Simple Collision Theory (OCR A Level Chemistry A): Revision Note
Exam code: H432
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
Examiner Tips and Tricks
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
Calculating Rates
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
Calculating the rate of reaction
Calculate the rate of reaction, in mol dm-3 s-1, when 0.0440 g of ethyl ethanoate, CH3COOC2H5, (Mr = 88.0 g mol-1) is formed in 1.00 minute from a reaction mixture of total volume 400 cm3
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
Time (mins) | [cyclopropane] mol dm-3 | [propene] mol dm-3 |
|---|---|---|
0 | 1.50 | 0.00 |
5 | 1.23 | 0.27 |
10 | 1.00 | 0.50 |
15 | 0.82 | 0.68 |
20 | 0.67 | 0.83 |
25 | 0.55 | 0.95 |
30 | 0.45 | 1.05 |
35 | 0.37 | 1.13 |
40 | 0.33 | 1.17 |
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:
Curve a shows the concentration of propene over the first five minutes and line b is the tangent drawn at the start of the curve, whose gradient gives the initial rate. This initial rate is very similar to the average rate over the first five minutes
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
At 1.5 mol dm-3 the tangent gives a rate of 1.5 ÷ (25 x 60) = 1.00 x 10-3 mol dm-3 s-1
At 1.0 mol dm-3 the tangent gives a rate of 1.4 ÷ (35 x 60) = 6.67 x 10-4 mol dm-3 s-1
At 0.5 mol dm-3 the tangent gives a rate of 1.04 ÷ (52.5 x 60) = 3.30 x 10-4 mol dm-3 s-1
The rate falls as the concentration falls
Examiner Tips and Tricks
Other suitable physical quantities you could monitor to measure reaction rate include gas volume and mass
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