Which molecules can react?

Those with energy equal to or greater than the activation energy.

How does the curve change when temperature increases?

The peak is lower and moves right, the tail is bigger, and the total area is unchanged.

Why does a small temperature rise increase rate a lot?

The proportion of molecules with energy at least Eₐ increases greatly, so many more collisions are successful.

How does a catalyst increase rate?

It provides an alternative route with a lower activation energy.

What happens to a Maxwell–Boltzmann curve when a catalyst is added?

The curve is unchanged but the Eₐ line moves to the left.

Does a catalyst change ΔH?

No. Reactants and products have the same energies.

What does a catalysed reaction profile with an intermediate look like?

Two lower peaks with a dip between them for the intermediate.

How do you find the activation energy of the second step on a two-peak profile?

Peak energy minus the energy of the intermediate.

Which step is rate-determining on a profile with two peaks?

The step with the highest peak (the highest activation energy).

How do catalysts make processes more sustainable?

They allow lower temperatures, so less energy, and can give routes with higher atom economy.

Maxwell-Boltzmann distribution and catalystsEdexcel International A Level Chemistry: Flashcards

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What does a Maxwell–Boltzmann distribution show?

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What does a Maxwell–Boltzmann distribution show?: The spread of molecular energies in a sample: number of molecules against energy.
What does the area under a Maxwell–Boltzmann curve represent?: The total number of molecules.
Which molecules can react?: Those with energy equal to or greater than the activation energy.
How does the curve change when temperature increases?: The peak is lower and moves right, the tail is bigger, and the total area is unchanged.
Why does a small temperature rise increase rate a lot?: The proportion of molecules with energy at least Eₐ increases greatly, so many more collisions are successful.
How does a catalyst increase rate?: It provides an alternative route with a lower activation energy.
What happens to a Maxwell–Boltzmann curve when a catalyst is added?: The curve is unchanged but the Eₐ line moves to the left.
Does a catalyst change ΔH?: No. Reactants and products have the same energies.
What does a catalysed reaction profile with an intermediate look like?: Two lower peaks with a dip between them for the intermediate.
How do you find the activation energy of the second step on a two-peak profile?: Peak energy minus the energy of the intermediate.
Which step is rate-determining on a profile with two peaks?: The step with the highest peak (the highest activation energy).
How do catalysts make processes more sustainable?: They allow lower temperatures, so less energy, and can give routes with higher atom economy.

Exam questions on Maxwell-Boltzmann distribution and catalysts

A chemist sketches Maxwell–Boltzmann distributions of molecular energies for the same sample of gas at 300 K and at 310 K. She notes that the mean kinetic energy of the molecules rises by only about 3% over this range, yet the rate of a reaction in the gas roughly doubles.
Explain, using the Maxwell–Boltzmann distribution, why a rise in temperature of only 10 K can double the rate even though the mean kinetic energy rises very little.2 marks
Hydrogen peroxide solution decomposes slowly at room temperature to form water and oxygen. When a small amount of solid manganese(IV) oxide is added, oxygen is released rapidly, and the manganese(IV) oxide can be recovered unchanged at the end.
Describe how the reaction profile for the catalysed route differs from the profile for the uncatalysed route, given that the catalyst forms an intermediate.2 marks
Ethanol can be made industrially by the catalysed hydration of ethene with steam, using phosphoric acid on a silica support at about 570 K. A laboratory alternative is to react bromoethane with aqueous sodium hydroxide, which gives sodium bromide as the only other product. A company wants to know which route is more sustainable.
Explain, in terms of activation energy, how a catalyst allows a process to be carried out at a lower temperature, and why this makes the process more sustainable.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).