Q.(a) Write the properties of ionic crystals.

(a) Ionic crystals owe their properties (high melting point, hardness/brittleness, conductivity only in molten/dissolved state, solubility in polar solvents) to the strong, non-directional electrostatic forces holding ions in a rigid lattice. (b) The adsorption theory explains heterogeneous catalysis as a five-step surface process: diffusion, adsorption, surface reaction, desorption, and diffusion away.

(a) Properties of ionic crystals

1. High melting and boiling points: the ions are held together by strong, non-directional electrostatic (Coulombic) forces throughout the lattice, so a large amount of thermal energy is needed to overcome the lattice energy and separate the ions.

2. Hard but brittle: the strong electrostatic bonding in all directions makes the crystal hard; however, a mechanical shearing force displaces one layer of ions relative to the next so that like-charged ions come to face each other — the resulting strong repulsion shatters the crystal, making ionic solids brittle rather than malleable.

3. Electrical conductivity: in the solid state the ions are locked in fixed lattice positions and cannot move, so ionic crystals do not conduct electricity. When melted, or dissolved in a polar solvent like water, the ions become free to move under an applied electric field, so the molten salt or its aqueous solution conducts electricity well (electrolytic conduction).

4. Solubility: ionic crystals are generally soluble in polar solvents such as water (the polar solvent molecules solvate/hydrate the individual ions, overcoming lattice energy) but are insoluble in non-polar solvents (which cannot stabilise separated ions).

5. Crystal geometry and lattice energy: ionic solids have a well-defined, ordered three-dimensional arrangement of cations and anions (crystal lattice) characteristic of the compound, and possess high lattice energy (the energy released when gaseous ions come together to form the solid lattice).

6. Low volatility: because of the strong inter-ionic forces, ionic solids have negligible vapour pressure at ordinary temperatures.

(b) Adsorption theory of catalysis

This theory explains how a solid (heterogeneous) catalyst speeds up a gas- or liquid-phase reaction occurring at its surface, in five sequential steps:

1. Diffusion of reactants: reactant molecules move from the bulk phase to the surface of the catalyst.

2. Adsorption: the reactant molecules are adsorbed (usually chemisorbed) onto "active sites" on the catalyst surface, held there by residual/unsatisfied valence forces of surface atoms. This adsorption increases the local concentration of reactants at the surface and can also weaken the bonds within the reactant molecules.

3. Reaction on the surface: the adsorbed reactant molecules, now in close proximity and with weakened internal bonds, react with each other on the surface to form an activated surface complex (or intermediate); this surface pathway has a much lower activation energy than the corresponding uncatalysed gas/liquid-phase pathway, which is why the reaction rate increases.

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