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Chemical Kinetics Question Paper

Chemical Kinetics

Course:Bachelor Of Science

Institution: Kenyatta University question papers

Exam Year:2009

KENYATTA UNIVERSITY
UNIVERSITY EXAMINATIONS 2008/2009
INSTITUTE OF OPEN LEARNING

EXAMINATION FOR THE DEGREE OF BACHELOR OF SCIENCE AND
BACHELOR OF EDUCATION

SCH 305 : CHEMICAL KINETICS

DATE: WEDNESDAY 12TH AUGUST 2009 TIME: 11.00 A.M. – 1.00 P.M.

INSTRUCTIONS
Answer ALL questions.

Constants R = 8.314Jk-1 mol-1

1.
a)
What do you understand by the term rate of a chemical reaction? (2 marks)

The table shows what happens to the concentration of phenolphthalein in a solution

that was initially 0.005M phenolphthalein in 0.61 M sodium hydroxide.

Conc. of
50.0 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 2.50 1.50 1.00
phenolphthalein/M
x 10--4
Time/Min
0.00 0.175 0.371 0.595 0.851 1.16 1.52 2.00 2.68 3.83 4.98 583 6.51

i)
What is the average rate of reaction?
ii)
What will be the rate at 3.00 minutes?
iii)
Give reasons as to why the sign is negative?

(6 marks)

Page 1 of 3

b)
The rate of disappearance of ozone is related to the appearance of

oxygen as shown below.

2 O 3 (g) ? 3 O 2 (g)

If the appearance of oxygen is 6.0 x 10-5MS-1 at a particular instant, what will

be the rate of disappearance of ozone at that time.

(2 marks)

2.
The data below was for the gas- phase reaction between nitric oxide and bromine at
273oC.

Br
?
2 (g) + 2 NO (g)
2 NOBr (g)

[NO]/Mol dm-1 0.10 0.10 0.2
0.30
[Br2]/Mol dm-1 0.10 0.20 0.10
0.10
Rate of appearance 12 24 48
108
of NOBr M S-1

i)
Determine the rate law.

(2 marks)

ii)
Calculate the value of the rate for the appearance of NOBr. (2 marks)

iii)
How is the rate of appearance of NOBr related to the rate of disappearance of

bromine.

(3 marks)

3.
a)
what is half-life and what is unique about the half-life of a second order

reaction?

(3 marks)

b)
The thermal decomposition of N2O5 (g) to form NO2 (g) and O2 (g) is a first

order reaction. The rate constant for the reaction is 5.1 X 10-4 at 318 K.

What is the half-life of the
process?
(2
marks)

Page 2 of 3

4.
a)
Write an expression of the Arrhenius equation.

(2 marks)

The following kinetic data is for the reaction,

2N
?
2O5 (g)
2NO2 (g) + 3O2 (g)

Temperature / K 195
230
260 298
369

Rate M S-1 x 109 1.08
5.19 7.89 12.0
35.5

1
i)
Plot a graph of ln K against .
T
ii)
Determine the activation energy at 300 K.

(4 marks)

5. The following mechanism has been proposed for the gas-sphere reaction.

K
??
? 1
STEP 1

Cl 2(g)
2Cl .
?
??
?
(g)
Fast
K -1

STEP 2
2Cl . (g) + CHCl 3(g)
?
? ?
HCl + CCl . 3(g)
Slow
K 2

STEP 3
Cl . (g) + CCl . 3(g)
?
? ?
CCl4(g)
Fast
K3

i)
What
is
the
overall
reaction?
(1
mark)

ii)
Which are the intermediates in the mechanism?

(2 marks)
iii)
Write
the
rate
determining
step.
(1
mark)

iv)
What is the rate law predicted by the mechanism.

(3 marks)
Page 3 of 3

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