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
- 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 particles with energy greater than the Ea 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
- Therefore, the Ea in endothermic reactions is relatively larger than in exothermic reaction
The diagram shows that the reactants are higher in energy than the products in the exothermic reaction, so the energy needed for the reactants to go over the energy barrier is relatively small
The diagram shows that the reactants are lower in energy than the products in the endothermic reaction, so the energy needed for the reactants to go over the energy barrier is relatively large
- 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
Examiner Tip
When questions mention a doubling of concentration make sure you mention double the number of particles per unit volume and double the frequency of effective collisions