HALOGENATION OF PROPANONE

Hint

The rate of a fast reaction will be represented by a large number, and the rate of a slow reaction will be represented by a small number.

The rate of any given reaction is a function of the rate constant and the reactant concentrations.

Therefore for a rate to be a large number, either the rate constant and/or the reactant concentrations must be large. Conversely for a rate to be a small number, either the rate constant and/or the reactant concentrations must be small.

In the case of the Halogenation of Propanone example, no limits have been placed on the reactant concentrations (i.e. the mechanism should remain valid for both low and high concentration scenarios). Thus it may be assumed that any fast reaction in this mechanism must have a large rate constant, and any slow reaction must have a small rate constant.

Example:

Comparing k_x[A] and k_y[B]², where k_x is large and k_y is small.

k_x[A] = (Large Number) x (Number) = Large Number

k_y[B]² = k_y[B][B] = (Small Number) x (Number) x (Number) = Small Number

Therefore, it can be assumed that k_x[A] >> k_y[B]²

Simplifying the Expression for the Overall Rate of Consumption of Iodine

The overall rate of consumption of iodine has been found to be

-d[I₂]/dt = + k₁k₂[H⁺][I₂][P_k] / ( k_-1[H⁺] + k₂[I₂])

It has been previously stated that the reversible reaction

So comparing the terms (k_-1[H⁺] and k₂[I₂]) on the bottom of the rate equation would lead to the simplifying assumption that

k_-1[H⁺] is approximately equal to k₂[I₂]

k_-1[H⁺] is much larger than k₂[I₂]

k_-1[H⁺] is much smaller than k₂[I₂]

Nothing can be assumed about the relative magnitudes of k_-1[H⁺] and k₂[I₂]