U5.7 · Introduction to Reaction MechanismsPractice · 反应机理入门

Add the steps and cancel $\ce{A2B}$, which appears on both sides: $\ce{A2 + 2B -> 2AB}$. $\ce{A2B}$ is made in step 1 and used in step 2, so it is the intermediate. B is a reactant in both steps, so it is an overall reactant.

Adding the steps (step 2 twice) gives $\ce{O2 + 2SO2 -> 2SO3}$. NO is put in at the start and regenerated at the end, so it is a catalyst; $\ce{NO2}$ is formed and then used up, so it is an intermediate. Neither appears in the overall reaction.

The two steps must add up to the overall reaction. Try adding each choice to step 1: only $\ce{NO2Cl + Cl -> NO2 + Cl2}$ makes Cl cancel and gives exactly $\ce{2NO2Cl -> 2NO2 + Cl2}$. Cl is the intermediate.

$\ce{NOBr2}$ is made in step 1 and used in step 2, and it isn’t in the overall reaction: it is the intermediate. NO and $\ce{Br2}$ are reactants, and NOBr is the product.

An intermediate is made first and used later; a catalyst is the reverse, used first and regenerated. An intermediate sits in a valley of the energy profile and exists for a while, whereas a transition state is at a peak. Experimental rate laws contain reactants (sometimes catalysts or products), never intermediates.