# 1.7.1 Collision Theory

### Collision Theory

#### 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:

#### Calculating the rate of reaction

Calculate the rate of reaction for 0.0440 g of ethyl ethanoate  (CH3COOC2H5) formed in 1.00 min from a reaction mixture of volume 400 cm3

• Step 1: Calculate the amount of ethyl ethanoate formed in mol

= 0.0005 mol

• Step 2: Calculate the volume of the reaction mixture in dm3

400 cm3 = 0.400 dm3

• Step 3: Calculate the concentration change of product formed

= 0.00125 mol dm-3

• Step 4: Calculate the time in seconds

1.0 min = 60.0 s

• Step 5: Use the equation to calculate the rate

= 2.08 x 10-5 mol dm-3 s-1

#### 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 (Ea)

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 collision per unit time take place, the number of particles with energy greater than the Ea increases
• This causes an increase in 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 an alternative mechanism with a lower activation energy

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

### 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 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

#### Exam Tip

The activation energy is the energy needed to ‘activate’ the reactant particles in order for them to collide effectively and cause a chemical reaction.

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