LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

LM 41

M.Sc. DEGREE EXAMINATION – CHEMISTRY

THIRD SEMESTER – APRIL 2007

CH 3801 – CHEMICAL KINETICS

 

 

 

Date & Time: 26/04/2007 / 9:00 – 12:00      Dept. No.                                       Max. : 100 Marks

 

 

Part A

Answer ALL the questions.                                                    (10 x 2 = 20 Marks)

1. Differentiate between time order from true order.
2. At 518 K, the half life for the decomposition of a gaseous compound initially at 363 torr

was 410 s.  When the pressure was 169 torr, the half life was 880 s.

Determine the order of the reaction.

3. The rate constant for the reaction of the type A^2- + B^– à X^3- + Y is 0.2 M^-1 min^-1 in zero ionic strength. Calculate the rate constant of the above reaction in water at 25⁰C in an ionic strength of 0.5 M.
4. What is the significance of electrostriction?
5. Write down the expression for the rate constant of a reaction between two linear molecules forming a linear transition state on the basis of ARRT.
6. What is meant by ‘turnover number’?
7. For the reaction between 2NO(g) + O₂(g) à 2 NO₂(g), the observed rate decreases with increase in temperature.
8. What is meant “capillary condensation” ?
9. Calculate the Hammett acidity function (H₀) if pK_BH⁺ for the base 4 Chloro 2 nitro

aniline (B) is –3.3 and [BH⁺]/[B] = 8 x 10^-4

10. Mention the steps involved in a chain reaction with an example.

PART – B

Answer ANY EIGHT questions                                            (8 x 5 = 40 Marks)

11. Describe the potential energy surface of the following reaction. H_AH_B + H_C <=>  H_BH_C + H_A.
12. The rate constant for the decomposition of a certain substance is 1.7 x 10^-2 dm³ mol^-1 s^-1 at 24⁰C and 2.01 x 10^-2 at 37⁰ Evaluate the activation energy of the reaction.
13. Calculate the number of collisions per s between oxygen molecules in 1 cm³ at a total pressure of 1 atm and at a temperature of 27⁰ The molecular radius oxygen is 1.46×10^-8 cm.
14. Explain the kinetics of unimolecular reactions using Lindemann’s Theory.
15. Explain the significance of volume of activation for reactions in solution phase.
16. The pre-exponential factor for a certain first order reaction is 3.98 x10¹³ s^-1 and E_a = 180 KJ mol^-1. Calculate DH^#, DS^# and DG^# for the reaction at 200⁰C (R=8.314 J K^-1 mol^-1)
17. Differentiate van’t Hoff intermediates from Arrhenius intermediates.
18. Write BET equation. Mention the terms involved.  How is it verified?
19. How is Stern-Volmer constant evaluated?
20. For the following parallel first order reaction, the percentage yields of he two products B and C are 1.2 and 98.8 respectively.  The overall half life is 21.8 y.  Calculate he rate constant for each of the two parallel paths.                                                 B

A

C

21. Outline the various steps involved in the thermal decomposition of acetaldehyde.
22. Discuss the principle of flash photolysis.

PART – C

Answer ANY FOUR questions                                              (4 x 10 = 40 Marks)

23. Compare the rate constants calculated by the ARRT and the Collision Theory for the reaction between two atoms. Explain.
24. Using double sphere model for a reaction between two ions deduce the relation connecting the rate constant and dielectric constant of the medium and explain.
25. Explain an two of the following a) Relaxation methods b) Skrabal plots  c) kinetics of explosions
    26. Explain Michaelis-Menton kinetic scheme for an enzymatic reaction. Deduce the rate law and explain
    27. Discuss the mechanisms of bimolecular surface reactions with a specific example.

Deduce the rate expression for each case.

28. In the study of the kinetics of chlorination of phenol and mono substituted phenols by t-butyl hypochlorite in aqueous alkaline medium, the following data were obtained.

 

(i) At constant [substrate]₀, and [OH^–], the plots of log[t-BuOCl] vs time were linear. (ii) At constant [t-BuOCl]₀ and [OH^–], the rate increased with increase in [substrate] with first order dependences with all phenols.  The plots of k_obs vs [phenol] were linear with zero intercepts on the ordinates.  (iii) The rate decreased with increase in [OH^–] at fixed [t-BuOCl]₀ and [phenol] for the chlorination of all phenols. The plots of k_obs vs 1/[OH^–] gave straight line passing through the origin (iv) Variation of ionic strength of the medium had negligible effect on the rates of chlorination.  (v) The decrease in the dielectric constant of the medium increased the rate.   Propose a suitable mechanism to account for the above data and explain.

 

 

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