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:

Worked example: Calculating the rate of reaction

Answer

• 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 (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

(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 E_a 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