Define : (a) Isotonic solution (b) Molecularity of reaction(ii) State and explain Hess’s law of constant heat summation.

Answer:

(i)(a) Isotonic solutions: Two or more solutions having the same osmotic pressure at the same temperature are called isotonic solutions (e.g. 0.9% NaCl solution is isotonic with blood). No osmosis occurs between them when separated by a semipermeable membrane.

(b) Molecularity of a reaction: The number of reactant molecules (atoms or ions) that take part in an elementary step and collide simultaneously to bring about the reaction is called the molecularity of that elementary reaction. It is always a whole number (1, 2 or rarely 3), e.g. 2HI → H₂ + I₂ is bimolecular.

(ii) Hess’s law of constant heat summation: The overall enthalpy change of a reaction is the same whether the reaction takes place in one step or in several steps. In other words, the enthalpy change of a reaction is the sum of the enthalpy changes of the individual steps: \(\Delta H = \Delta H_1 + \Delta H_2 + \Delta H_3 + \dots\)

Explanation: Enthalpy is a state function, so \(\Delta H\) depends only on the initial and final states and not on the path. Example — formation of CO₂ from carbon:

Route I (one step): C_(graphite) + O_2(g) → CO_2(g); ΔH = −393.5 kJ

Route II (two steps):
C_(graphite) + ½O_2(g) → CO_(g); ΔH₁ = −110.5 kJ
CO_(g) + ½O_2(g) → CO_2(g); ΔH₂ = −283.0 kJ

ΔH₁ + ΔH₂ = −110.5 + (−283.0) = −393.5 kJ = ΔH, as Hess’s law predicts. The law lets us calculate enthalpy changes (such as the enthalpy of formation of CO) that cannot be measured directly, by adding and subtracting thermochemical equations like algebraic equations.