Collision Theory

Key Stage 4

Meaning

Collision Theory is a scientific theory which explains that reactions take place via the collision of particles of reactants.

About Collision Theory

Collision Theory is based on the Kinetic Theory which explains the motion of particles in matter.

In Collision Theory particles are said to collide with one another and that if two reactants in a reaction collide with enough energy this will cause them to react.

Collision Theory indicates that the particles of reactant must collide for the reaction to occur.

Temperature and Collision Theory

Kinetic Theory explains how increasing the temperature of a substance causes the particles to gain more kinetic energy so they travel faster. When particles travel faster (due to higher temperatures) they collide with more energy and they collide more frequently (more often). This means at higher temperatures particles of the reactants are more likely to collide with each other and they are more likely to have enough energy to react when they do. This increases the rate of reaction.

Pressure and Collision Theory

The Particle Model explains how increasing the pressure on a gas, and compressing it, causes the particles in that gas to collide more frequently (more often). This means at higher pressures the particles of reactants will collide with each other more frequently, increasing the chances of them reacting and increasing the rate of reaction.

NB This may not be true in the case of some reversible reactions (see Dynamic Equilibrium).

Concentration and Collision Theory

The Particle Model explains how a higher concentration of gas, liquid or solute causes particles to collide more frequently (more often). This means at higher concentration the particles of reactants will collide with each other more frequently, increasing the chances of them reacting and increasing the rate of reaction.

Surface Area and Collision Theory

The Particle Model explains how particles in a solid vibrate around fixed positions. This means most of those particles cannot collide with other reactants. It is only the particles on the surface of the solid that can have other reactants collide with them. Therefore the larger the surface area the more frequently reactant particles in a gas or liquid will collide with the reactant particles in the solid, increasing the rate of reaction.

Catalysts and Collision Theory

Catalysts can provide different reaction pathways so that particles of reactants may not need to collide for a reaction to occur. Instead the reactants may collide with the catalyst. For instance a reactant may pass on an electron to a catalyst which then passes that electron on to the other reactant resulting in the same product, but without the two reactants colliding with one another.