Question

Why are Kf values dependent upon temperature? Why are activity coefficients dependent upon temperature?

Answer 1

Activity is the measure of the effective concentration of any species in a mixture or solution.

The chemical potential of any species in a mixture depends on its activity in a real solution. This can be related to the chemical potential of any species dependent on its concentration in an ideal solution.

The reason of temperature dependence of activity is due to the fact that molecules in ideal mixtures (gases or solutions ) interact with each other. They either tend to repel or attract each other. At higher temperatures the kinetic energy of molecules is high so they interact less with each other. And so the effective concentration decreases (less availability for chemical reaction due to more scattered state of molecules) and hence the activity of the species. In ideal conditions,

Generally , activity of pure substances in condensed phases ( be it solid or liquid) is taken as unity .

Activities are generally considered to define equilibrium constants like kf(formation constant) in any chemical reaction.

Equilibrium constant =ratio of activities of product species to that of the reactant species, (both raised to the power of their coefficient in the balanced chemical equation).

For example, formation of metal-aqua complex,

Mⁿ⁺ +6H2O↔[M(H2O)]ⁿ⁺

kf=a([M(H2O)]ⁿ⁺)/a (Mⁿ⁺⁾

activity=ai= yi [C]i            where yi= activity coefficient of the species i and [C]i is the concentration of the ith species in solution.

When yi=1 (under ideal conditions) ai=[C]I, activity is replaced by concentration term

As activity is temperature dependent(explained above) except under ideal conditions, so is kf (defined by activity ) also temperature dependent.