Activation Energy - UNSOLVED PRACTICE SET

Q7. Define activation energy. Draw a rough energy profile diagram for an exothermic reaction and label the activation energy.

Q8. Explain why not all collisions between reactant molecules lead to a chemical reaction, even when the molecules have sufficient energy.

Q9. A catalyst lowers the activation energy of a reaction from 80 kJ/mol to 50 kJ/mol. Draw rough energy profile diagrams for the catalyzed and uncatalyzed reactions on the same axes.

Q10. Why does a spark ignite a matchstick, even though the matchstick and oxygen are always in contact at room temperature?

Q11. The activation energy for the forward reaction is 50 kJ/mol and for the backward reaction is 70 kJ/mol. Is the reaction exothermic or endothermic? Calculate ΔH for the reaction.

Q12. Explain the concept of the activated complex with a suitable example. Why is it called a "transition state"?

Q13. Explain the concept of activation energy with a neat energy profile diagram. Distinguish between the activation energy of the forward and backward reactions for both exothermic and endothermic reactions.

Q14. (a) What is activation energy? How does it affect the rate of a chemical reaction?

(b) The rate constant of a reaction at 300 K is 2.0 × 10⁻³ s⁻¹ and at 320 K is 8.0 × 10⁻³ s⁻¹. Calculate the activation energy. (R = 8.314 J K⁻¹ mol⁻¹)

(c) Draw an energy profile diagram showing how a catalyst affects the activation energy.

Q15. (a) Define the following terms:

(i) Activation energy

(ii) Activated complex

(iii) Threshold energy

(b) The activation energy for a reaction is 60 kJ/mol. At what temperature will the fraction of molecules having energy equal to or greater than activation energy be twice that at 300 K? (Use the Arrhenius factor concept—set up the equation.)

Q16. For a reaction, the following data is given:

Activation energy (uncatalyzed) = 100 kJ/mol

Activation energy (catalyzed) = 60 kJ/mol

Temperature = 300 K

R = 8.314 J K⁻¹ mol⁻¹

(a) Using the Arrhenius equation, calculate the ratio of rate constants (k_catalyzed / k_uncatalyzed) at 300 K.

(b) By what factor does the catalyst increase the rate?

(c) If the frequency factor A is the same for both, what does this tell you about the mechanism?

Q17. The activation energy for the decomposition of hydrogen peroxide is 75 kJ/mol without a catalyst and 25 kJ/mol with a catalyst (enzyme catalase).

(a) Calculate the ratio of rate constants at 298 K with and without the catalyst.

(b) Explain why hydrogen peroxide decomposes rapidly when a small cut exposes it to catalase in blood.

(c) Draw energy profile diagrams for both catalyzed and uncatalyzed reactions.

(d) Why doesn't the catalyst change the overall enthalpy change of the reaction?

Q18. In a school chemistry lab, students compare the rate of decomposition of H₂O₂ with and without MnO₂ as a catalyst.

(a) What role does MnO₂ play? Explain in terms of activation energy.

(b) If the activation energy without catalyst is 75 kJ/mol and with MnO₂ is 55 kJ/mol, calculate how many times faster the reaction is at 300 K. (Use Arrhenius equation.)

(c) The students observe that the reaction with MnO₂ produces more bubbles initially but eventually produces the same total volume of O₂. Explain why.