(\mathrm{s})\)$$ is $$\(+21.0 \mathrm{~kJ} \mathrm{~mol}^{-1}\)$$. Use this information and the data in Table 1.1 to calculate the enthalpy change of hydration of the potassium ion, $$\(\mathrm{K}^{+}(\mathrm{g})\)$$. $$\[ \Delta H_{\text {hyd }} \text { of } \mathrm{K}^{+}(\mathrm{g})= \]$$ ........................................ $$\(\mathrm{kJ} \mathrm{mol}^{-1}\)$$ (iv) Solid $$\(\mathrm{PbI}_{2}\)$$ forms when $$\(\mathrm{KI}(\mathrm{aq})\)$$ is mixed with $$\(\mathrm{Pb}^{2+}(\mathrm{aq})\)$$ ions. The solubility product, $$\(K_{\mathrm{sp}}\)$$, of $$\(\mathrm{PbI}_{2}\)$$ is $$\(7.1 \times 10^{-9} \mathrm{~mol}^{3} \mathrm{dm}^{-9}\)$$ at $$\(25^{\circ} \mathrm{C}\)$$. Calculate the solubility, in $$\(\mathrm{moldm}^{-3}\)$$, of $$\(\mathrm{PbI}_{2}(\mathrm{~s})\)$$. solubility of $$\(\mathrm{PbI}_{2}(\mathrm{~s})=\)$$ ....................................... $$\(\mathrm{moldm}^{-3}\)$$ (v) The ionic radius of $$\(\mathrm{Pb}^{2+}\)$$ is 0.120 nm compared to 0.133 nm for $$\(\mathrm{K}^{+}\)$$. Suggest how the $$\(\Delta H_{\text {latt }}^{\ominus}\)$$ of $$\(\mathrm{PbI}_{2}(\mathrm{~s})\)$$ differs from $$\(\Delta H_{\text {latt }}^{\ominus}\)$$ of $$\(\mathrm{KI}(\mathrm{s})\)$$. Explain your answer. ....................................................................................................................................... . ....................................................................................................................................... . .................................................................................................................................
Exam No：9701_m24_qp_42 Year：2024 Question No：1(b)
Answer:
Knowledge points:
23.2.1 define and use the term enthalpy change with reference to hydration, and solution, Δ$\mathrm{H}_{sol}$
23.2.2 construct and use an energy cycle involving enthalpy change of solution, lattice energy and enthalpy change of hydration
23.2.3 carry out calculations involving the energy cycles in 23.2.2
23.2.4 explain, in qualitative terms, the effect of ionic charge and of ionic radius on the numerical magnitude of an enthalpy change of hydration
3.6.1.1 describe hydrogen bonding, limited to molecules containing N–H and O–H groups, including ammonia and water as simple examples
3.6.1.2.1 its relatively high melting and boiling points
3.6.1.2.2 its relatively high surface tension
3.6.1.2.3 the density of the solid ice compared with the liquid water
3.6.2 use the concept of electronegativity to explain bond polarity and dipole moments of molecules
3.6.3.1 describe van der Waals’ forces as the intermolecular forces between molecular entities other than those due to bond formation, and use the term van der Waals’ forces as a generic term to describe all intermolecular forces
3.6.3.2.1 instantaneous dipole – induced dipole (id-id) force, also called London dispersion forces
3.6.3.2.2 permanent dipole – permanent dipole (pd-pd) force, including hydrogen bonding
3.6.3.2.3 tdescribe hydrogen bonding and understand that hydrogen bonding is a special case of permanent dipole – permanent dipole force between molecules where hydrogen is bonded to a highly electronegative atom
3.6.4 stae that, in general, ionic, covalent and metallic bonding are stronger than intermolecular forces
Solution:
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