Before two things can react and turn into something new, they have to get over an energy hill — like shoving a heavy cart up and over a hump before it can roll down the far side. If the hill is tall, hardly anything makes it across, so the reaction crawls. A catalyst is a clever helper that digs a lower path over the very same hill. Suddenly far more reactions get across and the whole thing speeds up enormously. And the best bit: the catalyst is not used up, so it can help over and over. Toggle the catalyst in the simulator and watch the energy hill drop and the reaction rate leap.
Most people think a catalyst is used up driving the reaction, like fuel. In fact it offers a lower path over the same energy hill and emerges unchanged, so a tiny pinch can usher endless reactant through.
What's actually happening
Plenty of reactions that 'want' to happen simply don't, because they are stuck at the bottom of an energy hill. Wood next to oxygen would rather be carbon dioxide and ash, but your bookshelf isn't bursting into flame — the molecules need a shove of energy (a lit match) to get the reaction over its hump before it can run downhill on its own. That hump is called the activation energy, and it is the gatekeeper for the speed of nearly every chemical process.
A catalyst is anything that lowers that hump without being consumed. It works not by pushing harder but by offering a cleverer route — holding the reacting molecules in just the right position, or breaking the journey into easier steps, so the barrier they must climb is smaller. The payoff is wildly out of proportion to the change, because the rate depends on the barrier exponentially: shave the hill by a modest amount and the reaction can speed up not twofold but thousands or millions of times. And because the catalyst is regenerated at the end, a tiny pinch of it can usher endless tonnes of reactant through. The catalytic converter in a car, a palm-sized honeycomb coated in platinum, scrubs poisonous exhaust gases for the life of the vehicle without running out.
Life itself is one vast catalysis machine. Almost every reaction in your body would take years at body temperature if left to climb its barrier unaided; enzymes, protein catalysts, knock those barriers down so the reactions finish in milliseconds. One enzyme in your blood, carbonic anhydrase, processes up to a million molecules every second. Without enzymes lowering the hills, the chemistry of being alive would simply be too slow to keep you alive. The same trick built the modern world: catalysts make most industrial chemicals, from fertiliser to plastics to fuels, economically possible.
A catalyst lowers the energy hill without being consumed, and because rate depends on it exponentially, a small drop can multiply speed millionfold.
- 1Pour a little hydrogen peroxide (the brown-bottle kind) into two glasses. One is your control.
- 2Drop a small piece of raw potato or liver into the second glass and watch it foam — the enzyme catalase in the tissue is tearing the peroxide apart into water and oxygen.
- 3Fish the potato back out and it still works in a fresh glass: the catalyst did its job without being used up, exactly as a catalyst should.
Common questions
Because rate depends on the barrier exponentially, following rate ∝ e^(−Ea/RT). Shaving the energy hill by a modest amount can speed a reaction up not twofold but thousands or millions of times.
No. It only lowers the activation barrier and provides an easier route. The reactants, products and overall energy released are all unchanged.
Enzymes are biological catalysts, protein molecules that knock down the barriers of reactions in your body so they finish in milliseconds. Carbonic anhydrase in your blood can process up to a million molecules every second.