Define - Polymorphism.(b) State and explain Hess’s law of constant summation. Write its one application.

Answer:

(a) Polymorphism: the existence of a substance (compound) in two or more different crystalline forms, e.g. calcium carbonate as calcite and aragonite; SiO₂ as quartz, tridymite and cristobalite. (For elements it is called allotropy.)

(b) Hess’s law: if a chemical reaction can be carried out in one step or in a series of steps, the total enthalpy change of the overall reaction is the same, irrespective of the path, i.e. the enthalpy change is the algebraic sum of the enthalpy changes of the individual steps.

Explanation: enthalpy is a state function, so ΔH depends only on the initial and final states. Example: formation of CO₂ from carbon.

Path I (one step): C(graphite) + O₂(g) → CO₂(g); ΔH = –393.5 kJ

Path II (two steps): C(graphite) + ½O₂(g) → CO(g); ΔH₁ = –110.5 kJ
CO(g) + ½O₂(g) → CO₂(g); ΔH₂ = –283.0 kJ

ΔH₁ + ΔH₂ = –110.5 + (–283.0) = –393.5 kJ = ΔH. The total enthalpy change is the same by both paths.

Application (any one): to calculate enthalpy changes that cannot be measured directly, e.g. the enthalpy of formation of CO, enthalpies of formation of compounds from enthalpies of combustion, enthalpies of phase transitions (graphite → diamond) and lattice energies.