Which of the following statements about catalyst is false?

Analysis of the Question

This question tests your understanding of the fundamental properties of catalysts in chemical kinetics. We need to identify which of the four statements is scientifically incorrect.

Concept Explanation

A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. Here is how they work:

• Activation Energy: Catalysts provide an alternative reaction pathway with a lower activation energy ($E_a$), allowing more reactant molecules to have enough energy to collide effectively.
• Reaction Mechanism: By providing this alternative pathway, catalysts do change the mechanism of the reaction.
• Rate Law: Because the mechanism changes, the catalyst often appears in the rate law expression (its concentration affects the rate).
• Balanced Equation: Catalysts are not consumed, so they do not appear as net reactants or products in the overall balanced chemical equation.
• Enzymes: These are protein molecules that act as catalysts in biological systems.

Step-by-Step Evaluation

• Option A: True. Enzymes are indeed biological catalysts that speed up biochemical reactions in living organisms.
• Option B: True. Since a catalyst is regenerated during the reaction, it is not a net reactant or product, so it is omitted from the overall stoichiometric equation.
• Option C: True. This is the primary function of a catalyst; by lowering the activation energy, it makes it easier for the reaction to proceed.
• Option D: False. This statement claims catalysts do not alter the mechanism and never appear in the rate law. In reality, catalysts do alter the reaction mechanism (that is precisely how they lower the activation energy), and because they participate in the rate-determining step, they often appear in the rate law expression.

Conclusion

Since the question asks for the false statement, Option D is the correct choice because it contains incorrect information regarding the role of catalysts in reaction mechanisms and rate laws.

Exam Tip

When you see questions about catalysts, remember the "Alternative Pathway" rule: If a catalyst changes the speed of a reaction, it must be doing so by changing the "path" (mechanism) the molecules take to become products. If the path changes, the rate law (which describes the path) must also change.