What do the axes of the distribution show?

Vertical: number of molecules. Horizontal: energy.

Why does the curve never touch the energy axis at high energy?

There is no maximum energy a molecule can have, so a few always have very high energy.

What does the area under the curve represent?

The total number of molecules, which does not change with temperature.

What does the peak of the curve represent?

The most probable energy of the molecules.

Which molecules can react when they collide?

Those in the area to the right of the activation energy: energy ≥ Ea.

What happens to the peak when temperature rises?

It becomes lower and moves to a higher energy.

Why does a rise in temperature increase the rate?

A greater proportion of molecules have energy ≥ Ea, so more collisions are successful.

Does temperature change the activation energy?

No. Only the distribution changes; Ea is fixed for a given reaction pathway.

How does a catalyst appear on the distribution?

The curve is unchanged and the activation energy line moves left.

Why does a catalyst increase the rate at constant temperature?

It lowers Ea, so a greater proportion of molecules have energy ≥ the new Ea.

How do the curves for two temperatures compare in the tail?

The tail of the higher temperature curve is higher, so it has a greater area beyond Ea.

Maxwell-Boltzmann distributionsEdexcel A-Level Chemistry: Flashcards

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What does the Maxwell–Boltzmann distribution show?

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What does the Maxwell–Boltzmann distribution show?: The spread of molecular energies in a sample of gas at a given temperature.
What do the axes of the distribution show?: Vertical: number of molecules. Horizontal: energy.
Why does the curve start at the origin?: No molecules have zero energy.
Why does the curve never touch the energy axis at high energy?: There is no maximum energy a molecule can have, so a few always have very high energy.
What does the area under the curve represent?: The total number of molecules, which does not change with temperature.
What does the peak of the curve represent?: The most probable energy of the molecules.
Which molecules can react when they collide?: Those in the area to the right of the activation energy: energy ≥ Ea.
What happens to the peak when temperature rises?: It becomes lower and moves to a higher energy.
Why does a rise in temperature increase the rate?: A greater proportion of molecules have energy ≥ Ea, so more collisions are successful.
Does temperature change the activation energy?: No. Only the distribution changes; Ea is fixed for a given reaction pathway.
How does a catalyst appear on the distribution?: The curve is unchanged and the activation energy line moves left.
Why does a catalyst increase the rate at constant temperature?: It lowers Ea, so a greater proportion of molecules have energy ≥ the new Ea.
How do the curves for two temperatures compare in the tail?: The tail of the higher temperature curve is higher, so it has a greater area beyond Ea.

Exam questions on Maxwell-Boltzmann distributions

A student sketches the Maxwell–Boltzmann distribution of molecular energies for a gas at a fixed temperature, with the number of molecules on the vertical axis and energy on the horizontal axis, and marks the activation energy of a reaction on the energy axis.
Explain why the curve starts at the origin and why it never touches the energy axis at high energy.2 marks
A reaction mixture of gases is heated from 300 K to 310 K, and the rate of the reaction approximately doubles. The student sketches the distribution of molecular energies at the two temperatures on the same axes.
Explain, using the distributions, why the reaction is faster at 310 K.2 marks
A car's catalytic converter contains platinum and rhodium on a solid support. At the temperature of the exhaust gases it converts carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen: 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g). Without the catalyst the reaction is far too slow at this temperature.
Explain, with reference to the Maxwell–Boltzmann distribution, how the catalyst increases the rate of the reaction at the same temperature.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).