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Chem 323: Chemical Kinetics Question Paper
Chem 323: Chemical Kinetics
Course:Bachelor Of Education Science
Institution: Chuka University question papers
Exam Year:2013
CHUKA
UNIVERSITY
UNIVERSITY EXAMINATIONS
EXAMINATION FOR THE AWARD OF
DEGREE IN BACHELOR OF EDUCATION SCIENCE
CHEM 323: CHEMICAL KINETICS
STREAMS: BED (SCI) TIME: 2 HOURS
DAY/DATE: WEDNESDAY 14/8/2013 8.30 AM – 10.30 AM
INSTRUCTIONS:
Answer Question ONE and any other TWO Questions
USEFUL DATA
I Cal = 4.184 J
A = ?10?^(-10) m
Speed of light (c)=3×?10?^10 cm ?sec?^(-1)
h=plank^'' s constant=6.624×?10?^(-27) erg sec
N=Avagadro^'' s constant=6.02 ×?10?^23
0?=273 K
in=2.303 Log
R=8.3145 J? k?^(-1) ?mol?^(-1)
Question One (30 Marks)
(a) (i) Distinguish between the half life t_(1/2) and time constant T of a reaction for N_2 0_5 at 25? [1 mark]
Calculate the
(I) Half life [½ mark]
(II) Time constant [½ mark]
(III) The fraction decomposed after 8 hours {k=3.38×?10?^(-5) s^(-1) y}
[2 marks]
(b) (i) Derive the rate law for the decomposition of N_2 0_5
2N_2 0_5 (g) ? ?4N0?_2 (g)+0_2 (g) on the basis of the following mechanism:
N_2 O_5??NO?_2+?NO?_3- - -K_a
?NO?_2+?NO?_3?N_2 O_5---K_a^1
?NO?_2+?NO?_3??NO?_2+O_2+NO---K_b
NO+N_2 O_5??NO?_2+?NO?_2+?NO?_2---K_C [4 marks]
(ii) Show that the pre-equilibrium mechanism in which A+B?I followed by I?p results in an overall second order reaction without ignoring the fact that I is slowly leaking a way as it forms P [2 marks]
(iii) Explain using Lindeman’s theory how change in pressure affects the reaction rate. [4 marks]
(c) (i) Briefly discus the Transition State Theory (TST) [4 ½ marks]
(ii) Discuss the theory of heterogeneous catalysis. [4 ½ marks]
(d) (i) When propionaldehyde is irradiated with light of ?=3020 it is decomposes to form carbon monoxide ?CH?_3 ?CH?_2 CHO+hr??CH?_3 ?CH?_3+CO . The quantum yield for the reaction is 0.54. The light energy absorbed is 15000 erg mol in a given time. Determine the amount of carbon monoxide formed in the same time. [3 marks]
(ii) Write short notes on a chain reaction mechanism. [4 marks]
Question Two (20 Marks)
(a) (i) Derive an equation of the velocity constant of a second and third order reaction.
[3 marks]
(ii) The rate constants for third order are less than those for second order reactions. Explain.
(iii) At a certain temperature ethyl acetate on saponification gave the following results.
T(min) 0 5 25 55 120 ?
Ml of 0.1M acetic acid used in titrate 10ml of unreacted alkali 16 10.2 4.3 2.3 1.1 0
Show that it is a second order reaction. How much fraction of the ester will be decomposed in 40 minutes? [7 marks]
(b) Explain the following with examples
(i) Parallel reactions [2 ½ marks]
(ii) Reversible reactions [2 ½ marks]
(iii) Consecutive reactions [3 marks]
Question Three (20 Marks)
(a) (i) Briefly discuss quenching as used in photochemistry. [5½ marks]
(ii) Determine the quenching rate constant using the data given below.
[fe(H_2 O)_6^(3+) ]/(?10?^(-4) mol?dm?^(-3) ) 0 1.6 4.7 7 9.4
t/(?10?^(-7) s) 6 4.05 3.37 2.96 2.17
[7 marks]
(b) The rate of second order decomposition of acetaldehyde (ethanol, CH3CHO) was measured over the temperature range 700 – 1000k, and the rate constants are given in the table below. Determine activation energy (E_a) and A.
T/K 700 730 760 790 810 840 910 1000
k/(?dm?^3 ?mol?^(-1) s^(-1) 0.011 0.035 0.105 0.343 0.789 2.17 20.0 14.5
[7 ½ marks]
Question Four (20 Marks
(a) (i) Calculate ? ? and ? for the second order reaction.
?2NO?_2 (g)?2NO(g)+O_2 (g) at 500K . Given A=2.0×?10?^9 s^(-1) and the energy of activation = 111KJ mol-1. [5 marks]
(ii) The thermal decomposition of acetaldehyde vapour ?CH?_3 CHO??CH?_4+CO is an irreversible process whose rate at 500? is given by the following two sets of data for the pressure change at constant volume and temperature.
Initial pressure of CH3CHO Total pressure after 100 sec
400 mm of Hg 500 mm of Hg
200 mm of Hg 226 mm of Hg
(I) Find the order with respect to CH3CHO [4 ½ marks]
(II) Calculate the rate constant at 500? [1 ½ marks]
(III) At what temperature will be rate constant be twice of what it is at 500? {activation energy = 180 KJ y} [1 mark]
(b) In a particular reaction the time required to complete half of the reaction was found to increase nine times when the initial concentration of the reactant was reduced to one third. What is the order of the reaction? [3 marks]
(c) Zero order reaction must be multistep. Explain. Does such a reaction go to completion?
[5 marks]
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