Why Petrol Sits Quietly Next to Air

A tank of petrol and the surrounding air are in a state of considerable thermodynamic tension. The reaction between them releases a great deal of energy, and the products are far more stable than the reactants.

They sit together indefinitely.

The reason is that getting from here to there requires breaking bonds before new ones form, and breaking bonds costs energy. There is a hill between the reactants and the products, and the reaction cannot happen until something supplies enough energy to get molecules over it. That is the activation energy, and a spark supplies it locally, after which the energy released carries the rest.

This separates two questions that beginners run together constantly. Whether a reaction is favourable (whether the products are lower in energy) is thermodynamics. Whether it will actually happen in your lifetime is kinetics. Diamond is thermodynamically unstable relative to graphite and the activation barrier is so high that the conversion takes essentially forever.

Which is what makes catalysis such an important idea, and it is worth being precise about what a catalyst does and does not do.

A catalyst provides an alternative route with a lower barrier, usually by binding reactants in an orientation that makes the rearrangement easier. It is not consumed. It supplies no energy. And crucially it does not change where the equilibrium lies, because it speeds the forward and reverse reactions equally; it changes how fast you arrive, not where.

Roughly 90% of industrial chemical processes use a catalyst, and every enzyme in your body is one, frequently accelerating a reaction by factors of a million or more.

The single most consequential application is worth telling properly.

Nitrogen makes up 78% of the atmosphere, and plants cannot use it. The nitrogen molecule is held together by a triple bond, one of the strongest in chemistry, and breaking it is hard enough that biologically available nitrogen was the limiting factor on agriculture for the whole of human history. Crop rotation with legumes, manure, guano and Chilean nitrate deposits were all ways of gathering a scarce resource.

In 1909 Fritz Haber found conditions under which atmospheric nitrogen combines with hydrogen to make ammonia: high pressure, high temperature, and an iron catalyst. Carl Bosch scaled it to industrial production.

The Haber-Bosch process now fixes more nitrogen than all natural processes combined. Estimates suggest that roughly half the nitrogen atoms in the average person's body passed through it, which is another way of saying that around half the people alive are eating food that would not exist without it.

It is also how Germany made explosives after the British blockade cut off Chilean nitrate, which is the same chemistry, and Haber went on to direct the chemical weapons programme in the First World War. He received the Nobel Prize in 1918. His wife, also a chemist, killed herself shortly after the first chlorine attack.

Two further concepts that do a great deal of work.

Equilibrium is dynamic rather than static. When concentrations stop changing it is because the forward and reverse reactions are proceeding at equal rates, not because anything has stopped. Isotope labelling shows this directly: mark some reactant molecules and they turn up among the products long after the concentrations have settled. This is why disturbing a system at equilibrium produces a compensating response, which is Le Chatelier's principle, and why Haber could improve his yield by removing ammonia as it formed.

And oxidation, which is a much bigger idea than its name suggests. It was named for reactions with oxygen, and when chemists worked out that oxygen's role was to take electrons, the concept was redefined as electron transfer. Oxidation is loss of electrons; reduction is gain; and they always happen together, because the electrons have to go somewhere.

Defined that way it covers rusting, combustion, photosynthesis, cellular respiration, bleaching and every battery ever made. A battery is a redox reaction with the two halves physically separated so the electrons have to travel through a wire to get from one to the other, doing work on the way. That is the whole trick, and it is why the concept earns its place.