Introduction to Reaction Mechanisms

A balanced equation only tells you where a reaction starts and ends. It doesn't tell you how it got there. Most reactions don't happen in one clean collision between all the written reactants at once; that would need too many particles lined up perfectly at the same instant. Instead the reaction happens in elementary steps, each one a single molecular event: one collision or one breakup. A mechanism is the proposed sequence of these steps.

To check that a mechanism is valid, add up its elementary steps like algebra equations. Reactants and products that appear on both sides cancel. Anything made in an early step and fully used up in a later step is an intermediate: it never appears in the overall equation because it cancels out when you sum the steps. Spot it by scanning the steps for a species that shows up as a product first, then as a reactant later, and checking that its coefficients match so it disappears completely.

Don't confuse an intermediate with a catalyst, even though both seem to vanish from the overall equation. An intermediate is built by the mechanism itself, partway through, then consumed later in that same mechanism. A catalyst is already present at the start, gets used up in an early step, and is regenerated in a later step, so it speeds up the reaction without any net change in its amount. This distinction matters because catalysts lower activation energy and show up in rate laws for certain steps, while intermediates are just internal bookkeeping. If a proposed intermediate is never actually detected experimentally, chemists revise the mechanism, since a trustworthy mechanism should be consistent with every species it predicts.