Activation energy

A match can burn, yet it sits quietly in its box and does nothing. Only when you strike it across the rough strip does it flare up. Wood, paper, candle wax: there is energy in all of them, but none of it just bursts into flame. Here you'll find out why many reactions need a push first.

The hill before the valley

Picture a ball resting in a small dip on a mountain. It would be much happier down in the valley, but it won't roll off on its own: there is a hill in front of it. Only once you push it over the crest does it roll down by itself. Many reactions work the same way: the starting substances first have to get over an energy hill before the reaction runs. This push of energy is called the activation energy. The height of that hill is given in kilojoules, kJ for short.

Why wood doesn't burn on its own

Wood and the oxygen in the air could react at any time, and that would release a lot of energy. Yet your shelf is not on fire. The reason: at room temperature the particles are too slow. Almost no collision is hard enough to get over the energy hill. A match delivers the needed push of energy at one small spot. Then the burning releases so much heat that the neighbouring particles make it over the hill too: the fire keeps itself going.

The ignition temperature

For flammable substances there is a handy number: the ignition temperature. That is the temperature at which a substance starts to burn by itself, with no flame or spark held to it. Paper ignites at around 240 °C, wood at around 300 °C. As long as a substance stays cooler than that, the particles' energy is not enough to clear the hill. That is why a hot stove plate next to a tea towel is dangerous, but a warm radiator is not.