Define activation energy.

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

Why do most collisions not lead to a reaction?

Most colliding particles have energy less than the activation energy, so they rebound.

What is a successful collision?

One in which the particles have energy equal to or greater than the activation energy.

Explain why a higher concentration increases the rate.

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

Does increasing concentration change the activation energy?

No, the activation energy is unchanged.

Does increasing concentration change the proportion of particles with energy ≥ Ea?

No, this depends on temperature, not concentration.

Explain why increasing the pressure of a gas increases the rate.

There are more gas particles per unit volume, so collisions are more frequent and there are more successful collisions per second.

Why does adding argon at constant volume not change the rate?

It does not change the concentrations of the reactant gases, so collision frequency between reactants is unchanged.

Why does pressure have almost no effect on reactions of solids and liquids?

They cannot be compressed, so the particles per unit volume do not change.

Why is the rate related to 1/t in the thiosulfate and acid experiment?

The same amount of sulfur forms each time to hide the cross, so the rate is proportional to 1/time.

Collision theory, concentration and pressureAQA A-Level Chemistry: Flashcards

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State collision theory.

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State collision theory.: A reaction can only occur when particles 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 do most collisions not lead to a reaction?: Most colliding particles have energy less than the activation energy, so they rebound.
What is a successful collision?: One in which the particles have energy equal to or greater than the activation energy.
Explain why a higher concentration increases the rate.: There are more particles per unit volume, so collisions are more frequent and more successful collisions occur per second.
Does increasing concentration change the activation energy?: No, the activation energy is unchanged.
Does increasing concentration change the proportion of particles with energy ≥ Ea?: No, this depends on temperature, not concentration.
Explain why increasing the pressure of a gas increases the rate.: There are more gas particles per unit volume, so collisions are more frequent and there are more successful collisions per second.
What happens to the concentration of a gas if its pressure doubles at constant temperature?: It doubles.
Why does adding argon at constant volume not change the rate?: It does not change the concentrations of the reactant gases, so collision frequency between reactants is unchanged.
Why does pressure have almost no effect on reactions of solids and liquids?: They cannot be compressed, so the particles per unit volume do not change.
Why is the rate related to 1/t in the thiosulfate and acid experiment?: The same amount of sulfur forms each time to hide the cross, so the rate is proportional to 1/time.

Exam questions on Collision theory, concentration and pressure

A student investigates the reaction between marble chips (calcium carbonate, in excess) and 50.0 cm³ of hydrochloric acid, CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g), by measuring the volume of carbon dioxide produced using a gas syringe. She repeats the experiment using 2.0 mol dm⁻³ acid in place of 1.0 mol dm⁻³ acid. All other conditions, including the temperature and the mass and size of the marble chips, are kept the same.
Explain why most collisions between particles do not lead to a reaction.2 marks
Nitrogen monoxide reacts with oxygen in the gas phase at constant temperature: 2NO(g) + O₂(g) → 2NO₂(g). A mixture of the gases is held in a sealed cylinder fitted with a movable piston.
Explain, in terms of particles, why halving the volume of the cylinder increases the rate of reaction.2 marks
A student investigates how concentration affects the rate of the reaction between sodium thiosulfate and hydrochloric acid: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l). In each experiment 5.0 cm³ of 1.0 mol dm⁻³ hydrochloric acid is added to the thiosulfate solution at 20 °C, and the time taken for a cross viewed through the mixture to disappear is measured. Experiment 1 uses 50.0 cm³ of 0.20 mol dm⁻³ sodium thiosulfate solution and takes 36 s. Experiment 2 uses 25.0 cm³ of 0.20 mol dm⁻³ sodium thiosulfate solution mixed with 25.0 cm³ of water, and takes 72 s.
Explain, in terms of collisions, why the cross disappears more slowly in experiment 2.3 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).