Question

123 A strong correlation has been found between volumes and entropies of activation for reactions in aqueous solution (K. Laidler and D. T Y. Chen, Can. Chen. 37,599 (1959)J. What is the nature of this correlation? What is its chemical significance?

Answer 1

Io VOLUME OF ACTIVATION 0 (cm2 per mole) -20 -40 20 20 ENTROPY OF ACTIVATION (cal. deg: mole')

From the graph of Volume of activation VS Entropy of activation for reaction in aqueous solutions show fairly good linear correlation.

Chemical significance :

for reactions in which ions or strong dipoles are concerned the solvent effects are generally more important than the structural ones. The volume of activation depends on electrostriction while the activation entropy depends on the strength of the chemical bonds.

If a reaction takes place such that ions of the same sign approach or ions of opposite sign separate, there is an intensification of the electric field. Thus electrostriction increases leading to a lowered volume of activation. There is also a decrease in entropy due to the loss of freedom of the solvent molecules.

In case Ions of opposite charge approach then the electric field is weakened when the activated complex is formed, there is some release of bound solvent molecules and the volume of activation and entropy of activation are positive.

In suggesting if a reaction is slow normal or fast the volume of activation is more reliable compared to the entropy of activation as entropy is highly dependent on loosening and strengthening of chemical bonds while volume is dependent on electrostriction. Thus volume of activation is more dependable.