ConceptConceptDocsDocuments

CAIE A-Level Chemistry 8.2 Temperature, Rate and Activation Energy

Practise using activation energy and Boltzmann distributions to explain how temperature changes reaction rate.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • define activation energy as the minimum collision energy needed for a reaction to occur
  • sketch the lower, broader and right-shifted Boltzmann curve produced by increasing temperature
  • link the larger area above EA to a higher frequency of effective collisions and a faster reaction

Covered syllabus points

8.2.1—Activation energy, EA8.2.2—Sketch and use the Boltzmann distribution8.2.3—In terms both of the Boltzmann distribution

8.2 Temperature, rate and activation energy question 1

[Maximum number: 4]

A large excess of 2-bromo-2-methylpropane is added to 0.0010 mol of NaOH(aq), which contains a few drops of phenolphthalein indicator. A stopwatch is started as soon as the substances are mixed. The time taken for the pink colour to disappear is recorded.

The experiment is repeated at different temperatures, keeping all concentrations and volumes of reagents constant.

Table for Question 8.2 Temperature, rate and activation energy question 1 — CAIE A-Level Chemistry AS

Question (a)

(a)

The graph shows the energy distribution of molecules in a sample of 2-bromo-2-methylpropane at 25∘C25^{\circ} \mathrm{C}.
EaE_{\mathrm{a}} represents the activation energy for the reaction.

Figure for Question (a) — CAIE A-Level Chemistry AS
[ 4 ]

Question (i)

(i)

Label the graph to show the proportion of 2-bromo-2-methylpropane molecules which have sufficient energy to react.

[ 1 ]

Question (ii)

(ii)

Use the same axes to sketch the distribution of energies of molecules in a sample of 2-bromo-2-methylpropane at 50∘C50^{\circ} \mathrm{C}.

[ 2 ]

Question (iii)

(iii)

State the effect of an increase in temperature on EaE_{\mathrm{a}} for this reaction.

[ 1 ]

8.2 Temperature, rate and activation energy question 2

[Maximum number: 5]

Ammonia, NH3\mathrm{NH}_{3}, is manufactured from nitrogen and hydrogen by the Haber process.

N2( g)+3H2( g)⇌2NH3( g)ΔH=−92 kJ mol−1\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92 \mathrm{~kJ} \mathrm{~mol}^{-1}

Question (a)

(a)

The Haber process is usually carried out at a temperature of approximately 400∘C400^{\circ} \mathrm{C} in the presence of a catalyst. Changing the temperature affects both the rate of production of ammonia and the yield of ammonia.

The Boltzmann distribution for a mixture of nitrogen and hydrogen at 400∘C400^{\circ} \mathrm{C} is shown. Ea represents the activation energy for the reaction.

Figure for Question (a) — CAIE A-Level Chemistry AS
[ 5 ]

Question (i)

(i)

Using the same axes, sketch a second curve to indicate the Boltzmann distribution at a higher temperature.

[ 2 ]

Question (ii)

(ii)

With reference to the Boltzmann distribution, state and explain the effect of increasing temperature on the rate of production of ammonia.

[ 3 ]

8.2 Temperature, rate and activation energy question 3

[Maximum number: 2]

Calcium, magnesium and radium are Group 2 elements. Radium follows the same trends as the other members of Group 2.

Cold water reacts slowly with a piece of Mg to produce bubbles of H2( g)\mathrm{H}_{2}(\mathrm{~g}). Cold water reacts rapidly with burning Mg to produce H2( g)\mathrm{H}_{2}(\mathrm{~g}) in an explosive mixture.

Mg+2H2O→Mg(OH)2+H2\mathrm{Mg}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Mg}(\mathrm{OH})_{2}+\mathrm{H}_{2}

Explain why the rate of reaction of cold water with burning magnesium is greater.

All question bank results loaded