Reactions can happen only when the reactant particles collide. All the collisions which occur are not successful in forming product molecules. The reason is that particles have a wide range of kinetic energy but only a small fraction of particles have enough kinetic energy to break bonds and bring about chemical change. The minimum kinetic energy required for reaction is known as the activation energy.
The small number of high kinetic energy collisions between particles which do produce a chemical change are called ‘fruitful _collisions’. Here the reactant molecules collide with enough kinetic energy to break the original bonds and form new bonds in the product molecules.
It must be noted that the rate-controlling factors described below are to do with the collision frequency (chance of collision) or the energy of reactant particle collision (energy activation energy) which can summed up as the ‘chance of a fruitful collision’ leading to product formation.
THE EFFECT OF CONCENTRATION
If the concentration of any reactant in a solution is increase, the rate of reaction is increase. Increasing the concentration, increases the probability of a collision between reactant particles because there are more of them in the same volume and so increases the chance of a fruitful collision forming products.
For example, Increasing the concentration of acid molecules in a reaction of zinc with HCl, increase the frequency or chance at which they hit the surface of marble chips to dissolve them faster.
THE EFFECT OF TEMPERATURE
When gases or liquids are heated the particles gain kinetic energy and move faster (see diagrams below.) The increased speed increases the chance (frequency) if collision between reactant molecules and the rate increases.
It must be noted that this is not the main reason for the increased reaction speed, so be careful in your theory explanations if investigating the effect of temperature.
THE EFFECT OF SURFACE AREA-PARTICLE SIZE OF A SOLID REACTANT
If a solid reactant or a solid catalyst is broken down into smaller pieces the rate of reaction increases.
The speed increase happens because smaller pieces of the same mass of solid have a greater surface area compared to larger pieces of the solid.
Therefore, there is more chance that a reactant particle will hit the solid surface and react.