Question

Predict the effects of (a) increasing the dielectric constant of a solvent, (b) increasing the ionic strength, and (c) increasing the pressure on the reactions of the following types:

A2+ +B- X+

A+ +B2- X3+

A +B A+ B-

Give a clear explanation in each case. What can you say about the entropy of activation to be expected in each case?

Answer 1

(a) Increasing the dielectric constant of a solvent:

Higher the dielectric constant of a solvent indicates that it is more polar. A polar solvent support the ionic species hence as the dielectric constant of solvent increases, the rate of ionic reactions also increases.

b) Increasing the ionic strength:

Ionic strength represents the total dissolved salt ions in solution. The opposite charged ions attract each other therefore an anion is surrounded by cation and a cation is surrounded by anion. As the ionic strength increases, the attraction force between ions also increases hence the ability of ion to react with other reactant decreases that reduces the rate of reaction for all the following reactions.

(c) Increasing the pressure on the reactions:

At high pressure the reaction will move in that direction which can reduce the pressure of reaction mixture. Hence it is applicable only for the gaseous reactions not for reactions in solution or liquid.

The entropy of activation of a reaction gives idea about molecularity of the rate determining step in a reaction. The positive values entropy of activation indicates that entropy increases during the formation of transition state. Hence the dissociative mechanism of activated complex will feasible. The negative values for entropy of activation indicate that entropy decreases during the formation of transition state. Hence the associative mechanism will be feasible.