A catalyst speeds a reaction by
Answer: A
It offers an alternative route with a smaller barrier and is regenerated at the end of each cycle. It cannot change where the equilibrium lies, only how quickly that point is reached. It emerges unchanged from each cycle.
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Assertion (A): A catalyst cannot shift the position of a chemical equilibrium.
Reason (R): It lowers the barrier in both the forward and the reverse direction equally.
Answer: B
Both are true and the reason gives the argument: the ratio of the two rate constants is unchanged, and that ratio is exactly the equilibrium constant. Both directions are accelerated equally. Only the approach to equilibrium is hastened.
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Arrange the steps of a heterogeneous catalytic reaction in order.
1. adsorption, surface reaction and desorption
2. diffusion of reactant from the bulk to the outer surface
3. diffusion of product back out to the bulk
4. diffusion into the pores of the particle
Answer: C
The sequence is a series of resistances, and the slowest step controls the observed rate. Which one that is decides how the reactor should be designed and what will improve it. The slowest step controls the observed rate.
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If the observed rate of a catalytic reaction is limited by pore diffusion, the most effective remedy is
Answer: D
A shorter diffusion path lets the reactant reach the interior, whereas a more active catalyst simply makes the limitation worse. Raising the temperature helps the reaction more than the diffusion and also makes matters worse.
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Assertion (A): A more active catalyst does not always give a faster observed rate.
Reason (R): If the rate is already limited by diffusion, increasing the intrinsic activity changes nothing.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the cause. Where the effectiveness factor is already low, reactant is consumed in a thin outer shell and the interior sees none; raising the intrinsic activity consumes it in a thinner shell still and leaves the observed rate almost unchanged. The remedy is smaller particles or wider pores, and the diagnostic is that a diffusion-limited rate shows a low apparent activation energy - about half the true value.
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The effectiveness factor of a catalyst particle is
Answer: B
A value near one means the whole pellet is being used and a small value means only the outer layer is. It falls as the particle grows or the intrinsic rate rises. One means the whole pellet is used. Small values point to larger particles being the fault.
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Consider the following statements about the effectiveness factor.
1. A value near one indicates no significant diffusion limitation.
2. It falls as the particle size increases.
3. It rises as the intrinsic reaction rate increases.
Answer: C
The first two are correct. The third is inverted, since a faster intrinsic reaction consumes the reactant nearer the surface and leaves the interior starved. Particle size is the main design lever. Intrinsic activity is not always the answer.
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A catalyst pellet has an effectiveness factor of 0.4. What fraction of its potential activity is being realised?
Answer: D
The factor is by definition the ratio of actual to potential rate. A value this low means most of the pellet interior is contributing very little and smaller particles would be better. Most of the interior sits idle at this value.
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Match each catalyst component with its role.
List I
A. Active component
B. Support
C. Promoter
D. Poison
List II
1. improves activity or selectivity without acting alone
2. the material on which the reaction occurs
3. binds to active sites and destroys activity
4. provides surface area and mechanical strength
Answer: A
A commercial catalyst is a formulated material rather than a single substance, and each component earns its place. The support usually accounts for most of the mass and almost all of the area. Each component earns its place.
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A catalyst support is used mainly to
Answer: B
Alumina or silica gives hundreds of square metres per gram on which a small amount of expensive metal can be spread. It also provides the mechanical strength the pellet needs in a bed. Hundreds of square metres per gram are usual.
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