Topic: Chemical Kinetics

Transcription

1 PHYSICAL CHEMISTRY by: SHAILENDRA KR. Classes at: - SCIENCE TUTORIALS; Opp. Khuda Baksh Library, Ashok Rajpath, Patna PIN POINT STUDY CIRCLE; House No. 5A/65, Opp. Mahual Kothi, Alpana Market, Patna Topic: Chemical Kinetics 1. The following rate data were obtained at 25 C for the given reaction. What is the rate-law expression for the reaction : 2 BrO I + 6 H + 3 I Br + 3 H 2 O Exp. [BrO 3 [I [H + initial rate (M/sec) x x x x a) rate = k[bro 3 2 [I [H + b) rate = k[bro 3 2 [I 2 [H + c) rate = k[bro 3 2 [I [H + d) rate = k[bro 3 2 [I e) rate = k[bro 3 [I [H The following rate data were obtained at 20 C for the reaction of hemoglobin (Hb) carbon monoxide reaction. What is the rate-law expression for this reaction : 4 Hb + 3 CO Hb 4 (CO) 3 Exp. [Hbmol/L [CO mol/l Rate of disappearance of Hb (M/sec) a) rate = k[hb b) rate = k[hb 2 [CO 2 c) rate = k[hb 4 [CO 3 d) rate = k[hb [CO e) rate = k[hb [CO 2 3. The following rate data were obtained at 37 C for the indicated reaction. What is the rate-law expression for the reaction : 2 MnO 4 + H 3 2 MnO 4 + IO OH Exp. [MnO 4 [H 3 initial rate (M/min) Meq. Approach Page No.: 1

2 a) rate = k[mno 4 2 [H 3 2 b) rate = k[mno 4 2 [H 3 c) rate = k[mno 4 [H 3 d) rate = k[mno 4 2 e) rate = k[mno 4 [H Given K = s 1 for a reaction which is first order in A, what is the initial rate of the reaction (M/s) when the initial concentration of A is M : a) b) c) 1.2 d) e) Given K = s 1 for a reaction which is first order in A, what is the initial rate of the reaction (M/s) when the initial concentration of A is.0060 M : a) b) c) 7.8 d) e) The initial rate of a reaction which is first order in A is M/s when the initial concentration is M. What is the first order rate constant (s 1 )? a) b) c) d) e) The initial rate of a reaction which is second order in A is M/sec when the initial concentration is M.What is the second order rate constant (M 1 sec 1 )? a) b) c) d) e) The initial rate of a reaction which is second order in A is M/sec when the initial concentration is m M. What is the second order rate constant (M 1 sec 1 )? a) 6.9 b) c) 7.6 d) e) Given k = 62.1 M 1 s 1 for a reaction which is second order in A, calculate the initial rate of the reaction (M/sec) when the initial concentration of A is M. a) b) c) d) e) The rate constant of a first order reaction is s 1. What concentration of reactant will remain after 74.4 sec for an initial concentration of M? a) b) c) d) e) Page No.: 2

3 11. What is the rate constant in s 1 for a first order reaction with an initial concentration of M if after 158 sec, the concentration is M? a) b) c) d) e) What is the rate constant in s 1 for a first order reaction with an initial concentration of M if after 142 sec, the concentration is M? a) b) c) d) e) What is the rate constant in M 1 s 1 for a second order reaction with an initial concentration of M if after 210 sec, the concentration is M? a) b) c) d) e) The rate constant of a second order reaction is M 1 s 1. What concentration of reactant will remain after 142 sec for an initial concentration of M? a) b) c) d) e) The rate constant of a second order reaction is M 1 s 1. What concentration of reactant will remain after 74.4 sec for an initial concentration of M? a) b) c) d) e) What is the half life in sec for a first order reaction wiht K = s 1? a) 323 b) 369 c) 341 d) 422 e) The half life of a first order reaction is 45.6 sec. What is the first order rate constant (sec 1 )? a) b) c) d) e) What is the half life in sec for a first order reaction with K = s 1? a) 165 b) 180 c) 152 d) 226 e) What is the half life in sec for a second order reaction in reactant A with the rate constant K = M 1 s 1 if the intial concentration is M? Page No.: 3

4 a) 8.59 b) 14.1 c) 16.8 d) 12.2 e) The half life of a second order reaction in reactant A with an initial concentration equal to M is 45.6 sec. What is the second order rate constant (M 1 s 1 )? a) b) c) d) e) The half life of a second order reaction in reactant A with an initial concentration equal to M is 49.5 sec. What is the second order rate constant (M 1 s 1 )? a) b) c) d) e) The reaction of NO 2 with F 2 is believed to occure according to the following mechanism. What would be the rate expression if the mechanism is correct? 2 NO 2 + F 2 2 NO 2 F (balanced equation); NO 2 + F 2 NO 2 F + F (slow); F + NO 2 NO 2 F (fast) a) rate = k [NO 2 2 [F 2 b) rate = k [NO 2 [F 2 c) rate = k [NO 2 [F d) rate = k [NO 2 e) rate = k [F Nitric oxide, NO, is believed to react with chlorine according to the following mechanism. What would be the rate expression if the mechanism is correct? 2 NO 2 + Cl 2 2 NOCl (balanced equation); NO + Cl 2 2 NOCl 2 (fast); NOCl 2 + NO 2 NOCl (slow) a) rate = k [NO 2 b) rate = k [NO 2 [Cl 2 c) rate = k [NO [NOCl 2 d) rate = k [NO [Cl 2 e) rate = k [NO 2 [Cl Ozone is believed to decompose according to the following mechanism. What would be the rate expression if the mechanism is correct? 2 O 3 3 O 2 (balanced equation); O 3 O 2 + O (fast) ; O 3 + O 2 O 2 (slow) a) rate = k [O 2 3 b) rate = k [O 3 2 / [O 2 c) rate = k [O 3 2 d) rate = k [O 3 2 [O e) rate = k [O Consider the following hypothetical reaction and the established rate law. Select an acceptable mechanism A 2 + B 2 x + y rate = K [A 2 [B 2 Page No.: 4

5 a) A 2 C (fast); C x + y (slow) b) A 2 2 A (fast); C + A X + y (fast); B 2 + A C (slow) c) B 2 2 B (fast); C Y (fast); B C + X (slow) d) A 2 C + X (fast); B 2 + C Y (slow) e) B + A 2 C (fast); C + B X + Y (fast); B 2 2 B (slow) 26. Consider the following hypothetical reaction and the established rate law. Select an acceptable mechanism. A 2 x + y rate = K [A 2 [B 2 / x a) A 2 C + X (fast); B 2 + C Y (slow) b) A 2 2 A (fast); C + A X + y (fast); B 2 + A C (slow) c) C + B X + Y (fast); B + A 2 C (fast); B 2 2 B (slow) d) A 2 C (fast); C X + Y (slow) e) B 2 2 B (fast); C Y (fast); B C + X (slow) 27. Consider the following hypothetical reaction and the established rate law. Select an acceptable mechanism. A 2 x + y rate = K [B 2 1/2 a) B 2 2B (fast); C Y (fast); B C + x (slow) b) A 2 2 A (fast); C + A X + y (fast); B 2 + A C (slow) c) C + B X + Y (fast); B + A 2 C (fast); B 2 2 B (slow) d) A 2 C + X (fast); B 2 + C Y (slow) e) A 2 C (fast); C X + Y (slow) 28. The enthalpy of activation of a first order reaction is 111 kj/mole. The rate constant at 60 C is 2.02 x 10 4 s 1. What is the rate constant (s 1 ) at 45 C? (R= 8.31 J/mole K) : a) 7.75 x 10 6 b) 6.15 x 10 6 c) 3.05 x 10 5 d) 2.50 x 10 5 e) 1.25 x The enthalpy of activation of a first order reaction is 123 kj/mole. The rate constant at 80 C is 1.73 X 10 2 s 1. What is the rate constant (s 1 ) at 35 C? (R= 8.31 J/mole K) : a) 7.75 x 10 5 b) 6.02 x 10 5 c) 3.81 x 10 5 d) 4.50 x 10 5 e) 2.55 x At 25 C and 70 C the first order rate constants of a reaction are 1.79 x 10 5 and 4.53 x 10 4 s 1 respectively. What is the enthalpy of activation (kj/mole)? (R= 8.31 J/mole K) : a) 128 b) 125 b) 117 d) 120 e) Consider the rate constants for the following two reactions and determine the relationship between their respective enthalpy of activation. Page No.: 5

6 Reaction 1 Reaction x 10 6 s 40 C 2.0 x 10 5 s 40 C 4.0 x 10 5 s 80 C 8.0 x 10 5 s 70 C a) H 1 > H 2 b) H 1 < H 2 c) H 1 = H Consider the rate constants for the following two reactions and determine the relationship between their respective enthalpy of activation. Reaction 1 Reaction x 10 6 s 35 C 3.0 x 10 5 s 35 C 4.0 x 10 5 s 70 C 5.0 x 10 4 s 70 C a) H 1 < H 2 b) H 1 > H 2 c) H 1 = H Consider the rate constants for the following two reactions and determine the relationship between their respective enthalpy of activation. Reaction 1 Reaction x 10 6 s 50 C 2.0 x 10 5 s 30 C 3.0 x 10 5 s 70 C 6.0 x 10 4 s 60 C a) H 1 < H 2 b) H 1 > H 2 c) H 1 = H Which of the following is false? a) catalysts shift reaction equilibria toward the side of the products b) activation energy is required for both exothermic and endothermic reactions c) reaction rates depend on temperature, reactant structure, concentration of reactants and the presence of catalysts d) enzymes are catalysts in living organisms 35. Which of the following statements is false about a catalyst? a) a catalysts increases the kinetic energy of the reactants b) a catalyst does not change the equilibrium constant for a reaction c) a catalyst in living systems are called enzymes d) a catalyst lowers the activation energy of a reaction e) a catalyst does not undergo any net chemical change 36. The reason for an increase in the rate of reaction at higher temperatures may be due to : i) an increase in the number of collisions between reactants ii) lowering of the actiation energy for the reactions iii) an increase in the average kinetic energy of the reactant molecules a) both i and iii are correct b) iii c) i, ii and iii are correct d) i e) ii 37. Given the following initial rate data for the reaction in aqueous solution determine the rate law : CH 3 + Br 2 CH 3 COCH 2 Br + H + + Br Page No.: 6

7 [CH 3 0 [ Br 2 0 [ H + 0 [ Br 2 / t (M/s) x x x x 10 3 a) Rate = k[br 2 [H + b) Rate = k [CH 3 [Br 2 [H + c) Rate = k [CH 3 [Br 2 d) Rate = k [CH 3 [H e) Rate = k [CH 3 [Br 2 [H The kinetics of the reaction A + 3 B C + 2 D were studied and the following results obtained, where the rate law is : [A / t = k [A n [B m For a run where [A 0 = 1.0 x 10 3 M and [B 0 = 5.0 M, a plot in 1n [A versus t was found give a straight line with slop = 5.0 x 10 2 s 1 : For a run where [A 0 = 1.0 x 10 3 M and [B 0 = 10.0 M, a plot in 1n [A versus t was found give a straight line with slop = 7.1 x 10 2 s 1 : What is the value of n? a) 0.0 b) 1.5 b) 0.5 d) 1.0 e) The kinetics of the reaction A + 3 B C + 2 D were studied and the following results obtained, where the rate law is [A / t = k [A n [B m For a run where [A 0 = 1.0 x 10 3 M and [B 0 = 5.0 M, a plot in 1n [A versus t was found to give a straight line with slope = 5.0 x 10 2 s 1 : For a run where [A 0 = 1.0 x 10 3 M and [B 0 = 10.0 M, a plot in 1n [A versus t was found to give a straight line with slope = 7.1 x 10 2 s 1 : Calculate the value of k (ignore units). a) 1.0 x 10 2 b) 1.1 x 10 1 c) 5.0 x 10 2 d) 2.2 x10 2 e) none of these 40. The rate constant for a reaction at 40 C is exactly three times that at 20.0 C. Calculate the Arrhenius energy of activation for the reaction : a) 41.9 kj/mol b) none of these c) 3.0 kj/mol d) 3.20 kj/mol e) 366 kj/mol 41. The reaction 2 N 2 (g) O 2 (g) + 4 NO 2 (g) is first-order in N 2. For this reaction at 45 C, the rate constant K = 1.0 x 10 5 s 1, where the rate law is defined is : Rate = [N 2 / t = k [N 2 For a particular experiment ([N 2 0 = 1.0 x 10 3 M), calculate [N 2 after 1.0 x 10 5 seconds. a) 3.7 x 10 4 M b) 1.0 x 10 3 c) 5.0 x 10 4 M Page No.: 7

8 A N S W E R S 1. d 16. c 31. b 2. d 17. c 32. b 3. d 18. c 33. b 4. d 19. d 34. a 5. d 20. d 35. a 6. d 21. d 36. a 7. e 22. b 37. d 8. e 23. b 38. d 9. e 24. b 39. d 10. c 25. a 40. a 11. c 26. a 41. a 12. c 27. a 13. a 28. c 14. a 29. c 15. a 30. c Page No.: 8