Q.(a)

(a) Molecular crystals are weakly-bound, soft, low-melting non-conductors, while ionic compounds are hard, brittle, high-melting solids that conduct only when molten or dissolved. (b) Substituting $P=[\,]RT$ into the $K_p$ expression gives $K_p=K_c(RT)^{\Delta n}$.

(a)(i) Molecular crystals: In a molecular crystal, the lattice points are occupied by discrete, individual molecules (not ions, and not a continuous covalent network), held together by comparatively weak intermolecular forces - London/van der Waals dispersion forces (e.g. in $I_2$, dry ice/$CO_2$), dipole-dipole forces (e.g. in solid $SO_2$), or hydrogen bonds (e.g. in ice). Because these binding forces are much weaker than ionic or covalent bonds, molecular crystals are typically soft, have low melting and boiling points, are often volatile, and are poor conductors of electricity in every physical state (since there are no free ions or delocalised electrons). Common examples: ice ($H_2O$), dry ice ($CO_2$), iodine ($I_2$), naphthalene.

(a)(ii) Six characteristics of ionic compounds:

1. High melting and boiling points, due to the strong electrostatic (Coulombic) forces of attraction holding the oppositely charged ions together in the crystal lattice.
2. Hard but brittle - hard because of the strong lattice forces, but brittle because a slight displacement of ion layers brings like-charged ions face to face, causing repulsion and fracture.
3. Exist as crystalline solids at room temperature (never as liquids or gases under normal conditions), with a well-defined, ordered lattice geometry.
4. Non-volatile, with negligible vapour pressure at room temperature.
5. Generally soluble in polar solvents (like water, which can solvate/hydrate the ions), but insoluble (or poorly soluble) in non-polar solvents (like benzene or petrol).
6. Do not conduct electricity in the solid state (ions are fixed in the lattice), but do conduct when molten or dissolved in water, since the ions then become mobile and free to carry charge - i.e. they are electrolytes.

(b) Derivation of the relation between $K_p$ and $K_c$:

For a general gaseous equilibrium reaction:

$aA(g) + bB(g) \rightleftharpoons cC(g) + dD(g)$

The equilibrium constants in terms of partial pressure and molar concentration are:

$K_p = \frac{P_C^{\,c}\,P_D^{\,d}}{P_A^{\,a}\,P_B^{\,b}}, \qquad K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$

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