autocatalysis
/ aw-toh-KAT-uh-liss-iss /
Think of a rumour that spreads faster the more people already know it: at first only a few tell it, but each new person who hears it becomes another teller, so the more it has spread, the faster it spreads further. A reaction can behave the same way when one of its own products helps speed it up. That self-feeding behaviour is autocatalysis.
More precisely, autocatalysis is a reaction in which one of the products acts as a catalyst for the reaction itself. Because making more product creates more catalyst, the reaction tends to start slowly (when there is little product around to help), then accelerate as product builds up, and finally slow again as the reactants run out. The result is a distinctive S-shaped progress curve, quite unlike the steady slowdown of an ordinary reaction.
Why it matters: autocatalysis underlies many striking phenomena — chemical clocks that suddenly change colour, oscillating reactions, the spread of corrosion, and ideas about how self-replicating molecules might have started life. The honest caveat is that the runaway speed-up is self-limiting: it cannot accelerate forever, because the reactants are being consumed, so the reaction always eventually stalls as it approaches completion.
When permanganate oxidises oxalic acid, the reaction crawls at first — then, once a little manganese(II) ion has formed, it suddenly takes off, because that very ion catalyses the reaction. Adding a pinch of the product at the start makes it begin briskly instead of stalling.
A slow start that suddenly accelerates as its own product builds up.
An autocatalyst is a product that catalyses its own formation, so it is regenerated and grows in amount — unlike an ordinary catalyst, whose amount stays fixed. The tell-tale sign is a reaction that speeds up partway through instead of steadily slowing.