Chemistry 65.100 A and V
Final Exam
April, 1998
Part A. Answer each question with a few sentences or equations where required (5 marks each)
1. Define all terms in the equation DGo
= -RT ln(Keq). Be precise.
2. Which of CH4, SH4 and XeF4,
if any, are NOT tetrahedral. How do you know?
3. Why is diamond so much harder than graphite? Answer in terms
of the types of bonding in each.
4. Which of Ar(g) or HCl(g) will more
closely obey the ideal gas law? Why?
5. Some solid substances have positive heats of solution in
water. Why then do they dissolve?
6. Why is the rate of a catalyzed reaction greater than the
uncatalyzed one?
7. The electronic configuration of copper is [Ar] 4s1
3d10. Why is it not [Ar] 4s2 3d9?
8. Describe how cathodic protection works. Which of the following
metals can cathodically protect iron? Mg, Zn, Cu, Pb.
Part B. Answer any seven (7) of the following ten questions. If you answer more than seven questions, the best seven will be used to calculate your mark (20 marks each).
1. The electrochemical cell with the shorthand notation:
Pt(s), H2(g)(1 atm) | H+(aq) (x M) || H+(aq) (1 M) | H2(g)(1 atm), Pt(s)
is assembled to measure the concentration of H+(aq).
At 50oC, the cell potential is Ecell =
0.108 V.
a. Write the oxidation, reduction and overall reactions
occurring in the cell.
b. Calculate the unknown concentration of H+(aq),
i.e. find the value of x.
2. During production, chlorine is stored in a container that is isolated from other products to prevent unwanted reactions. If a 15.0 L container holds 0.580 kg Cl2(g) at 200oC,
a. Calculate the pressure of the container, assuming ideal gas
behaviour.
b. Calculate the pressure again using the van der Waals equation.
For Cl2, a = 6.49 atm L2 mol-2
and b = 0.0562 L mol-1
c. Comment on why the van der Waals pressure is different
than the ideal gas pressure.
3. One reaction used to produce small quantities of pure hydrogen is:
CH3OH(g) ¾ CO(g) + 2 H2(g)
CH3OH(g) |
CO(g) |
H2(g) | |
DHo, kJ mol-1 |
-238.7 |
-110.5 |
0 |
So, J K-1 mol-1 |
127 |
197.6 |
130.6 |
a. Using the given data, calculate DHo,
DSo and DGo
for the process.
b. Calculate the equilibrium constant at 400oC
c. If 1.00 mol of CH3OH(g) is placed in a
vessel at 400oC, what are the equilibrium amounts (in
mol) of the three gases?
4. (a) Calculate the pH of an aqueous solution of 0.55 M
formic acid (HCOOH) and 0.63 M sodium formate (HCOONa). pKa
for formic acid is 3.74.
(b) Calculate the pH of 1 L of the above solution after addition
of 0.020 mol of HNO3(aq) (HNO3 is a strong
acid).
5. Iridium crystallizes in a face centred cubic unit cell with an edge length of 383.3 pm.
(a) What is the radius of an Ir atom in pm?
(b) What is the density of Ir in g/cm3?
6. One compound commonly found in urban smog is peroxyacetyl nitrate ("PAN"). This molecule dissociates into peroxyacetyl radicals and NO2(g) in a first order reaction with a half-life of 32 minutes (at 30oC):
CH3COOONO2(g) à CH3COOO + NO2(g)
a. Calculate the rate constant, k, for the reaction. Include
the correct units for k.
b. If the initial concentration of PAN in an air sample is 5.0 x
1014 molecules L-1, what will be the
concentration 1.50 h later (answer in molecule L-1)?
c. If the half life for the reaction is 16 minutes at 40oC,
what is the activation energy for the process?
7. Radioactive 47Ca has a half life of 4.5 days.
(a) 32P decays via b-
decay. Show a balanced nuclear reaction for the process, clearly
showing what the products are.
(b) Calculate the decay constant of 47Ca. Express in
day-1.
(c) Use the decay constant to calculate the mass of 47Ca
left from an original sample of 1.0 mg 47Ca after 19.0
days.
8. (a) Give correct names for the following compounds:
(i) (ii) (iii) (iv) CH2(NH2)2
(b) Predict the products (draw or give correct names) of the
following reactions:
(i) CH2CHCH2 + I2
(ii) methylbenzene + chlorine ( (three different products!!)
(iii) diethylamine + HCl(aq)
(iv) butanoic acid + methanol
(v) pentanoic acid + ethylamine
9. Use VSEPR theory to predict the shapes of the following species. Wrong name of shape = zero marks!
a. PCl6-
b. SiCl3-
c. NH4+
d. H3PO4 (predict the shape around
the P atom)
10. A molecular orbital diagram for the B2 molecule is shown at right
a. Calculate the bond order of B2 and predict
whether or not the molecule is stable.
b. How many electrons would have to be added to make the
molecule unstable? How many would have to be removed to
make the molecule unstable? Why?
c. Place B2-, B2 and B2+
in increasing order of bond length and explain.
d. Which, if any, of B2-, B2 and
B2+ are paramagnetic? Why?
Some Useful Information
R = 0.082 L atm K-1 mol-1 = 8.314 J K-1 mol-1
F = 96500 C mol-1
NAV = 6.022 x 1023 mol-1
P = nRT/(V-nb) – a(n/V)2