Question

Species that are formed in one step of reaction mechanism and used up in another step are called

• A. catalysts
• B. inhibitors
• C. intermediates
• D. promotors

Hint:

Remember the difference:
- Catalyst: Added initially, consumed early, regenerated at the end.
- Intermediate: Not added initially, produced in an early step, consumed in a subsequent step.

Answer 1

The Correct Option is C

Concept:
Most chemical reactions take place through a sequence of elementary steps rather than in a single step.
The sequence of elementary reactions that leads from reactants to products is defined as the reaction mechanism, which often involves short-lived, transient chemical species.

Step 1: Definition of a Reaction Intermediate:
A reaction intermediate is a chemical species produced during an early elementary step of the mechanism that is subsequently consumed in a later elementary step.
Intermediates possess real physical existence and represent local energy minima on the potential energy diagram of the reaction coordinate.
Because they are consumed in later steps, their net stoichiometry cancels out, and they do not appear in the overall balanced equation for the reaction.

Step 2: Comparison with Other Reaction Components:
A catalyst is introduced with the starting reactants to provide an alternative pathway with a lower activation energy; it is consumed in an initial step and regenerated in a later step.
An inhibitor is a substance that interacts with reactants, catalysts, or intermediates to decrease the reaction rate.
A promoter is a substance that does not have catalytic activity on its own, but enhances the activity of a catalyst when present.

Step 3: Characterizing Intermediates:
Examples of reaction intermediates include free radicals, carbocations, carbanions, and carbenes.
Because they are generated during the reaction and subsequently used up, they match the description in the question.
Final Answer:
Such species are called intermediates, which corresponds to option (C).