LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

WD 46

FOURTH SEMESTER – April 2009

CH 4807 / 3801 – CHEMICAL KINETICS

 

 

 

Date & Time: 18/04/2009 / 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. What is your inference if the rate of a chemical reaction involving H₂(g) as one reactant was decreased when hydrogen was substituted with Deuterium?
3. The racemisation of pinene is a first order reaction. In the gas phase, the specific rate constant was found to be 2.2×10^-5 min^-1 at 457.6K and 3.07 x10^-3 min^-1 at 510K. Estimate the energy of activation
4. The absolute rate theory predicts that the frequency factor (A) is proportional to Tⁿ, where T is temperature. Calculate the value of exponent ‘n’ for the following reaction:                                  atom + atom         à   linear activated complex
5. Predict the influence of ionic strength on the rate of the following reaction and estimate the sign of DS^#: [Co(NH₃)₅Cl]²⁺ + OH^– à [Co(NH₃)₅OH]²⁺ + Cl^–
6. What are Arrhenius and vant Hoff Intermediates?
7. What is meant by capillary condensation?
8. The turnover number of an enzyme with a single active site that metabolizes acetylcholine is     4 x 10⁴ s^-1.  How may grams of it can 2.16 x 10^-6 g of the enzyme metabolize in one hour.  The molecular masses of the enzyme and the substrate are 4.2 x 10⁴ and 146.21 g/mol respectively.
9. The thermal decomposition of acetaldehyde forming gaseous products CH₄ and CO through a chain mechanism follows the rate law d[CH₄]/dt = (k₁/k₄)^1/2.k₂[CH₃CHO]^3/2.  Deduce an expression for observed activation energy in terms of stepwise activation energies and stepwise rate constants k₁,k₂ and k₄
10. For the base 2,4 dinitro aniline (B) pK_BH+ = – 4.55. Evaluate H₀ if [BH⁺]/[B]= 45 x10^-6 in 1 M HClO₄

PART – B

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

 

11. The half life of ⁶⁴Cu which emits a beta particle is 12.8 h. (a) How many grams of Copper are required to produce 150 beta particles per minute? (b) For a biochemical experiment, it is calculated that the activity must not go below 150 beta particles per minute.  The duration of the experiment is 10 hours.  What is the minimum initial amount of ⁶⁴Cu and what is the minimum initial activity that one should start with?
12. Find the relation between half-life (t_1/2) and temperature and from this relation, show how E_a can be obtained.
13. For the second order gas phase reaction, H₂+I₂ à 2HI, the pre–exponential term at 300⁰C is     4 x 10¹⁰   dm³ mol^-1 s^-1 and E_a = 150 kJ per mole.  Calculate DH^#  and DS^#
14. Using the Collision theory of reaction rates, derive the expression for the Collision number between two different molecules A and B.
15. Write a note on Okamato-Brown Equation.
16. If the rate constant at high pressures for the isomerization of cyclo propane is 1.2×10^-5 s^-1 and that at low pressure is 5.14 x10^-6 torr^-1 s^-1, above what pressure will the isomerization be, for all practical purposes, a first order reaction?
17. The gas phase decomposition of acetic acid at 1189 K proceeds by way of two parallel reactions : i) CH₃COOH à CH₄ + CO₂                       k₁ = 3.74 s^-1
18. ii) CH₃COOH à H₂C==C==O + H₂O k₂ = 4.65 s^-1

Calculate the maximum percentage yield of the ketene obtainable and the overall half life period.

18. The equilibrium constant for the reaction, D⁺aq + OD^–aq ó D₂O (l) at 25⁰C is 4.08 x 10¹⁶ mol^-1 k_-1 = 2.5 x 10^-6 s^-1.  Calculate the relaxation time for a temperature jump experiment to a final temperature of 25⁰C.  The concentration of D₂O at 25⁰C is 55.13 M.
19. Discuss the kinetics of bimolecular quenching and derive Stern-Volmer equation.
20. Explain how kinetic parameters be evaluated for a single substrate enzymatic reaction.

 

21. Show that Bronsted catalytic law is a form of linear free energy relation.
22. Ina typical BET plot for the adsorption of N₂(g) on solid Al₂O₃, a straight line was obtained with slope = 123 x 10^-5 mm^-3 and y-intercept 3.98 x 10^-6 mm^-3. Assume that the volumes are corrected to STP conditions and each atom occupies an area of 0.16 nm².  Calculate the surface area of the solid catalyst.

 

 

PART – C

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

 

23. The following data were obtained for the reaction between benzoyl chloride and aniline in benzene at 25⁰ The initial concentration of benzoyl chloride was 5 x 10^-3 mol dm^-3 and that of aniline     1 x 10^-2 mol dm^-3. Calculate the second order rate constant for the reaction, C₆H₅COCl + 2C₆H₅NH₂ à C₆H₅CONHC₆H₅ + C₆H₅NH₂HCl

t/min                                        0          2.5       5.6       9.6       14.6

[C₆H₅COCl]10³/mol dm^-3       5.0       4.5       4.0       3.5       3.0

24. a) The pKa for the dissociation of phenol in water at 25⁰C is 9.85. The r value for the dissociation H₂O + X-C₆H₄OH à X-C₆H₄O^– + H₃O⁺ is 2.00. If  s^–_p value of –CH₃ is –0.31, determine Ka for the dissociation of p-methyl phenol in water at this temperature.                                                (4)
25. b) Derive the expression to show the effect of dielectric constant of the medium on the rate of a reaction between dipoles and explain it.                                                                                  (6)

25.a)    Discuss the mechanism of bimolecular surface reaqctions with specific reference to NH₃(g) + D₂(g) à NH₂D(g) + HD(g).  Given that NH₃ is molecularly adsorbed and D₂ is dissociatively adsorbed species.  Derive the relevant rate law consistent with the mechanism you choose and explain.                                                                                                                      (6)

1. b) Explain the PE diagrams for Arrehenius type intermediates.                                     (4)
2. The decomposition of nitramide in water, O₂N-NH₂ (aq) à N₂O(g) + H₂O(l)

follows the mechanism:

1. i) O₂N-NH₂ (aq)    à         O₂N-NH^– (aq) + H⁺(aq)       fast equilibrium
2. ii) O₂N-NH^–  (aq)    à     N₂O (g)  + OH^–(aq)                 slow

iii)        H⁺ (aq)+ OH^–(aq)à    H₂O(l)                                     fast

1. Derive the rate law for the reaction using i) equilibrium approach ii) steady state approximation     for O₂N-NH^–                                                                                                                              (6)
2. Express the observed rate constant in terms of stepwise rate constants in each case. Deduce an expression for overall activation energy in terms of stepwise activation energies as per equilibrium approach.                                                                                                                   (4)
   27. Explain any two of the following: a)  Determination of Hammett acidity function     b)   Kinetics of reversible first order reaction  c)  Kinetics of H₂-Br₂ chain reaction.  d)   Evaluation of catalytic coefficients in a general acid-base catalysed reaction.                                                               (2 x 5)
   28. For the oxidation of glyoxaylic acid (GOx) by peroxomonophosphoric acid (PMP) in acid perchlorate medium (pH = 3), the following data were obtained: i) A plot of initial rate vs [PMP] yielded a straight line passing through the origin ii) The plot of initial rate vs [GOx] also yielded a straight line passing through the origin   iii) The pseudo first order rate constants (k_obs) increased with increasing [lithium perchlorate]  iv)  The rate increased with increasing pH.  The plot of k (observed second order rate constant) vs 1/[H⁺] was a straight line with non-zero intercept.              v) The addition of  acrylonitrile had no effect on the rate of oxidation.                               (5)

Propose a suitable mechanism and derive the rate law to account for all the above experimental data.                                                                                                                                            (5)

 

 

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