Rate of Reaction & Collision Theory (CIE A Level Chemistry)

Reaction rate

• The rate of a reaction is the speed at which a chemical reaction takes place and has units mol dm^-3 s^-1
• The rate of a reaction can be calculated by:

Rate of reaction =

Collision theory

• The collision theory states that for a chemical reaction to take place, the particles need to collide with each other in the correct orientation and with enough energy
• The minimum energy that colliding particles must have for a collision to be successful and a reaction to take place is called the activation energy (E_a)

Collision theory table

• An ineffective collision is when particles collide in the wrong orientation or when they don’t have enough energy and bounce off each other without causing a chemical reaction

 Effective and ineffective collisions

(A) shows an ineffective collision due to the particles not having enough energy whereas (B) shows an effective collision where the particles have the correct orientation and enough energy for a chemical reaction to take place

Increase in reaction rate

• The collision frequency is the number of collisions per unit time
• When there are more collisions per unit time, the number of particles with energy greater than the E_a increases
• This causes an increase in the rate of reaction
• A catalyst is a substance that increases the rate of reaction without taking part in the chemical reaction by providing the particles with an alternative mechanism with a lower activation energy

How catalysts affect reaction pathways

A catalyst increases the rate of a reaction by providing an alternative pathway which has a lower activation energy

Concentration

• The more concentrated a solution is, the greater the number of particles in a given volume of solvent
• An increase in concentration causes an increased collision frequency and therefore an increased rate of reaction

How increasing concentration affects collisions

The higher concentration of particles in (B) means that there are more particles present in the same volume than (A) so the chance and frequency of collisions between reacting particles increase causing an increased rate of reaction

Pressure

• An increase in pressure in reactions that involve gases has the same effect as an increased concentration of solutions
• When the pressure is increased, the molecules have less space in which they can move
• This means that the number of effective collisions increases due to an increased collision frequency
• An increase in pressure therefore increases the rate of reaction

How increasing pressure affects collisions

The higher pressure in (B) means that the same number of particles occupy a smaller volume, resulting in an increased collision frequency and therefore increased rate of reaction