Question

What is the pseudo-rate constant of the iodination of acetone? What is the pseudo-rate constant of the hydroxylation of crystal violet?

Answer 1

Iodination of acetone is given by:

CH₃COCH₃(aq) + I₂(aq) + H⁺(aq) → CH₃COCH₂I(aq) + I^-(aq) + 2H⁺(aq)

The reaction requires hydrogen ions to take place. Here hydrogen ions act as a catalyst and influences the rate of reaction.The rate law for the reaction is:

Rate = k[Ac]^m[H⁺]ⁿ[I₂]^p

Rate of reaction with respect to iodine is zero. So the rate is given as:

Rate = k[Ac]^m[H⁺]ⁿ

If the initial concentrations of acetone and hydrogen ion are much greater than iodine, then as reaction proceeds, the concentrations of acetone and hydrogen ion will not change during the reaction. the product of true rate constant k, and the initial concentrations of two reactants that do not change over time is known as pseudo rate constant.

The reaction between crystal violet and sodium hydroxide is given as:

CV⁺(aq) + OH^-(aq) → CVOH(aq)

The rate law for the reaction is given as:

Rate = k[CV⁺]^m[OH^-]ⁿ

Here m is the order with respect to crystal violet and n is the order with respect to the hydroxide ion. Since the hydroxide ion concentration is much more than the concentration of crystal violet, concentration of OH- will not change during the experiment. So we will find the order with respect to crystal violet, but not with respect to hydroxide. Therefore the rate constant determined will be pseudo rate constant.