Reactions That Go Both Ways

Five parts paying off the double arrow that the reactions track drew and deliberately left unexplained. Most reactions, as they are first taught, run one way and stop; a few run back again, and heating blue copper sulfate white and then adding water to turn it blue once more is the version you can hold in your hand. The second part is the one that changes what the word means: a reaction at equilibrium has not stopped at all. Both directions are still running, at the same rate, which is why nothing appears to change — and that needs the rates track, which is why this one waited for it. From there the behaviour becomes predictable. Change a concentration, a temperature or a pressure and the balance moves in whichever direction opposes what was done to it, including the result nearly everybody gets backwards: heating a reversible exothermic reaction gives less product, not more. The fourth part puts a catalyst against that and finds it shifts nothing, because it speeds both directions equally — the sharpest available test of whether rate and position have been told apart. And the last spends all of it on one reactor, where the conditions that favour the yield and the conditions that favour the speed want opposite temperatures and the plant has to choose.