Collision theory
Collision theory says that for substances to react with one another they must collide. The rate of the reaction depends on
Experimental observations consistent with this model are described below.
Reactions are faster if the reactants are present in higher concentration because particles collide more frequently.
The diagrams show green and blue particles as reactants in solution. Reaction between these is faster in Sample C than in Sample A because they collide more frequently in Sample C.
Heterogeneous reactions have the reactants in two phases (for example, a solid and a solution).
These occur at a faster rate if the particles of the solid are smaller (more surface area) because reaction occurs at the surface of the solid. These reactions are also faster if the mixture is stirred because more of the surface of the solid is exposed to reactants that are in solution.
Reaction occurs on collision of reacting particles only if the collision is sufficiently energetic to overcome the activation barrier for the reaction.
Reactions are faster at higher temperature due to the increased speed of the particles at higher temperature. This means that more collisions occur that have sufficient energy to exceed the activation barrier for the reaction.
- the frequency of the collisions
- the energies of the colliding particles
- the orientation of the particles when they collide
Experimental observations consistent with this model are described below.
Reactions are faster if the reactants are present in higher concentration because particles collide more frequently.
Sample A  | |
| higher temperature  Sample B | higher concentration  Sample C |
The diagrams show green and blue particles as reactants in solution. Reaction between these is faster in Sample C than in Sample A because they collide more frequently in Sample C.
Heterogeneous reactions have the reactants in two phases (for example, a solid and a solution).
These occur at a faster rate if the particles of the solid are smaller (more surface area) because reaction occurs at the surface of the solid. These reactions are also faster if the mixture is stirred because more of the surface of the solid is exposed to reactants that are in solution.
In Sample E the reaction of the green particles in solution with the brown solid is faster than in Sample D because the brown solid has been crushed to expose its particles at the surface. This makes them available for collision with the green particles. Also the mixture is being stirred.
   | |
| Sample D | Sample E |
Reactions are faster at higher temperature due to the increased speed of the particles at higher temperature. This means that more collisions occur that have sufficient energy to exceed the activation barrier for the reaction.
Reaction of the blue and green particles in Sample B is faster than in Sample A.