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In today's T-Level Chemistry lesson, we are going to study A eleven point two, Temperature and Rate of Reaction.

By the end of this lesson, you should be able to explain why increasing temperature increases particle kinetic energy and speed, how that changes collision frequency and the proportion of successful collisions, what happens when temperature falls, and how to read that change on a Maxwell–Boltzmann distribution.

First, retrieve collision theory. What two conditions are required for a successful collision?

Sufficient collision energy, and the correct spatial orientation.

Activation energy is the minimum collision energy required for a successful reaction.

If particles move faster, they collide more frequently. If they have more kinetic energy, a greater proportion can have energy equal to or greater than the activation energy.

Specification point A eleven point two point one. Temperature is linked to the kinetic energy of particles.

When temperature increases, particles have greater average kinetic energy, so they move faster.

Watch the particle model. At higher temperature the particles travel more quickly through the mixture.

That faster movement has two effects. First: collision frequency.

Faster particles encounter other reactant particles more often, so there are more collisions per unit time, and more opportunities for successful collisions.

Second: the proportion of successful collisions.

The particle-energy distribution changes. A greater proportion of particles have energy equal to or greater than the activation energy, so a greater proportion of collisions can satisfy the energy requirement.

Correct orientation is still required. Increased temperature does not remove that condition.

Put the chain together. Higher temperature means greater average kinetic energy. Particles move faster, so collisions occur more frequently. A greater proportion of particles have energy at least equal to activation energy. Successful collisions therefore occur more frequently, and the rate of reaction increases.

Now specification point A eleven point two point two: what happens when temperature decreases?

Lower temperature means lower average kinetic energy. Particles move more slowly. Collision frequency decreases. A smaller proportion of particles have energy equal to or greater than activation energy. Successful collisions occur less frequently, so reaction rate decreases.

Avoid a common shortcut. It is incomplete to say only that particles collide less. A full explanation includes fewer collisions per unit time and a smaller proportion of collisions with sufficient energy.

The activation energy of the same reaction has not changed simply because temperature changed. Orientation remains a requirement at all temperatures.

Specification point A eleven point two point three. The Maxwell–Boltzmann distribution shows how particle energies are distributed in a sample at a particular temperature.

The horizontal axis is particle energy. The vertical axis is the number, or fraction, of particles with a given energy. Activation energy can be shown as a vertical threshold, E a.

Now compare two temperatures. The higher-temperature curve is broader, has a lower peak, and is shifted toward higher energies.

The total number of particles is unchanged, so the total area under each distribution is the same for the same sample.

The key rate comparison is the area to the right of E a.

Particles to the right of the activation-energy line have energy greater than or equal to E a.

At higher temperature, that area is larger, so a greater proportion of particles have sufficient energy for a successful collision. That helps explain the increased reaction rate.

What changes with temperature? The shape of the energy distribution, the average kinetic energy, and the proportion with energy at least E a.

What does not change for the same uncatalysed reaction? The activation-energy threshold itself.

The graph supports the energy part of collision theory. Successful collisions still also require correct orientation.

Guided interpretation. Curve A is taller and narrower. Curve B is lower and broader.

The activation-energy line is fixed. Which curve is the higher temperature?

Curve B. It is broader and shifted toward higher energies, with a lower peak.

Curve B has the larger area to the right of E a, so a greater proportion of particles have sufficient energy. Together with increased collision frequency, that gives a greater reaction rate.

Correct four errors before the examination.

Heating does not lower activation energy for the same reaction. It changes the particle-energy distribution.

The peak does not get higher when temperature increases. The distribution becomes broader and the peak becomes lower.

Collision frequency is not the only change. The proportion of particles with energy at least E a also increases.

And every collision above E a is not automatically successful. Correct spatial orientation is still required.

Examination-style question. A reaction mixture is heated from two hundred and ninety eight kelvin to three hundred and eighteen kelvin.

Part a. Explain how the increase in temperature affects particle kinetic energy and collision frequency. Two marks.

Average kinetic energy increases. Particles move faster and collide more frequently.

Part b. Explain why the proportion of successful collisions increases. Two marks.

A greater proportion of particles have energy equal to or greater than E a, so more collisions satisfy the energy requirement.

Part c. Describe how the Maxwell–Boltzmann distribution changes. Three marks.

The distribution becomes broader, its peak becomes lower, and a greater area lies at higher energies, to the right of E a.

Part d. Explain why the reaction rate increases, using collision theory. Three marks.

Collision frequency increases and a greater proportion of collisions have sufficient energy, so successful collisions occur more frequently and rate increases. Correct orientation is still required.

Before we finish, complete the model.

Higher temperature increases average kinetic energy and particle speed. Collision frequency increases. The Maxwell–Boltzmann curve becomes broader and lower, with a larger area beyond E a. Successful collisions occur more frequently, so rate increases. Lower temperature does the reverse. Temperature does not, by itself, change E a for the same uncatalysed reaction.

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