ASChemistryCambridge (CIE)Exam Questions8. Reaction KineticsEffect of Temperature on Reaction Rates & the Concept of Activation Energy

Effect of Temperature on Reaction Rates & the Concept of Activation Energy (Cambridge (CIE) AS Chemistry): Exam Questions

Exam code: 9701

1 hour20 questions

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Effect of Temperature on Reaction Rates & the Concept of Activation Energy

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1
1 mark

Which factor affects the activation energy of a reaction?

  • the addition of a catalyst

  • a change in the concentration of the reactants

  • a change in pressure

  • a change in temperature

Choose your answer

2
1 mark

The Maxwell-Boltzmann energy distribution curve below describes a mixture of two gases at a given temperature.

Maxwell-Boltzmann energy distribution curve for a mixture of two gases with two activation energy values, Ea1 and Ea2, marked on the x-axis

Of the two activation energy (E~a~) values shown, one is for a catalysed reaction, the other for an uncatalysed one.

When a catalyst is used, which pair of statements is correct?

A

Ea1 uncatalysed reaction

fewer effective collisions

Ea2 catalysed reaction

more effective collisions

B

Ea1 catalysed reaction

fewer effective collisions

Ea2 uncatalysed reaction

more effective collisions

C

Ea1 uncatalysed reaction

more effective collisions

Ea2 catalysed reaction

fewer effective collisions

D

Ea1 catalysed reaction

more effective collisions

Ea2 uncatalysed reaction

fewer effective collisions

    Choose your answer

    3
    1 mark

    The diagram represents the reaction pathway for the following reaction:

    Z (g) + Y (g) → X (g) + W (g)

    Energy profile diagram for the reaction Z (g) + Y (g) → X (g) + W (g), showing reactants at higher energy than products with a transition state peak, indicating an exothermic forward reaction

    Which row describes the properties of the reverse reaction, W (g) + X (g) → Y (g) + Z (g)?

    Activation energy

    ΔH

    A

    smaller

    negative

    B

    smaller

    positive

    C

    larger

    negative

    D

    larger

    positive

      Choose your answer

      1
      1 mark

      The distribution of molecular energies in a sample of a gas at a given temperature is shown by the Boltzmann distribution graph.

      Boltzmann distribution curve for a gas at a given temperature, with point X marked at the peak representing the most probable energy

      The temperature is increased. Which statement describes the new position of X?

      • Fewer molecules possess the most probable energy value so X will shift to the right.

      • Fewer molecules possess the most probable energy value so X will shift to the left.

      • More molecules possess the most probable energy value so X will shift to the left.

      • More molecules possess the most probable energy value so X will shift to the right.

      Choose your answer

      2
      1 mark

      The diagram shows a reaction pathway for an endothermic reaction.

      Which arrow represents the activation energy for the forward reaction?

      Endothermic reaction pathway diagram showing energy against reaction progress, with four arrows A, B, C and D indicating different energy spans between reactants, products and transition state

        Choose your answer

        3
        1 mark

        The dotted-line curve on each graph below represents the corresponding distribution for a gas at 300 K.

        Which solid-line curve represents the distribution of molecular energies in the same mass of gas at 500 K?

        • Boltzmann distribution curves at 300 K (dotted) and 500 K (solid) where the solid curve is shifted to the left of the 300 K curve
        • Boltzmann distribution curves at 300 K (dotted) and 500 K (solid) where the solid curve has a higher, narrower peak at the same position as the 300 K curve
        • Boltzmann distribution curves at 300 K (dotted) and 500 K (solid) where the solid curve is shifted to the right with a higher peak than the 300 K curve
        • Boltzmann distribution curves at 300 K (dotted) and 500 K (solid) where the solid curve is shifted to the right with a lower, broader peak than the 300 K curve

        Choose your answer

        4
        1 mark

        The energies of the reactants, the products and the transition state of a reaction are shown in the reaction pathway diagram.

        Reaction pathway energy diagram showing three energy levels E1, E2 and E3 labelled on the y-axis, with the curve rising from the reactants through a transition state peak and falling to the products

        Which expression represents the activation energy of the forward reaction?

        • E1 - E2

        • E2 - E1

        • E2 - E3

        • E3 - E2

        Choose your answer

        5
        1 mark

        The diagram represents, for a given temperature, the Boltzmann distribution of the kinetic energy of the molecules in a mixture of two gases that react slowly together without a catalyst.

        The activation energy for the reaction, Ea, is marked for the uncatalysed reaction.

        Boltzmann distribution of molecular energies for a reacting gas mixture, with the uncatalysed activation energy Ea marked as a vertical line, and positions A, B, C and D labelled on the energy axis relative to Ea

        Which position represents the activation energy for the catalysed reaction?

          Choose your answer

          6
          1 mark

          Four gas-phase reactions with the same general equation are measured at the same temperature:

          X (g) + Y (g) ⇌ Z (g)

          Which reaction pathway diagram shows the reaction that would proceed most rapidly and with the highest equilibrium yield?

          • Reaction pathway energy profile showing an exothermic reaction with a high activation energy barrier
          • Reaction pathway energy profile showing an exothermic reaction with the lowest activation energy barrier
          • Reaction pathway energy profile showing an endothermic reaction with products at higher energy than reactants
          • Reaction pathway energy profile showing an endothermic reaction with a high activation energy barrier and products above reactants

          Choose your answer

          7
          1 mark

          The diagram shows a Boltzmann distribution of molecular energies for a gaseous mixture. The distribution has a peak, labelled Q on the diagram.

          Boltzmann distribution of molecular energies for a gaseous mixture with the peak labelled Q and the activation energy Ea marked as a vertical line on the energy axis

          Which row describes the changes when a catalyst is added to the mixture?

          Height of peak at Q

          Position of Ea

          A

          remains the same

          moves to the left

          B

          decreases

          moves to the left

          C

          remains the same

          moves to the right

          D

          decreases

          moves to the right

            Choose your answer

            8
            1 mark

            A graph shows the Boltzmann distribution of molecular energies at temperature T1 for a sample of gas. A vertical line represents the activation energy, Ea. A second curve is added for the same sample at a higher temperature, T2.

            Graph showing distribution of molecular energy. Two curves marked T1 and T2 with a vertical line at Ea. X-axis: molecular energy E, Y-axis: proportion of molecules.

            Which statement correctly describes the T2 curve compared to T1?

            • The area under the curve increases and the peak shifts to the right.

            • The activation energy decreases and the peak shifts to the right.

            • The peak shifts to the right and becomes lower.

            • The activation energy decreases and the area under the curve increases.

            Choose your answer

            1
            1 mark

            The following diagrams show Boltzmann distributions.

            Two Boltzmann distribution diagrams. Diagram 1 shows a distribution curve with two vertical lines labelled P and Q at different positions on the energy axis. Diagram 2 shows two distribution curves labelled X and Y with different widths and peak heights.

            In diagram 1, one line, P or Q, corresponds to the activation energy in the presence of a catalyst and the other line corresponds to the activation energy of the same reaction in the absence of a catalyst.

            In diagram 2, one curve, X or Y, corresponds to a higher temperature of the same sample of gas.

            Which row correctly identifies the presence of a catalyst and the higher temperature?

            Presence of catalyst

            Higher temperature

            A

            P

            X

            B

            P

            Y

            C

            Q

            X

            D

            Q

            Y

              Choose your answer

              2
              1 mark

              1 dm3 of gas X and 1 dm3 of gas Y are kept at the same temperature and pressure.

              The speed of the molecules is plotted against the fraction of molecules with that speed.

              Speed distribution curves for gases X and Y at the same temperature and pressure, showing the fraction of molecules at each molecular speed

              Which statement is correct?

              • The molar mass of X is larger than the molar mass of Y

              • The molar mass of Y is larger than the molar mass of X

              • The average kinetic energy of X is higher than that of Y

              • The average kinetic energy of Y is higher than that of X

              Choose your answer

              3
              1 mark

              The diagram shows a Boltzmann distribution of molecular energies with three regions, H, J and K, labelled.

              Boltzmann distribution curve with three regions H, J and K labelled on different parts of the curve

              The temperature is decreased. Which row describes what happens to the size of the areas labelled H, J and K?

              H

              J

              K

              A

              Decreases

              Decreases

              Increases

              B

              Decreases

              Increases

              Decreases

              C

              Increases

              Decreases

              Decreases

              D

              Increases

              Decreases

              Increases

                Choose your answer