Define activation energy.

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

Define rate of reaction.

The change in concentration (or amount) of a reactant or product per unit time.

Why does increasing concentration increase rate?

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

Why does increasing the pressure of a gas increase rate?

Gas molecules are closer together, so there are more molecules per unit volume and collisions are more frequent.

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

A larger surface area is exposed, so collisions with the other reactant are more frequent.

Give the two effects of increasing temperature.

Particles collide more often, and a greater proportion of collisions have energy ≥ activation energy. The second is more important.

Which factors change the proportion of collisions with energy ≥ Ea?

Temperature (and a catalyst, by lowering Ea). Concentration, pressure and surface area do not.

How do you calculate relative rate from the time taken?

Rate ∝ 1 ÷ time, e.g. 1 ÷ 50 s = 0.020 s⁻¹.

How is the rate at a given time found from a concentration–time graph?

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

How is the initial rate found from a graph?

The gradient of the tangent drawn at t = 0.

Why does the rate fall as a reaction proceeds?

The concentration of reactants falls, so collisions are less frequent.

Why is the tangent method better than 1/t for rate at a given moment?

1/t gives an average rate over the whole timed period; the tangent gives the rate at one instant.

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

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What must happen for two particles to react, according to collision theory?

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What must happen for two particles to react, according to collision theory?: They must collide with energy equal to or greater than the activation energy, and with the correct orientation.
Define activation energy.: The minimum energy that colliding particles must have for a reaction to occur.
Define rate of reaction.: The change in concentration (or amount) of a reactant or product per unit time.
Why does increasing concentration increase rate?: More particles per unit volume, so collisions are more frequent and more successful collisions occur per second.
Why does increasing the pressure of a gas increase rate?: Gas molecules are closer together, so there are more molecules per unit volume and collisions are more frequent.
Why does a powder react faster than lumps of the same solid?: A larger surface area is exposed, so collisions with the other reactant are more frequent.
Give the two effects of increasing temperature.: Particles collide more often, and a greater proportion of collisions have energy ≥ activation energy. The second is more important.
Which factors change the proportion of collisions with energy ≥ Ea?: Temperature (and a catalyst, by lowering Ea). Concentration, pressure and surface area do not.
How do you calculate relative rate from the time taken?: Rate ∝ 1 ÷ time, e.g. 1 ÷ 50 s = 0.020 s⁻¹.
How is the rate at a given time found from a concentration–time graph?: Draw a tangent to the curve at that time and find its gradient.
How is the initial rate found from a graph?: The gradient of the tangent drawn at t = 0.
Why does the rate fall as a reaction proceeds?: The concentration of reactants falls, so collisions are less frequent.
Why is the tangent method better than 1/t for rate at a given moment?: 1/t gives an average rate over the whole timed period; the tangent gives the rate at one instant.

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).