Chemistry 65.100 A and V
Mid-Year Examination
December 22, 1998
Part A. Answer each question (5 marks each).
1. How do we know that the permanganate ion, MnO4-(aq), will be a strong oxidant?
2. How many nodes are there in a 4s orbital?
3. Explain how the effective nuclear charge changes going DOWN a group in the periodic table, and explain why.
4. Molecular orbital (MO) theory can predict several phenomena not predicted by valence bond theory, hybrid orbital theory or VSEPR theory. Name one of these phenomena and briefly describe how MO theory explains it.
5. A reaction has a small positive value of DG. At equilibrium, what will be the composition of the mixture, in terms of the relative amounts of reactants and products?
6. A particular reaction has DHo>0 and DSo<0. Will this reaction be spontaneous and if so at what temperatures?
7. Why is the ideal gas law not valid at high pressure (two reasons)?
8. Why is CH4 a more effective greenhouse gas than the same amount of CO2?
9. In the ethylene molecule (C2H2), there is a triple bond between the two carbon atoms. For each of these three bonds, name the orbitals that each carbon atom uses to make the bond.
10. Name two ways that a system can exchange energy with its surroundings.
11. For the exothermic reaction H2(g) + ½ O2(g) ¾ H2O(g), which way will the equilibrium shift if the temperature is lowered?
12. For the reaction 2 NO(g) + Cl2(g) ¾ 2 NOCl(g), Kp = 1.9 x 103 atm-1 at 25oC. Suppose we have a mixture at 25oC with pNOCl = 12 atm, pNO = 0.1 atm and pCl2 = 1.0 atm. Is the system in equilibrium? If not, which way must the equilibrium shift so as to attain equilibrium?
Part B. Answer all 6 questions (20 marks each).
1. At 25oC, Keq = 1100 L mol-1 for the reaction:
Fe+3(aq) + SCN-(aq) ¾ FeSCN+2(aq)
Calculate the concentrations of Fe+3(aq), SCN-(aq) and FeSCN+2(aq) at equilibrium if 0.0200 mol Fe+3(aq) and 0.1000 mol SCN-(aq) are added to 1.000 L water at 25oC.
2. For the reaction 2 Cu(s) + ½ O2(g) ¾ Cu2O(s), calculate the equilibrium constant at 25oC using the following data:
Substance |
D Hof (kJ mol-1) |
So (J K-1 mol-1) |
Cu(s) |
0 |
33 |
O2(g) |
0 |
205 |
Cu2O(s) |
-170 |
93 |
3. (a) Calculate the wavelength of the lowest energy line in the Paschen series of the hydrogen atom. (These lines terminate at m = 3.)
(b) Calculate the energy of the photons (kJ/mol) of the shortest wavelength line in the Paschen series.
(c) Microwave radiation is used to heat 250 g water from 20oC to 100oC. If the wavelength of the radiation is 0.0155 m, how many photons are absorbed by the water? The specific heat capacity of water is 4.18 J g-1 oC-1.
4. (a) Nitrous oxide (N2O) has three possible resonance structures (all of which have a nitrogen atom at the centre!). Draw the Lewis structure of each, and using formal charge, show that two of these are more likely than the third.
(b) For the molecule tetrafluoromethane (CF4):
Predict the direction of dipole, if any, in each C-F bond.
Using VSEPR, predict the shape of the molecule
Using the results from (i) and (ii), predict the net direction and magnitude of the dipole of the molecule.
5. (a) Balance the following REDOX reaction in acidic solution:
S2O8-2(aq) + Cr+3(aq) Ž SO4-2(aq) + Cr2O7-2(aq)
(b) Calculate the pH of the solution (pH = -log10[H+(aq)], where [H+(aq)] means the concentration
of H+(aq) expressed in mol L-1) if 0.001 g Cr+3(aq) is reacted with an excess of S2O8-2(aq) in a 1.00 L
solution. (Note: your balanced reaction from part (a) should have H+(aq) on the right hand side!)
6. (a) Calculate the pressure exerted by 0.500 mol of N2 in a 1.00 L container at 25oC using the ideal gas law.
(b) Repeat the calculation using the van der Waals equation
(for N2, a = 1.39 atm L2 mol-2 and b =
0.0391 L mol-1).
(c) Compare and comment on the two values obtained in (a) and (b).
(d) Repeat the calculation in (a) using a 10.00 L container.
(e) Repeat the calculation in (b) using a 10.00 L container.
(f) Compare and comment on the two values obtained in (d) and (e).
Some Useful Data:
NAV = 6.02 x 1023 mol-1
1 atm = 760 mm Hg
R = 0.082 L atm K-1 mol-1 = 8.314 J K-1 mol-1 (gas constant)
R = 0.01097 nm-1 (Rydberg constant)
h = 6.63 x 10-34 J s
c = 3.00 x 108 m s-1