Collision theory and rates of reactionEdexcel A-Level Chemistry: Revision notes

Section 1

Collision theory

For a reaction to happen, reactant particles must collide, and the collision must have enough energy, at least the activation energy ( $E_a$ ), and the right orientation. Most collisions are unsuccessful and the particles just rebound.

The rate of reaction is the change in concentration (or amount) of a reactant or product per unit time. It increases when the frequency of successful collisions increases.

Key termscollision theoryactivation energyrate of reaction

Section 2

Concentration, pressure and surface area

These factors change how often particles collide; they do not change the energy of each collision.

Concentration of a solution: more particles per unit volume, so more frequent collisions.
Pressure of a gas: squeezing the same number of molecules into a smaller volume has the same effect as increasing concentration.
Surface area of a solid: crushing the solid exposes more particles, so more collisions with the other reactant per second.

The activation energy is unchanged. Doubling concentration roughly doubles the rate of many simple reactions.

Key termsfrequency of collisionssurface area

Common mistake

Saying the particles 'have more energy' when concentration or surface area increases. Only temperature (or a catalyst) changes the proportion of collisions with enough energy.

Section 3

Temperature

Raising the temperature has two effects:

Particles move faster, so they collide more often.
Particles have more energy, so a greater proportion of collisions have energy equal to or greater than $E_a$ .

The second effect is by far the more important. A rise of about 10 K often roughly doubles the rate of a reaction.

Key termsproportion of collisions

Exam tip

Always say 'a greater proportion of collisions have energy ≥ Ea'. Saying 'more collisions' on its own gains only part of the credit.

Section 4

Measuring rate from the time taken

For a reaction where an event can be timed (a cross disappearing, magnesium dissolving), the rate is proportional to $1/t$ , where $t$ is the time taken to reach the same stage.

Example: t = 40 s gives a relative rate of 1 ÷ 40 = 0.025 s⁻¹. Halving the time doubles the rate.

This gives an average rate, and is useful for comparing experiments, but not for finding the rate at a particular moment.

Key termsrelative rate

Section 5

Measuring rate from a graph

If concentration or volume of gas is plotted against time, the rate at any time is the gradient of the curve at that time. Draw a tangent to the curve at that point and calculate

$\text{rate} = \dfrac{\Delta y}{\Delta x}$

The initial rate is the gradient of the tangent at t = 0. The curve is steepest at the start and flattens as reactants are used up, so the rate falls with time because the concentration of reactant falls.

Example: a tangent through (0 s, 0 cm³) and (20 s, 30 cm³) gives an initial rate of 30 ÷ 20 = 1.5 cm³ s⁻¹.

Key termstangentinitial rate

Common mistake

Using the gradient of the whole curve or a chord instead of a tangent. Put the ruler so that it touches the curve at only the chosen time and make the triangle as large as possible.

Must Know

Reactions need collisions with energy ≥ activation energy
Concentration, pressure and surface area change collision frequency only
Temperature raises the proportion of collisions with energy ≥ Ea
Rate = 1/t for timed experiments
Rate at time t or initial rate = gradient of a tangent
Rate falls during a reaction as reactant concentration falls

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Collision theory and rates of reaction

A technician reacts marble chips (calcium carbonate) with dilute hydrochloric acid and wants to understand how the rate of the reaction can be controlled.
Define the term activation energy and explain why most collisions between particles do not lead to a reaction.2 marks
A class investigates how temperature affects the reaction between sodium thiosulfate solution and dilute hydrochloric acid, which produces a pale yellow precipitate of sulfur. They time how long it takes for the cloudiness to hide a cross marked on paper under the flask.
The cross disappears after 80 s at 20 °C and after 41 s at 30 °C. Calculate the rate of reaction, as 1/time, at each temperature and the factor by which the rate increases.2 marks
A student reacts an excess of marble chips with 50 cm³ of dilute hydrochloric acid and measures the volume of carbon dioxide collected in a gas syringe at regular intervals, then plots volume against time.
A tangent drawn to the curve at time zero passes through the points (0 s, 0 cm³) and (30 s, 33 cm³). Describe how the initial rate is found from the graph and calculate its value.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).