2000 Chemistry Spring 2001 Section B
Final Examination
Time: 3 hours
Aids allowed: Calculator. In particular, periodic tables and other written or printed materials are
excluded.
Additional data is given at the end of this paper.
Write all your answers in the booklets provided. Your answers need not be entered into the
booklets in order, provided all answers are labeled clearly with the question number. Answer
all questions in section 1. In other sections, you have some choice. Do not answer more than
the required number of questions. Extra answers will not be marked.
1 Answer all questions in this section.
Total value of questions in this section: 70 marks
1. The following data have been obtained for the reaction
N2 g
2H2 O g
2NO g
2H2 g
at 700 C:
rate mol L 1s 1
1 2 10 6
2 4 10 6
0 60 10 6
H2 (mol/L) [NO] (mol/L)
0 0050
0 025
0 010
0 025
0 010
0 0125
Determine the rate law and rate constant. [7 marks]
4 mol
10
L solution of potassium hydroxide in water at 25 C.
2. Calculate the pH of a 6 3
[4 marks]
3. Calculate the pH of a 6 3 10 4 mol L aqueous sodium sulte (Na2 SO3 ) solution at 25 C.
The Kb of the sulte ion is 1 6 10 7. [8 marks]
1
4. Which of the following two compounds
OH
H3C CH2
O
H3C
ethanol
CH3
dimethyl ether
would you expect to have the higher vapor pressure? Explain briey. [3 marks]
5. Pure solid NH4 HSe is placed in a 2.0 L container at 24 8 C. The container is evacuated. The
pressure then rises due to the reaction
H2 Se g
10 5 . What is the total pressure at
2H2 S g is in equilibrium at 700 C when PH2 16 6 atm, PS2
57 9 atm. What is the equilibrium constant for this reaction? [3
6. The reaction 2H2 g S2 g
8 98 10 5 atm, and PH2 S
marks]
The equilibrium constant for this reaction is 8 46
equilibrium? [6 marks]
NH3 g
NH4 HSe s
7. 2.5 g of solid sodium acetate (CH3 COONa) is dissolved in 100 mL of a 0.4 mol/L aqueous
HCl solution at 25 C. What is the pH of the resulting solution? The Ka of acetic acid is
1 8 10 5. [10 marks]
8. Calculate the solubility of zinc hydroxide (Zn OH 2 ) in a pH 8 buffer at 25 C. For this
compound, Ksp 1 8 10 14. [7 marks]
9. A 50 g piece of metal is heated to a temperature of 120 C, then immersed in 800 g of water
initially at 21 3 C in a sealed and well-insulated container. The equilibrium temperature is
23 0 C. What is the specic heat capacity of the metal? [4 marks]
(a) Calculate the enthalpy change for the neutralization of aqueous acetic acid by hydroxide. [3 marks]
(b) Given that H
55 815 kJ mol for the reaction
OH aq
!
H aq
H2 O l
#
$
10.
"
calculate the enthalpy change for the dissociation of acetic acid into its ions. [4 marks]
11. Calculate the solubility product of lead (II) chloride at 25 C. [5 marks]
2
12. Insects are cold-blooded, meaning that they dont regulate their internal temperatures. Since
all processes in living organisms rely on chemical reactions, many of their behaviors display
temperature sensitivity. Tree crickets chirp at a rate of about 200 chirps per minute at 27 C.
This rate falls to about 40 chirps per minute at C. 5 Assuming that the difference is mainly
due to a reduction in a rate constant, determine the activation energy for the underlying
chemical process. [6 marks]
2 Answer one question in this section
Total value of questions in this section: 10 marks
1. An aqueous sodium bromide solution has a vapor pressure of 27.8 torr at 30 C. The vapor
pressure of pure water at this temperature is 31.8 torr. What is the molality of the solution?
[10 marks]
(a) The normal boiling and freezing points of sulfur dioxide are 10 C and 72 7 C,
respectively. The triple point is 75 5 C and 1 65 10 3 atm. Draw a rough sketch of
the phase diagram of sulfur dioxide, labeling all regions and special points. [8 marks]
3
(b) Which is more dense, solid or liquid sulfur dioxide? [2 marks]
2.
3 Answer one question in this section.
Total value of questions in this section: 20 marks
1. Suppose that we want to make iron (III) chloride from iron (III) oxide by reacting the latter
with chlorine gas:
3
2FeCl3 s
O
Fe2 O3 s 3Cl2 g
2 2g
&
%
(a) Calculate the equilibrium constant for this reaction at 25 C. [4 marks]
(b) Is the reaction spontaneous at 25 C if the partial pressures of oxygen and chlorine are,
respectively, 0.02 atm and 0.9 atm? [4 marks]
(c) If the reaction isnt spontaneous under the conditions given above, suggest a set of
conditions which would make it spontaneous. If the reaction is spontaneous, calculate
the maximum pressure of oxygen such that the reaction remains spontaneous at xed
chlorine pressure. [4 marks]
(d) Calculate the equilibrium constant at 150 C. Is the reaction spontaneous at this temperature under the conditions of question 1b? [8 marks]
(a) For the reaction
CO2 g
H2 S g
33 kJ mol. SCOs structure is analogous to that of CO2 :
(
H
H2 O g
SCO g
#
'
S=C=O
Estimate the S=C bond enthalpy. [6 marks]
(b) The absolute entropies of the reactants and products are given in the following table:
H2 O g
H2 S g SCO g
Substance
CO2 g
S (J K 1 mol 1 ) 213.785 188.835 205.81 231.57
Calculate the standard entropy change for the reaction. [2 marks]
(c) Calculate the standard free energy change at 25 C. [2 marks]
(d) Calculate the equilibrium constant at 25 C. [2 marks]
(e) Calculate the equilibrium constant at 100 C. [4 marks]
0 03, PH2 S
4
0 04, PCO2
(f) Would the reaction be spontaneous at 100 C if PH2 O
and PSCO 0 05 atm? [4 marks]
02
2.
Useful data and equations
RT ln Q
RT ln K
!
At 25 C, Kw
10
G
T S
H
G
G
Ea RT
)
G
k e
k
14 .
To convert degrees Celsius to Kelvin, add 273.15.
8 314 510 J K 1mol
R
1
The specic heat capacity of water is 4 184 J K 1g 1 .
Molar masses
Element M (g/mol)
C
12 011
H
1 0079
Na
22 9898
O
15 9994
Bond enthalpies
Bond Hb (kJ/mol)
C=O
803
H-O
464
H-S
339
Standard thermodynamic properties at 25 C
G f
Species
H f
(kJ/mol)
(kJ/mol)
CH3 COO aq
485 60
376 89
485 26
404 09
CH3 COOH aq
Cl aq
167 080
131 218
FeCl3 s
399
334
Fe2 O3 s
824
742
H2 O l
285 830
237 129
OH aq
230 015
157 22
2
Pb aq
0 92
24 24
PbCl2 s
359 41
314 10
5
"