Which statement correctly defines an intermediate in a reaction mechanism?

Which statement correctly defines an intermediate in a reaction mechanism?

• A. A catalyst that speeds up the reaction
• B. A species formed and consumed within the mechanism
• C. A species that appears in the overall equation
• D. A species that remains unchanged from reactants to products

Answer: (B)

In reaction mechanisms, an intermediate is a transient species that is formed in one step and consumed in a later step. Because it appears and then disappears as the reaction progresses, it does not show up in the overall balanced equation (the steps cancel the intermediate out). This is why the statement describing a species formed and consumed within the mechanism is the correct definition. A catalyst, while it speeds up the reaction, is not consumed and is regenerated, so it isn’t an intermediate. A species that appears in the overall equation isn’t an intermediate because intermediates cancel when the steps are combined. A species that remains unchanged from reactants to products is simply a spectator or a reagent that isn’t involved in the chemistry of the steps.

In reaction mechanisms, an intermediate is a transient species that is formed in one step and consumed in a later step. Because it appears and then disappears as the reaction progresses, it does not show up in the overall balanced equation (the steps cancel the intermediate out). This is why the statement describing a species formed and consumed within the mechanism is the correct definition.

A catalyst, while it speeds up the reaction, is not consumed and is regenerated, so it isn’t an intermediate. A species that appears in the overall equation isn’t an intermediate because intermediates cancel when the steps are combined. A species that remains unchanged from reactants to products is simply a spectator or a reagent that isn’t involved in the chemistry of the steps.