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

Section 1

Collision theory and activation energy

For a reaction to happen, particles must collide and the collision must have enough energy. The minimum energy needed is the activation energy ( $E_a$ ). Collisions with energy below $E_a$ simply bounce apart. Collisions with energy equal to or greater than $E_a$ (and in the right orientation) are successful collisions.

The rate of reaction depends on the number of successful collisions per second.

Key termscollision theoryactivation energysuccessful collision

Section 2

Concentration and pressure

Increasing the concentration of a solution means more particles in each unit volume, so collisions are more frequent. Increasing the pressure of a gas squeezes the same number of molecules into a smaller volume, which has the same effect.

The proportion of particles with energy at least $E_a$ is unchanged, but there are more successful collisions per second, so the rate increases.

Key termsconcentrationpressure

Common mistake

Do not say concentration or pressure changes the activation energy, or the proportion of particles with enough energy. It only changes how often particles collide.

Section 3

Surface area

For a solid reacting with a liquid or gas, only particles at the surface can collide. Breaking a solid into smaller pieces increases its surface area, exposing more particles, so collisions are more frequent and the rate increases.

Key termssurface area

Section 4

Temperature

At a higher temperature particles move faster, so collisions are more frequent. More importantly, a greater proportion of collisions have energy equal to or greater than $E_a$ . A small rise in temperature can increase the rate a lot: for many reactions the rate roughly doubles for each 10 K rise.

Key termstemperature

Exam tip

Always give both reasons for temperature: more frequent collisions and a greater proportion with energy at least E_a. The second one earns the marks.

Section 5

Calculating rate of reaction

Rate is the change in a quantity per unit time.

From the time taken: relative rate $= \frac{1}{t}$ . If a cloudiness mark appears after 40 s, rate $= 1 \div 40 = 0.025\ s^{-1}$ .
From a graph: draw a tangent to the curve and find its gradient, $\frac{\Delta y}{\Delta x}$ . A tangent at $t = 0$ gives the initial rate. A tangent at time $t$ gives the rate at that time.

Worked example: a tangent passes through (20 s, 8 cm³) and (100 s, 56 cm³). Rate $= \frac{56 - 8}{100 - 20} = 0.60\ cm^3\ s^{-1}$ .

Key termsrelative ratetangentinitial rate

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Collision theory and rates of reaction

A student investigates the reaction between sodium thiosulfate solution and dilute hydrochloric acid, which forms a fine yellow precipitate of sulfur. She places the reaction flask on a printed cross and measures the time taken for the cross to disappear when viewed from above.
At a higher temperature the cross disappears after 22 s. Calculate the relative rate (1 ÷ time) at each temperature and the factor by which the rate has increased.2 marks
Marble chips (calcium carbonate) are added to excess dilute hydrochloric acid in a conical flask on a balance. Carbon dioxide escapes, so the mass of the flask and contents falls as the reaction proceeds.
The reaction is repeated at a higher temperature. Explain, using collision theory, why the rate increases.2 marks
A student reacts excess magnesium ribbon with 50 cm³ of dilute sulfuric acid and collects the hydrogen in a gas syringe. She plots the volume of gas against time and draws tangents to the curve.
Explain, using collision theory, why the gradient of the graph decreases as the reaction proceeds.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).