Question

For the parallel reaction, A→B and A→C, if B is the desired product and the reaction A→B is of positive order n1 and C is the undesired product and the reaction A→C is of negative order n2, then increasing the concentration of A _____

(a) Decreases desired product selectivity

(b) Increases desired product selectivity

(c) Initially increases and then decreases desired product selectivity

(d) Does not affect desired product selectivity

The question was posed to me by my school principal while I was bunking the class.

I would like to ask this question from Parallel Reactions Design in chapter Potpourri of Multiple Reactions, Design for Single & Parallel Reactions of Chemical Reaction Engineering

Answer 1

Right choice is (b) Increases desired product selectivity

To elaborate: Selectivity = \(\frac{Rate \, of \, B \, formed}{Rate \, of \, C \, formed} \)

Selectivity α\(\frac{C_A^{n1}}{C_A^{n2}}\) α C\(_A^{n1-n2}.\) As n2 is negative, increasing CA increases the selectivity of B.