Heterogeneous Reaction Kinetics Questions and Answers

Answer: A

The first two are correct. The third is wrong, since collisions with the wall impede the molecule and make Knudsen diffusion the slower of the two. Pore size decides which regime applies. The mean free path sets the boundary.

Answer: B

The wide pores carry the reactant into the pellet and the fine ones provide the surface area. It is a direct response to the conflict between area and transport. Wide pores carry, fine pores react. The two functions are separated by design.

Answer: C

Correct answer: the slowest step in the sequence

The rate determining step is simply the slowest, whichever it happens to be - adsorption, surface reaction or desorption - and which one it is depends on the system and on the conditions. Assuming it is always the surface reaction is a convenience of textbook derivations rather than a fact, and the assumption can fail at high pressure or with a strongly held product.

Answer: D

Both are true and the reason gives the origin: fitting data to several candidate expressions and choosing the best is how the controlling step is usually inferred. Each gives a distinct denominator. Discriminating between them takes real evidence.

Answer: A

Strong pore diffusion control halves the apparent activation energy, because the observed rate depends on the square root of the intrinsic rate constant. This makes the diagnostic straightforward. Halving is the signature to look for.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: a power law can fit data well while asserting nothing about mechanism, and a Langmuir–Hinshelwood form does embed assumptions about what adsorbs and what reacts. Statement 3 is the error the other two set up - several mechanisms commonly fit the same data within experimental error, so a good fit is consistent with a mechanism and never proves it.