Heterogeneous Reaction Kinetics Questions and Answers

Answer: C

Correct answer: raising the reaction temperature

Temperature makes matters worse, not better: the rate constant rises exponentially while the effective diffusivity rises only weakly, so the Thiele modulus increases and the effectiveness factor falls. The other three all help - wider pores raise the diffusivity, a less active catalyst lowers the rate constant, and smaller particles shorten the diffusion path, which is the usual first remedy.

Answer: D

Both are true and the reason gives the mechanism: the activation energy of the reaction is far larger than that of diffusion, so the balance tips further towards the transport resistance. Activation energies differ by a large factor.

Answer: A

The first two are correct and are the standard diagnostic. The third is exactly the point being made, since transport limitation halves or nearly eliminates the apparent activation energy. Each observation implicates a resistance.

Answer: B

The observed rate then depends on the square root of the rate constant, which halves the apparent activation energy. It is one of the simplest and most useful diagnostics available. A square root halves the exponent. The test costs nothing beyond the data already taken.

Answer: C

If the film around the particle is the resistance, then thinning it by faster flow speeds the reaction. A rate that does not respond to velocity has ruled that particular resistance out. Film thickness is the variable.

Answer: D

Both are true and the reason gives the mechanism, and a rate that does not change when the velocity is raised has ruled external transport out as the controlling resistance. The test is quick and needs no new equipment.

Answer: A

A short series of experiments settles which resistance controls, and each has its own remedy. Attacking the wrong one is a common and expensive mistake. Each has its own characteristic signature. A short series of tests settles it.

Answer: B

Each experiment isolates one resistance, and the activation energy confirms the conclusion. Drawing the conclusion first and testing afterwards is how the wrong remedy gets chosen. Each experiment isolates one resistance.

Answer: C

Adsorption is a chemical step on the surface rather than a transport process. The other three are all diffusional resistances in the series of steps that make up the overall reaction. Chemistry and transport are distinct.

Answer: D

When the pore is comparable with the mean free path the molecules collide with the wall more often than with each other. Wide pore supports are used where this becomes limiting. Wall collisions dominate in a fine pore.