Define activation energy.

The minimum energy that colliding particles must have for a reaction to occur.

Why does increasing concentration increase the rate?

More particles per unit volume, so collisions are more frequent and more successful collisions occur each second.

Why does increasing the pressure of a gas increase the rate?

Molecules are closer together, so the concentration is higher and collisions are more frequent.

Why does a powder react faster than lumps of the same mass?

Larger surface area, so more particles are exposed and collisions are more frequent.

Give two reasons why a higher temperature increases the rate.

Collisions are more frequent, and a greater proportion of collisions have energy at least equal to the activation energy.

Does increasing concentration change the activation energy?

No. It only increases the frequency of collisions.

How is the rate at time t found from a concentration or volume graph?

Draw a tangent to the curve at t and find its gradient.

What is the initial rate?

The rate at the start, from the gradient of the tangent at t = 0.

Why does the gradient of a gas-volume graph fall during a reaction?

Reactants are used up, so concentration falls and collisions become less frequent.

Collision theory and rates of reactionEdexcel International A Level Chemistry: Flashcards

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What must happen for a reaction to occur, according to collision theory?

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What must happen for a reaction to occur, according to collision theory?: Particles must collide with energy equal to or greater than the activation energy.
Define activation energy.: The minimum energy that colliding particles must have for a reaction to occur.
Why does increasing concentration increase the rate?: More particles per unit volume, so collisions are more frequent and more successful collisions occur each second.
Why does increasing the pressure of a gas increase the rate?: Molecules are closer together, so the concentration is higher and collisions are more frequent.
Why does a powder react faster than lumps of the same mass?: Larger surface area, so more particles are exposed and collisions are more frequent.
Give two reasons why a higher temperature increases the rate.: Collisions are more frequent, and a greater proportion of collisions have energy at least equal to the activation energy.
Does increasing concentration change the activation energy?: No. It only increases the frequency of collisions.
How is relative rate found from the time taken?: Relative rate = 1 ÷ time.
How is the rate at time t found from a concentration or volume graph?: Draw a tangent to the curve at t and find its gradient.
What is the initial rate?: The rate at the start, from the gradient of the tangent at t = 0.
A cross disappears after 50 s. What is the relative rate?: 1 ÷ 50 = 0.020 s⁻¹.
Why does the gradient of a gas-volume graph fall during a reaction?: Reactants are used up, so concentration falls and collisions become less frequent.

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).