Chemistry 65.100 A and V
Mid-year Exam
December, 2000

Part A. 60 marks. Answer each question (5 marks each).

1. AUFBAU would predict that a Cr atom has the electronic configuration [Ar]4s23d4. However, Cr has an anomalous configuration. What do you think the configuration is and why?
2. There is a double bond between the two carbon atoms in ethylene molecule, C2H4. For one of the carbon atoms, state what kind of orbitals it uses to bond to the other carbon and to the hydrogens.
3. Balance the reaction C2H6(g) + O2(g) ¾ CO2(g) + H2O(g), then predict whether DSo is likely to be positive or negative and state why.
4. Why do elements always have DHof = 0?
5. If a reaction must release energy to be spontaneous, how can an endothermic reaction be spontaneous?
6. Write the relevant chemical reactions to show how a single Cl atom can destroy many ozone molecules in the stratosphere.
7. Starting with NO(g), show how ozone is produced in photochemical smog.
8.For the reaction 2 POCl3(g) ¾ 2 PCl3(g) + O2(g), which way will the equilibrium shift if the pressure is lowered? Why?
9.For the reaction CH4(g) + 3 O2(g) ¾ CO2(g) + 2 H2O(g), which way will the equilibrium shift if the temperature is decreased? Why?
10. State and explain Dalton's Law of partial pressures.
11. A flask contains a saturated aqueous solution of NaCl in equilibrium with 10 g of powdered solid NaCl. The flask is stoppered and left undisturbed. A year later, the solution is found to be in equilibrium with a single, large 10 g crystal of solid NaCl. Explain how this demonstrates that equilibrium is a dynamic process.
12. VSEPR theory predicts the SbCl5-2 ion to have a square pyramidal shape. Would you expect the shape to be exactly square pyramidal? Why or why not?

Part B. 120 marks. Answer all 7 questions. The best 6 will be counted. (20 marks each)

1. Estimate DHof for magnesium fluoride using the following data:

Lattice energy of MgF2(s)       3916 kJ/mol
First ionization energy of Mg       735 kJ/mol
Second ionization energy of Mg       1445 kJ/mol
Electron affinity of F                      -328 kJ/mol
Bond energy of F2                        154 kJ/mol
Sublimation energy of Mg(s)       150 kJ/mol

2. Four possible Lewis structures of XeO3 are shown below. Using the concept of formal charge, predict which structure is most likely.

3. For the reaction CO(g) + H2O(g) ¾ CO2(g) + H2(g), the concentration equilibrium constant is 5.10 at 700 K. Calculate the equilibrium concentration of all four species (in mol L-1) if initially 1.00 mol of each of the four gases are mixed in a 1.00 L flask at 700 K.

4.(a) Calculate DGo at 1000oC for the reaction CaCO3(s) ¾ CaO(s) + CO2(g).
(b) Is the reaction spontaneous at 1000oC? Why or why not?
(c) Calculate the value of Kp at 1000oC.
(d) Calculate the equilibrium partial pressure (in atm) of CO2(g) at 1000oC.

 

DHof, kJ mol-1

So, J K-1 mol-1

CaCO3(s)

-1206.9

92.9

CaO(s)

-635.1

38.2

CO2(g)

-393.5

213.7

5. A compound is found to have the composition 38.4% C, 4.82% H and 56.8% Cl by weight. It takes 7.73 min for a given mass of the substance to effuse through a porous barrier, but only 4.37 min for the same mass of Ar to effuse at the same temperature and pressure. What is the molecular formula of the compound (not the empirical formula)?


6(a). Use the ideal gas law to calculate the pressure exerted by 2.00 mol of a gas in a 1.00 L container at 25oC.
(b) Use the van der Waals equation to calculate the pressure exerted by 2.00 mol of He(g) in a 1.00 L container at 25oC. For He, a = 0.0346 L2 atm mol-2 and b = 0.0238 L mol-1.
(c) Use the van der Waals equation to calculate the pressure exerted by 2.00 mol of SO2(g) in a 1.00 L container at 25oC. For SO2, a = 6.865 L2 atm mol-2 and b = 0.0568 L mol-1.
(d) You should have found that the value calculated in (b) is greater than that calculated in (a). Why is this so?
(e) You should have found that the value calculated in (c) is less than that calculated in (b). Why is this so? (Using VSEPR to find the shape of SO2 may help you to answer this question)

7. A molecular orbital diagram for oxygen (O2) is shown below.

(a) Calculate the bond order of O2.
(b) Why is oxygen paramagnetic?
(c) Why is oxygen also diamagnetic?
(d) Why are there two p2p bonding molecular orbitals, but only one s2p bonding molecular orbital?
(e) Would O2+2 be a stable molecule? Why?