Multiple Reactions and Selectivity Questions and Answers

Answer: A

Correct answer: low concentration favours the wanted reaction

Feeding a reactant slowly keeps its concentration low throughout, since it is consumed almost as fast as it arrives. That is the semi-batch reactor’s purpose: it delivers the low-concentration condition a stirred tank gives, while retaining batch operation and the control of a slow addition. It helps only when low concentration is what selectivity wants - where the unwanted reaction has the higher order.

Answer: B

Both are true and the reason gives the shared mechanism, and it is common for the same feed policy to be justified on both grounds at once: the slow addition limits how fast heat can be released and at the same time keeps the reactant concentration where selectivity is best.

Answer: C

The kinetics must be known before any policy can be chosen, and the reactor is selected to deliver the policy rather than the other way round. Reversing the order leads to a reactor chosen for habit. Kinetics come before hardware.

Answer: D

Chlorination and nitration schemes are the classic examples, where each successive substitution competes with the last. Both selectivity problems must be handled at once. Successive substitutions compete with each other.

Answer: A

The first two are correct. The third is optimistic, since the concentration and temperature that each favour may point in different directions and a compromise is then needed. Each policy follows from its own kinetics.

Answer: B

Correct answer: True

Where a wanted product is worth much more than the reactant, and the unconverted reactant can be separated and recycled, a reactor running at modest conversion with excellent selectivity is worth far more than one running to completion with poor selectivity. Selectivity losses are permanent - the material has become something else - while unconverted reactant is only a separation cost.

Answer: C

The reactor is operated where it performs best and the separation section does the rest. The cost is the recycle loop itself, which must be justified against the gain in selectivity. Separation and recycle carry their own cost.