What does the area under a Maxwell–Boltzmann curve represent?

The total number of molecules in the sample.

How does the area under the curve change when temperature increases?

It does not change, as the number of molecules is the same.

Where does the Maxwell–Boltzmann curve start, and does it touch the axis at high energy?

It starts at the origin and never touches the axis at high energy.

What happens to the peak when temperature increases?

It becomes lower and moves to a higher energy.

What does the area to the right of Ea represent?

The number (proportion) of molecules with energy equal to or greater than the activation energy.

Define the rate of reaction.

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

Why does a small rise in temperature give a large rise in rate?

Ea is in the high-energy tail, so a small shift greatly increases the proportion of molecules with energy ≥ Ea.

Why does collision frequency explain little of the effect of temperature on rate?

Collisions become only slightly more frequent, but many more collisions have energy ≥ Ea.

What does the rate equal in the thiosulfate and acid experiment?

The rate is proportional to 1/t, where t is the time for the cross to disappear.

State two variables to control in the temperature investigation.

The volumes and the concentrations of the solutions.

Maxwell-Boltzmann distribution and temperatureAQA A-Level Chemistry: Flashcards

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What do the axes of a Maxwell–Boltzmann curve show?

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What do the axes of a Maxwell–Boltzmann curve show?: Energy of molecules on the x-axis and the number of molecules with that energy on the y-axis.
What does the area under a Maxwell–Boltzmann curve represent?: The total number of molecules in the sample.
How does the area under the curve change when temperature increases?: It does not change, as the number of molecules is the same.
Where does the Maxwell–Boltzmann curve start, and does it touch the axis at high energy?: It starts at the origin and never touches the axis at high energy.
What happens to the peak when temperature increases?: It becomes lower and moves to a higher energy.
What does the area to the right of Ea represent?: The number (proportion) of molecules with energy equal to or greater than the activation energy.
Does the activation energy change with temperature?: No.
Define the rate of reaction.: The change in concentration of a reactant or product per unit time.
Why does a small rise in temperature give a large rise in rate?: Ea is in the high-energy tail, so a small shift greatly increases the proportion of molecules with energy ≥ Ea.
Why does collision frequency explain little of the effect of temperature on rate?: Collisions become only slightly more frequent, but many more collisions have energy ≥ Ea.
What does the rate equal in the thiosulfate and acid experiment?: The rate is proportional to 1/t, where t is the time for the cross to disappear.
State two variables to control in the temperature investigation.: The volumes and the concentrations of the solutions.

Exam questions on Maxwell-Boltzmann distribution and temperature

The distribution of molecular energies in a sample of a gas at 300 K is described by a Maxwell–Boltzmann distribution curve, in which the number of molecules is plotted against their energy. The same sample is then heated to 310 K. A reaction of this gas has an activation energy that is much greater than the most probable molecular energy.
Explain the significance of the area under the curve to the right of the activation energy.2 marks
A student investigates how temperature affects the rate of the reaction between sodium thiosulfate solution and dilute hydrochloric acid, which forms a precipitate of sulfur. She mixes 50.0 cm³ of 0.10 mol dm⁻³ sodium thiosulfate solution with 5.0 cm³ of 1.0 mol dm⁻³ hydrochloric acid and measures the time taken for a cross viewed through the mixture to disappear. At 20 °C the time is 64 s and at 30 °C the time is 31 s.
Suggest two ways in which the reliability of the investigation could be improved.2 marks
A gas-phase reaction between two gases is carried out at constant volume at 300 K and at 310 K. At 300 K, 1.0 × 10⁻⁸ of the molecules in the mixture have energy equal to or greater than the activation energy. At 310 K the proportion is 1.8 × 10⁻⁸.
Calculate the factor by which the proportion of molecules with energy equal to or greater than the activation energy increases, and use it to estimate the effect on the rate of reaction.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).