How Fast Does a Reaction Go

Five parts on speed, opening with a puzzle that has to be solved before anything else makes sense: petrol and air sit together for years and then react in milliseconds, and both facts need explaining at once. The particles have to meet — which rules out nothing, since they are already colliding billions of times a second — so what separates a reaction from a mixture sitting still is that a collision also has to be hard enough to start breaking bonds, and almost none of them is. That turns a rate into a count of successful collisions per second, and the second part uses it to show that the four factors everybody memorises are really two mechanisms: make the particles meet more often, or make more of those meetings count. There is no third number to adjust, which is why the list is short. The middle part reads a rate graph properly, since the curve answers two questions and they are easy to merge — steepness is the rate and the final height is the total, so a faster reaction reaches the same amount sooner rather than producing more. The fourth part takes the one lever that is not used up and refuses the usual explanation of it: a catalyst gives nothing any energy, and works by lowering the bar rather than raising the particles. The last part follows surface area out of the beaker, to why a sack of flour is harmless and a cloud of it is not.