2000 Chemistry Spring 2002 Midterm Examination
Name:
Student number:
Aids allowed: Calculator. No printed materials (e.g. periodic tables, calculator manuals) are allowed.
Formulas and data are given on page 7.
Instructions: Answer all questions in the spaces provided. If you run out of space for a particular
question, you can use the backs of the pages but make sure to clearly label any continued
work.
Graphs should be drawn on the graph paper attached and clearly labeled with the corresponding question number. A hand-drawn graph should occupy at least half the sheet of graph
paper. You can use a graphing calculator instead of hand-drawn graphs, but you should in
these cases provide a clearly labeled and reasonably accurate sketch of the graph. Note that,
because of the scale, calculator graphs are sometimes a little misleading so you use such a
tool at your own risk.
Clarity may be considered in evaluating your answers. If you are asked a direct question,
give a direct answer. If you use advanced features of your calculator to carry out a nontrivial
computation (e.g. to solve a quadratic equation), say so. If you determined something from
a graph, refer to the graph in explaining your answer.
Question Mark
1
2
3
4
5
6
7
DO
NOT OPEN THIS PAPER UNTIL INSTRUCTED TO DO SO .
1
C2 H6 g
CH3 NNCH3 g (mol/L) rate (mol L 1 s 1 )
2 4 10 2
6 0 10 6
3
2 0 10 6
8 0 10
Experiment
1
2
N2 g , the following initial rate data have been
2g
1. For the reaction CH3 N
obtained:
(a) Determine the rate law and rate constant. [4 marks]
(b) Under the conditions of experiment 1, roughly how long would it take to make 10 4 mol L
of ethane (C2 H6 g )? [2 marks]
Hint: This question can be answered very simply from the basic denition of rate.
2
2. In assignment 2, we analyzed certain aspects of the decomposition of benzylpenicillin (bp)
under the assumption that the kinetics were rst order. The following data were obtained at
180 C:
t (min)
2.21 5.16 13.26 22.84 40.52 60.41
% bp remaining 95
83
65
44
23
10
Are these data consistent with a rst-order reaction? If so, what is the rate constant? [10 marks]
3
3. Fluorinated organic compounds are currently chlorouorocarbons replacing as refrigerants
and anaesthetics. Little is known of their atmospheric chemistry and, in particular, of their
reactivity toward common stratospheric species such as hydroxide radicals. Consider the
elementary reaction
products
OH g CF3 CH2 OCHF2 g
1010 L mol 1 s
1
and the activation energy is
30 C, a typical stratospheric temperature. [2 marks]
(a) Calculate the rate constant at
for which the preexponential factor is 1 2
17.5 kJ/mol.
(b) Write down a mass-action equation for the rate of change of the concentration of hydroxide radicals. [2 marks]
(c) Show that if the initial concentrations of the two reactants are equal, your rate equation
reduces to a simple case studied in class. [2 marks]
CF3 CH2 OCHF2 g
0 0405 mol L. How long would
(d) Suppose that, initially, OH
it take for 90% of the hydroxide radicals to be used up at 30 C? [4 marks]
!
4
4. At 250 C, the equilibrium constant for the reaction PCl5 g
PCl3 g Cl2 g is 1.05. A
mixture of 0.177 atm PCl5 , 0.223 atm PCl3 and 0.111 atm Cl2 is prepared. In which direction
will the reaction proceed? [4 marks]
"$
#
5. Calculate the pH of a 1 3 10 4 mol L aqueous solution of potassium hydroxide at 10 C.
Report your answer to two decimal places. [4 marks]
5
6. Molecular bromine exists in equilibrium with bromine atoms in the gas phase:
2Br g
Br2 g
"$
%
When 1.05 mol of Br2 is placed in 0.980 L ask at 1600 C, 1.20% of the bromine molecules
dissociate. Calculate the equilibrium constant for this reaction. [10 marks]
6
7. Oxalic acid (C2 O4 H2 ) is a diprotic acid whose two protons have Ka s of 6 5 10 2 and
6 1 10 5 , respectively. A 0.043 mol/L solution of oxalic acid in water at 25 C is prepared.
What is the pH? Report your answer to two decimal places. [10 marks]
Useful information
kt .
'
1 mol L
1 m3
1000 L
To convert degrees Celsius to Kelvin, add
273.15.
1
7
15
101 325 Pa
T ( C)
Kw
10
2 9 10
25
1 0 10
1 atm
P
8 314 472 J K 1mol
1
R0
c
&
R
k e
k
Ea RT
1
R
kt .
For a second-order reaction,
ln R 0
For a rst-order reaction, ln R
14