Collisions and the energy hill

Section 1 of 4

In lesson 8 you saw that a reaction breaks old bonds and makes new ones. Before any of that can happen, the molecules have to meet. Some reactions are over in a flash and others take years, and this lesson is about what sets the speed.

Molecules in a gas or a liquid are always on the move. At room temperature, the nitrogen and oxygen molecules in the air around you fly at close to 500 metres a second, about twice as fast as a jet airliner, and each one hits another molecule billions of times every second. Most of these collisions change nothing: the molecules bounce apart and fly on. A collision leads to a reaction when the molecules meet the right way round, and hit hard enough.

Three ways a bromine molecule can meet an ethene molecule Three rows, each showing a bromine molecule, two orange-brown atoms labelled Br, about to hit an ethene molecule, two carbon atoms joined by a double bond with two hydrogens on each. Top, slow, on the C=C: the bromine moves slowly down toward the double bond, a short arrow; they bounce apart. Middle, wrong way round: the bromine moves toward a hydrogen at the end of the ethene; they bounce apart. Bottom, fast, on the C=C: the bromine moves fast down toward the double bond, a long arrow; they react. Br Br Slow, on the C=C: they bounce apart Br Br Wrong way round: they bounce apart Br Br Fast, on the C=C: they react
A bromine molecule, Br₂, meeting an ethene molecule. A slow bump and a hit on a hydrogen at the end both bounce apart. A fast, hard hit on the C=C double bond leads to a reaction.

Earlier, you saw bromine add across the C=C double bond of ethene. The double bond is the part of ethene that reacts, so a bromine molecule has to hit the ethene there. A bromine molecule that bumps into a hydrogen at the end meets only a C–⁠H bond, and the two bounce apart.

The energy hill

The old bonds have to start breaking before the new ones can form, and that takes energy. Chemists call it the activation energy, and draw it as a hill. Picture pushing a cart over a hill: you push it up the slope, and once it is over the top, it rolls down the other side by itself.

In a collision, the energy to climb comes from the movement of the two molecules. A slow, gentle bump carries too little, and the molecules bounce apart unchanged. A fast, hard hit gets over the top. In an exothermic reaction, the products end up lower than the reactants, and the difference comes out as heat.

The energy hill of an exothermic reaction A graph of energy against the progress of a reaction. A line starts on a level at the left, marked reactants, climbs a hill to a peak, then falls to a lower level at the right, marked products. A double arrow from the reactants level up to the peak is labelled energy needed to start. A double arrow from the reactants level down to the products level is labelled energy given out. the reaction goes on energy energy needed to start energy given out reactants products
The energy hill of an exothermic reaction. The reactants climb the hill, the energy needed to start, and then drop to the lower products, giving out heat.

Why a gas flame needs a match

Remember that a gas stove needs a spark or a flame to start. Methane and the oxygen in the air can sit together in a kitchen without reacting, because at room temperature almost no collisions have enough energy to climb the hill. A match flame heats a little of the gas, and there the molecules move fast enough to get over. Burning gives out more heat than it took to start, and that heat lifts the next molecules over the hill, so the flame keeps itself going.

Quick check

Bromine molecules fly at ethene molecules. Which collision is most likely to make them react?