LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034  M.Sc. DEGREE EXAMINATION – CHEMISTRY

AD 19

THIRD SEMESTER – NOV 2006

         CH 3801 – CHEMICAL KINETICS

 

 

Date & Time : 01-11-2006/9.00-12.00         Dept. No.                                                       Max. : 100 Marks

 

 

PART – A                          (10 ´ 2 = 20 marks)

 

Answer ALL the questions.

 

1. Differentiate time order from true order.
2. The half life of C–14 is 5770 years. Starting with 100 mg of C–14, how much of
   it would remain after 17,310 years.  Also calculate the decay constant.
3. What is the effect of ionic strength on the rate constant of the following reaction
   in solution?

[Fe(CN)₆]^4– + S₂O₈^2– ®  products

4. Show that the Collision theory of activation energy is less than the energy of
   activation calculated from Arrhenius equation.
5. Write down the expression for the rate constant of a reaction between two linear
   molecules forming a linear activated complex on the basis of ARRT.
6. What are “Skrabal plots”?
7. For an enzyme catalysed reaction K_M = 25 ´ 10­^–3 M and the turnover number

is 4 ´ 10⁷ s^–1, calculate the limiting rate of the reaction if [E]₀­ = 1.6 ´ 10^–8 M.

8. Mention all the steps involved in the thermal decomposition of acetaldehyde
   following a chain reaction.
9. “Adsorption if spontaneous is generally exothermic”. Explain.
10. Explain why conventional methods cannot be used for the study of kinetics of
    very rapid reactions.

 

PART – B

Answer any EIGHT questions.                                        (8 ´ 5 = 40 marks)

 

11. The rate of a reaction of the type A + B ® products was studied in solution phase
    at 298 K with initial concentration of each reactant at 0.02 M.  From the
    following data determine the order of the reaction and calculate the rate constant.

Time (min.)                                 20                            30                 40

Conc. of ‘A’ reacted

in ‘t’  (mol’/l)                      8.76 ´ 10^-3              1.07 ´ 10^–2    1.21 ´ 10^–2

12. Derive an expression for the total number of collisions of all molecules of ‘A’
    with all molecules of ‘B’.
13. Explain the kinetics of unimolecular gaseous reaction using Lindemann’s theory.
14. For hydrolysis of sulphamic acid the rate constant is 1.16 ´ 10^–3 l mol^–1 s^–1 at
    90°C with E_a = 127.61 kJ mol^–1.  Calculate DG^±,  DH^±  and  DS^± of the reaction
    occurring in solution phase.
15. Explain the significance of potential energy surfaces with an example.
16. What is the importance of “volume of activation” in the study of kinetics of
    reactions in solution.  Explain.
17. Explain the concept of Arrhenius and Van’t Hoff type intermediates.

 

 

 

18. Show that “Bronsted–Catalytic law” is a form of LFER (Linear Free Energy
    Relation)
19. The decomposition of PH₃(g) on a tungsten filament follows first order kinetics at
    low gas pressure and zero order kinetics at very high pressure.  Explain.
20. ²²⁷Ac has a half life of 21.8 years with respect to radioactive decay. The decay
    follows two parallel paths one leading to Th–227 and the other to Fr–223.  The
    percentage yields of these two daughter nuclides are 1.2% and 98.8%
    respectively.    Calculate the rate constant in y^–1 for each of the paths.
21. How is surface area of a solid determined using “BET equation”.
22. Explain the principle of “flash photolysis”.

 

 

PART – C

Answer any FOUR questions.                                         (4 ´ 10 = 40 marks)

 

23.(a)How is order of a reaction determined using dimensionless parameters.
Explain.                                                                                                  (6)

(b)Calculate the translational partition function of CO(g) in the standard state of
1 mol/l at 27° C.                                                                                    (4)

24.(a) Explain the “single sphere model” for a reaction between two ions in

solution.                                                                                                            (5)

(b) Discuss the importance of Hammett equation with one example.       (5)

25. Explain the kinetics of bimolecular surface reactions with a specific example.

Derive the relevant rate laws in each mechanism.                               (5 + 5)

26. Discuss the kinetic scheme for a single substrate enzymatic reaction and

Explain how the kinetic parameters can be evaluated.                        (5 + 5)

27. Explain any two of the following: (5 + 5)

(a) Kinetics of reversible reactions (both I order)

(b) Determination of   k_H⁺  for an acid catalysed reaction.

(c) Expression for relaxation time for a I order reaction

(d) Stern–Volmer equation

28. (a) The decomposition of ozone 2O₃ ® 3O₂ proceeds through the mechanism

 

(i) O₃ Û O₂ + O with k₁ is the rate constant for the forward reaction and k₂ is the rate constant for the backward reaction

(ii) O₃ + O  Û 2O₂ (slow step)

Derive expression for the rate of reaction using

• equilibrium approach for step (i)
• Steady state treatment for O . (7)

(b) How is the energy of activation for the overall reaction above related to the
energies of activation for the individual steps?                                 (3)

 

 

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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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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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LM 53

LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

FOURTH SEMESTER – APRIL 2007

CH :4807– CHEMICAL KINETICS

 

 

 

Date & Time : 16.04.2007/9.00-12.00      Dept. No.                                                      Max. : 100 Marks

 

 

Part A

 

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

1. Explain electrostriction.
2. The rate of a reaction between X and Y^– has been investigated in aqueous solution at 300 K and the second order rate constant (k₂⁰ ) at zero ionic strength is found to be 0.681 M^-1 s^-1. When ionic strength of the medium (m) is 1.60 x 10^-3 M,  the second order rate constant (k₂) is 0.901 M^-1 s^-1.  Predict the charge on X.
3. Show that collision theory of activation is less than the energy of activation calculated using the Arrhenius equation.
4. Write down the expression for the rate constant of a reaction between two linear molecules forming a non-linear transition state on the basis of ARRT.
5. 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 values of –CH₃ is –0.31, determine pKa for the dissociation of p-methyl phenol in water at this temperature.
6. What are Skrabal plots?
7. For the reaction 2NO(g) + O₂(g) à2NO₂(g), the observed rate of the reaction decreases with increase in temperature.
8. What is the significance of “Hammett acidity functions”??
9. In a typical Stern-Volmer plot graph of F₀/F (Y-axis) versus the concentration of quencher gave a straight line with slope = 640 l/mol and the rate constant of quenching is 10¹⁰ l mol^-1 s^-1. Calculate the Stern-Volmer constant and the life time.
10. Mention any four relaxation techniques employed for the study of kinetics of fast reactions.

PART – B

 

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

 

11. Using the Collision theory of reaction rates, derive the expression for the Collision number between two different molecules A and B
12. Calculate the rotational partition function for CO₂ in the standard state of 10³ mol m^-3 at 25⁰  The carbon-oxygen distance is 1.16 x 10^-10 m
13. Explain the need for activation energy based upon potential energy surfaces.
14. If the rate constant at high pressures for the isomerization of cyclopropane is 2 x 10^-4 s^-1 and that at low pressures is 5.14 x 10^-6 torr^-1 s^-1, below what pressure will the isomerization be for all practical purposes a second order  reaction?
15. Discus the double-sphere model for the influence of dielectric constant on the rate of an ion-ion reaction
16. Heckt and Conrad measured the rate constant (k₂) of the reaction of methyl iodide and sodium ethoxide in ethanol as 2.45 x 10^-4 M^-1 s^-1 at 285 K.  If E_a = 85.20 kJ/mol.  Calculate DG^#, DH^# and DS^# of the reaction.   (k_B=1.37 x 10^-23 J molecule^-1)
17. How is surface area of a solid deermined using BET equation.
18. For an enzyme catalyzed reaction, K_M = 25 x 10^-3 Calculate the rate of the reaction if the limiting rate is 0.64 M s^-1 and the  the initial substrate concentration of 4.32 x 10^-6 M.
19. Represent all the possible steps in a bimolecular photophysical process. Indicate the rate expression for each step.
20. For the following parallel first order reaction,  the rate constants of the two parallel paths are 3.75 and 4.65 s^-1  Calculate the percentage yield of B and also the overall half life of the reaction.                 B

A

C

21. Discuss the principle of flash photolysis.
22. Explain Bronsted catalytic law.

 

PART – C

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

 

23. a) How will you determine the order of a reaction using dimensionless parameters?
24. b) The volume of activation for a certain reaction is – 4 x 10^-6 m³ mol^-1 at 300 K.  Calculate the pressure required to double the rate constant if the initial pressure is 10⁵
25. a) Calculate the number of collisions per second in 1 cm³ of oxygen at  27⁰C and 101.3 kPa pressure given the molecular radius of oxygen to be 1.46 x 10^-8 cm
26. b) Explain the influence of internal pressure on the rate of a reaction between neutral molecules in solutions
27. Explain the kinetics of bimolecular surface reactions with a specific example. Derive the relevant rate law for each mechanism.
28. Explain any two of the following: a) Evaluation of kinetic parameters for an enzymatic reaction b) kinetics of chain reactions  c) van’ Hoff intermediates
    27. a) Derive an expression for relaxation time for a I order reaction.
    28. b) Outline the salient features of the kinetics of explosive reactions.
    29. For the oxidation of aresenous acid by enneamolbdomanganate (IV) in perchloric acid medium, the following data were obtained: (i) Under constant [HClO₄], the second order rate constants were fairly constant. (ii) Presence of acrylonitrile  has no effect on the rate of the reaction (iii) It was found that the graph of k_obs against [H⁺]² was found to be curved passing through origin  (iv) The rate of the reaction was found to be unaffected by the increase of the ionic strength of the medium (vi) The entropy of activation was –5 J K^-1 mol^-1.  Propose a suitable mechanism to account for the above data, write the rate expression, and explain your answer.

 

 

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      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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LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

THIRD SEMESTER – NOVEMBER 2012

CH 3812 / 4807 – CHEMICAL KINETICS

 

 

Date : 08/11/2012            Dept. No.                                        Max. : 100 Marks

Time : 9:00 – 12:00

 

Part A

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

1. In the reaction 2NO + 2H₂à N₂ + 2H₂O, mixture of reactant gases at an initial pressure of 336 mm of Hg, was half changed in 108 sec and when the initial pressure was 288 mm of Hg, it was half changed in 147 sec. Determine the order of reaction.
2. Show that the energy of activation according to the collision theory is less than the energy of activation given by Arrhenius theory.
3. If the rate constant at high pressure for the isomerization of cyclopropane is 1.2 x 10^-4 s^-1 and that at low pressure is 5.14 x 10^-6 torr^-1 s^-1, below what pressure will the isomerization be for all practical purposes a second order reaction?
4. Explain electrostriction.
5. Addition of acetonitrile in an atmosphere of N₂, has no effect on the oxidation of thioacids by benzyl trimethyl ammonium chlorobromate. What is your inference regarding the mechanism of this reaction?
6. What is meant by turn over number? What is its significance?
7. The observed activation energy for the gas phase recombination of iodine atoms in the presence of argon is negative. Explain.
8. Draw the concentration-time profile for the consecutive reaction, A à B à C, with proper labeling.
9. What is Hammett acidity function?
10. In water the fluorescence quantum yield and the observed fluorescence life time of tryptophan are 0.2 and 2.6 ns respectively. Calculate the fluorescence rate constant.

PART – B

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

11. Explain Guggenheim’s method for first order reactions
12. Hydrogen iodide gas has a viscosity of 39.66 x 10^-5 poise at 560 K and 101.3 kPapressure. Calculate the collision diameter of hydrogen iodide molecule.
13. Explain the need for activation energy based on potential energy surfaces. Illustrate your answer by taking the following reaction as example:

H^a+  H^b  – H^g    -à  H^a  –  H^b  + H^g

14. The pre-exponential term for a unimolecular gas reaction occurring at 200⁰C is 2.75 x 10¹⁵ s^-1. Calculate the entropy of activation.  How does the result depend upon the standard state used?
15. Using double sphere model explain the effect of dielectric constant of the medium on the rate of the reaction between ions in solutions.
16. 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. The s^–_p values of –NO₂  and – COOH are + 1.27 and + 0.73 respectively and the s⁺_p values of – CH₃ and – Cl^– are –0.31 and + 0.11 respectively. Arrange the substituted phenols in the order of increasing acid strength.
17. What are Skrabalplots?Explain.
18. The rate of the reaction HA + H₂O   H₃O⁺ + A^– is studied using T-jump method.  Calculate the expected relaxation time if a 0.1 M solution has a pH of 4 and k_-1 is 10¹⁰ M^-1 s^-1.
19. The rate law for the reaction between 2- bromo butane and OH^-1 in 75% ethanol (aq) at 30⁰C is given by rate = 3.2 x 10^-5 [RBr][OH^-1] + 1.5 x 10^-6 [RBr].  What percentage of reaction will take place by an S_N2 path way if a) [OH^-1] = 1 M b) [OH^-1] = 10^-3
20. Discuss the salient features of the kinetics of H₂-Br₂ chain reaction (No derivation is needed).
21. Explain the kinetic scheme for a bimolecular quenching process and hence derive Stern-Volmer equation.
22. How are kinetic parameters evaluated for a single substrate enzymatic reaction?

PART – C

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

23. a) The isotope  ⁹⁰Sr emits β^–-radiation and has a half life of 28 years.  If 2 micro gram of⁹⁰Sr is absorbed by a newly born child, how much of it is left behind after (a) 18 years (b) 70 years. (5)
24. b) Derive the expression for the number of collisions between two different gas molecules A and B.(5)
25. a) The rate constant k for the reaction between persulphate ions and iodide ions in aqueous solution at 25⁰C varies with ionic strength (as follows.  Estimate Z_AZ_B.

x 10^-3/ mol dm^-3                              2.45                        4.45                        8.45

k/ 10^-4 dm³ mol^-1 s^-1                 1.05                 1.16                 1.26                 (6)

1. b) How are the reactions classified on the basis of the volume of activation? (4)
2. a) Explain how the catalytic constants for a reaction catalyzed by both general acid and general base are evaluated. (6)
3. b) Show that Bronsted catalytic law is a form of LFER.                                                   (4
4. Explain any two of the following: (5+5)a) Kinetics of H₂-O₂ explosive reaction b) van’t Hoff type and Arrhenius type intermediates.
5. c) Surface area measurement using BET equation d) Kinetics of reversible reactions
6. a) Discuss the Rideal-Eley mechanism for a bimolecular surface reaction with a specific example. (6)

b) The decomposition of O₃, 2O₃à 3 O₂ is observed to obey the rate law – d[O₃]/dt = 2 k [O₃]²/[O₂], suggest a suitable mechanism and explain.                                                            (4)

28. For the oxidation of hexacyano ferrate (II) by peroxodisulphate in aqueous perchloric acid medium, the following data were obtained: (i) Plots of log [Fe(CN)₆^4-] vs time were linear at constant ionic strength and [H⁺] (ii) The pseudo first order rare constants (k_obs) vary linearly with peroxodisulphate concentration at constant ionic strength and [H⁺]  (iii) At constant [Fe(CN)₆^4-] [S₂O₈^2-] and ionic strength, increasing [perchloric acid] increased the rate.  Plots of [S₂O₈^2-] /k_obsvs 1/[H⁺] were linear (iv)  Increasing the ionic strength of the medium, increased the rate (v)  ΔS# is negative.  Explain the data.                                                                                   (5)

Propose a suitable mechanism to account for the above data and write the rate expression.    (5)

 

 

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