Simple Collision Theory (OCR AS Chemistry A): Revision Note
Exam code: H032
Effect of Concentration
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
Collision Theory Table
Effective collision | Ineffective collision | |
|---|---|---|
Orientation | Correct | Incorrect |
Energy | Enough energy (Ea) | Not enough energy |
Chemical reaction | Yes | No |
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
(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 more collisions per unit time take place, the number of collisions per unit time with energy greater than or equal to the Ea (effective collisions) increases
This causes an increase in the rate of reaction
Activation Energy
For a reaction to take place, the reactant particles need to overcome a minimum amount of energy
This energy is called the activation energy (Ea)
In exothermic reactions the reactants are higher in energy than the products
In endothermic reactions the reactants are lower in energy than the products
In an endothermic reaction the Ea must be at least as large as the enthalpy change, but the size of Ea depends on the particular reaction, not on whether it is exothermic or endothermic
The diagram shows that the reactants are higher in energy than the products in the exothermic reaction, and the activation energy is the energy needed for the reactants to go over the energy barrier
The diagram shows that the reactants are lower in energy than the products in the endothermic reaction, and the activation energy is the energy needed for the reactants to go over the energy barrier
Even though particles collide with each other in the same orientation, if they don’t possess a minimum energy that corresponds to the Ea of that reaction, the reaction will not take place
Therefore, for a collision to be effective the reactant particles must collide in the correct orientation AND possess a minimum energy equal to the Ea of that reaction
Effect of concentration
The more concentrated a solution is, the greater the number of particles in a given volume of solvent
An increase in concentration causes in an increased collision frequency and therefore an increased rate of reaction
The diagram shows a higher concentration of particles in (b) which means that there are more particles present in the same volume than (a) so the chances and frequency of collisions between reacting particles is increased causing an increased rate of reaction
Effect of pressure
An increase in pressure in reactions that involve gases has the same effect as an increase in the concentrations 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
The diagram shows a higher pressure in (b) which means that the same number of particles occupy a smaller volume, resulting in an increased collision frequency and therefore increased rate of reaction