16.1.5 Activation energy from rate data Quiz: Pearson Edexcel Chemistry, Unit 16

The 20 questions

1. What does the activation energy represent?

   • The minimum energy that colliding particles must have for a collision to lead to reaction
   • The total energy released when the products are formed from the reactants
   • The average kinetic energy of all the particles in the reaction mixture
   • The energy required to break all bonds in one mole of the reactant

2. What effect does a catalyst have on activation energy?

   • It increases the enthalpy change of the reaction, so the products are more stable than before the catalyst was added
   • It lowers the activation energy by providing an alternative pathway, without changing the enthalpy change of the reaction
   • It has no effect on the activation energy, but increases the number of collisions per second in the mixture
   • It raises the activation energy for the forward reaction only, so fewer particles can react at the same temperature

3. In the Arrhenius equation k = A e^(-Ea/RT), what does A represent?

   • The temperature in kelvin at which the reaction starts, which is set by the external heat source in the lab
   • The activation energy in joules per mole, which is the energy barrier that colliding particles must overcome
   • The gas constant, which has a value of 8.31 J K-1 mol-1 and appears in the exponential term of the equation
   • The pre-exponential factor, which relates to the frequency of collisions with the correct orientation

4. A graph of ln k against 1/T is a straight line. What is its gradient?

   • R/Ea
   • Ea/R
   • -A
   • -Ea/R

5. The Arrhenius equation is used with R in J K-1 mol-1 and Ea in J mol-1. What is the unit of Ea/R?

   • kJ mol-1
   • K
   • J mol-1 K
   • mol K J-1

6. Which rearrangement of the Arrhenius equation gives y = mx + c with y = ln k and x = 1/T?

   • ln k = ln A - (Ea/R)(1/T)
   • k = ln A - Ea/RT
   • ln k = Ea/RT - ln A
   • ln k = ln A + (Ea/R)(1/T)

7. Why does raising the temperature increase the rate of a reaction?

   • The total number of particles in the vessel increases as the temperature rises, so there are more collisions
   • The activation energy falls as the temperature rises, so the barrier is lower and every collision succeeds
   • More particles have energy at or above the activation energy, so a greater fraction of collisions are successful
   • The enthalpy change becomes more negative as the temperature rises, so the reaction releases energy faster

8. Two reactions have the same pre-exponential factor. At the same temperature, which has the larger rate constant?

   • The reaction with the lower activation energy, since more molecules exceed the lower energy barrier
   • The reaction with the higher enthalpy change, since it releases more energy
   • Both have the same rate constant, since A is the same
   • The reaction with the higher activation energy, since more collisions have the energy needed

9. On a Maxwell-Boltzmann distribution, what does adding a catalyst do?

   • It increases the number of particles with energy greater than the activation energy, without changing the shape of the curve
   • It moves the peak of the curve to a higher energy and increases the total area under the whole distribution curve
   • It lowers the peak energy of the curve and reduces the total number of particles present in the reaction mixture
   • It makes the curve steeper at the peak while keeping the activation energy line in the same position on the graph

10. A plot of ln k against 1/T has a gradient of -4.8 x 10^3 K. What is the activation energy?

    • 580 kJ mol-1
    • 40 kJ mol-1
    • 4.8 kJ mol-1
    • -40 kJ mol-1

11. The rate constant doubles when temperature rises from 298 K to 308 K, approximately. Which activation energy best fits?

    • About 53 kJ mol-1
    • About 530 kJ mol-1
    • About 5.3 kJ mol-1
    • About 106 kJ mol-1

12. A reaction has Ea = 75 kJ mol-1. By what factor does k increase when temperature rises from 298 K to 308 K?

    • About 10
    • About 1.5
    • About 2.7
    • About 5.4

13. Ea = 50 kJ mol-1. By what factor does k increase when temperature rises from 300 K to 320 K?

    • About 3.5
    • About 10
    • About 7.0
    • About 1.3

14. A catalyst lowers Ea from 80 kJ mol-1 to 50 kJ mol-1. By what factor does the rate constant increase at 300 K, assuming A is unchanged?

    • About 1.6
    • About 1.7 x 10^3
    • About 30
    • About 1.7 x 10^5

15. Points on a ln k against 1/T plot are (3.00 x 10^-3 K-1, -10.0) and (2.80 x 10^-3 K-1, -6.0). What is Ea?

    • About 166 kJ mol-1
    • About 20 kJ mol-1
    • About 83 kJ mol-1
    • About 2.0 kJ mol-1

16. A student reports Ea = -45 kJ mol-1 from a plot. Which evaluation is best?

    • The value cannot be right, since Ea is always positive, so the sign of the gradient has been mishandled
    • The value is right, since the plot had a positive gradient for an endothermic reaction
    • The value is right, since catalysts make the activation energy negative
    • The value is right, since a negative activation energy shows the reaction is exothermic

17. Why does a plot of ln k against 1/T have a negative gradient?

    • The pre-exponential factor A is negative for all exothermic reactions
    • The gas constant R is negative, which makes ln k fall as temperature rises
    • The Arrhenius equation includes a negative sign that always reverses the gradient
    • k rises with T, so ln k rises as 1/T falls, which gives a negative gradient

18. What is the value of the gas constant R used in the Arrhenius equation?

    • 9.81 m s-2
    • 8.31 J K-1 mol-1
    • 1.38 x 10^-23 J K-1
    • 6.02 x 10^23 mol-1

19. Ea = 60 kJ mol-1. By what factor does k increase when temperature rises from 300 K to 310 K?

    • About 1.3
    • About 10
    • About 4.5
    • About 2.2

20. Why does a catalyst not change the position of an equilibrium?

    • It increases the enthalpy change of the reaction, which favours the products and so shifts the equilibrium to the right
    • It raises the temperature of the mixture locally, which shifts the equilibrium towards the endothermic side of the reaction
    • It removes products as they form, which pulls the equilibrium forward and so changes the final position of the mixture
    • It lowers the activation energy of the forward and reverse reactions equally, so equilibrium is reached faster at the same position

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