Which statement best describes a reaction intermediate?

• A. A description of energy changes during a reaction
• B. A substance that initiates or accelerates a chemical reaction without being consumed
• C. A measure of reaction rate constant
• D. A species that forms during a reaction, but is consumed along the process

Answer: (D)

A reaction intermediate is a transient species formed during the sequence of elementary steps that make up a reaction mechanism, and it is consumed as the mechanism proceeds. This is why the correct description is the one that says it forms during the reaction but is consumed along the process—the intermediate appears in the step-by-step pathway but does not appear in the overall net equation because it is generated and then used up. The other ideas describe different concepts: energy changes describe the energy profile and activation barriers rather than a concrete species; a substance that initiates or accelerates a reaction without being consumed refers to a catalyst; a measure of a rate constant is a kinetic parameter, not a chemical species.