3.5.1 Simple Collision Theory

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 E_a 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 (E_a)
• 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 E_a 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 E_a 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 E_a 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

Exam Tip

Reaction rate

• The rate of reaction is the speed at which a chemical reaction takes place
• The units are mol dm^-3 s^-1 or mol dm^-3 min^-1
• The rate of a reaction can be calculated using:

Rate of reaction = 

Worked Example

Answer

Step 1: Calculate the number of moles of ethyl ethanoate:

• • Number of moles =
  • Number of moles =

Step 2: Calculate the concentration of the product:

• • Concentration of ethyl ethanoate (mol dm^-3) =
  • Concentration of ethyl ethanoate (mol dm^-3) =

Step 3: Calculate the rate:

• • Rate of reaction = 
  • Rate of reaction  =

Measuring a rate from a graph

• During a reaction, the reactants are used up and changed into products
• This means that as the reaction proceeds, the concentration of the reactants is decreasing and the concentration of the products is increasing
• Therefore, the rate of the reaction is not the same throughout the reaction but changes
• The rate of reaction during the reaction can be calculated from a concentration-time graph
• The isomerisation of cyclopropane to propene is used as an example:

Isomerisation of cyclopropane

• The concentrations of reactant (cyclopropane) and product (propene) over time can be measured by experiment

Concentrations of Cyclopropane & Propene Table

• When taking the measurements, the temperature should be kept constant as a change in temperature will change the rate of reaction
• A concentration-time graph for the concentration of propene as well as cyclopropane can be obtained from the above results
  • As an example, the concentration-time graph for propene is shown below:

The graph shows that the concentration of propene increases with time

Calculating the rate at the start of a reaction

• At the start of the reaction, the concentration-time curve looks almost linear:

Line a shows the average rate over the first five minutes whereas line b shows the actual initial rate found by drawing a tangent at the start of the curve. The calculated rates are very similar for both methods

• The rate at this point can therefore be found by treating the curve as a linear line and by using:

Rate of reaction = 

• The average rate of the reaction over the first 5 minutes for propene is:

Rate of reaction = 

Calculating the rate as the reaction proceeds

• The curve becomes shallower with time which means that the rate decreases with time
• The rate of reaction can be calculated by taking short time intervals
  • For example. you can calculate the rate of reaction from 15 to 20 minutes during which the concentration of propene increases from 0.68 to 0.83 mol dm^-3 

Rate of reaction = 

• The smaller the time intervals, the more accurate the reaction rate value is
• It is even more accurate to find the rate of reaction at different concentrations of reactant or product at particular time points
• This can be done by drawing tangents at several points on the graph
  • As an example, the rates of reaction at different concentrations of cyclopropane are calculated by drawing the appropriate tangents:

The rate of reaction at three different concentrations of cyclopropane is calculated by drawing tangents at those points in the graph

Exam Tip