Collision Theory

To react, particles have to collide

All matter is made of particles (atoms, molecules, or ions) that are constantly moving. In a gas or a solution they move freely and collide with one another millions of times per second.

A chemical reaction is a rearrangement of atoms: bonds in the reactants break and new bonds form in the products. For bonds to break, the particles have to meet. That is why, without collisions, there is no reaction.

But not every collision produces a reaction. Imagine a court packed with people: they bump into each other all the time and almost always just brush past. For a reaction to occur, the collision must be effective.

Surface area and concentration

The first two factors increase how many collisions there are.

Surface area. In a reaction between a solid and a liquid or gas, only the particles on the surface react, because those inside cannot be reached. If you crush the solid, you expose more particles and there are more collisions per second.

The drawing is flat: it shows the 8 small cubes as squares, but each one has 6 faces, and all of them are exposed when the block is split.

This is why flour suspended in the air burns explosively and a compact sack does not, and why we chew food to digest it better.

Concentration. If there are more particles in the same volume, they are closer together and meet more often. In gases, increasing the pressure is equivalent to increasing the concentration.

Temperature

Temperature measures how fast particles move on average. When you heat a substance, the kinetic energy of its particles increases, and this has two effects:

1. They move faster, so they collide more times per second.

2. A much larger fraction of the collisions carries the minimum energy needed to break bonds.

The second effect is the more important one: that is why, with a modest increase in temperature, the rate grows enormously. As a rough rule, in many reactions the rate doubles or triples for every 10 °C that the temperature rises.

To the right of the dashed line are the particles with enough energy. At higher temperature, that region is larger: there are more effective collisions.

Catalysts

A catalyst is a substance that speeds up a reaction without being consumed. It offers a different pathway in which the minimum energy required is lower (you will see this in detail in ENER-EACT). This way, a larger fraction of the collisions becomes effective, even at the same temperature.

Biological catalysts are called enzymes: for example, the ones in saliva that begin to digest the starch in a tortilla. In car exhaust systems, the catalytic converter speeds up the transformation of polluting gases such as CO into CO₂.

Summary of factors

Summary