Chemistry 65.100 A and V

Christmas Exam

December 18, 1997, 2 PM

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

1. Which of HNO_3(aq) or HNO_2(aq) would be a stronger oxidant? How do you know?

2. Explain why Niobium (Nb) has the valence electron configuration 5s¹ 4d⁴.

3. Why is the ionization potential of Indium (In) lower than that of Cadmium (Cd)?

4. VSEPR theory predicts the SbCl₅^-2 ion to have a square pyramidal shape. Would you expect the shape to be exactly square planar? Why or why not?

5. Under what circumstances is it possible for an endothermic process to proceed spontaneously?

6. A particular reaction has DH<0 and DS>0. At what temperatures will the reaction be spontaneous?

7. Why is the ideal gas law not valid at high pressures?

8. Why is there a maximum in the concentration of ozone in the stratosphere?

9. Why can small amounts of CFCs destroy large amounts of ozone?

10. What is the difference between effusion and diffusion?

11. For the reaction H_2(g) + ½ O_2(g) ¾ H₂O_(g), which way will the equilibrium shift if the pressure is lowered?

12. For the reaction CH_4(g) + 3 O_2(g) ¾ CO_2(g) + 2 H₂O_(g), which way will the equilibrium shift if the temperature is increased?

Part B. Answer each question (20 marks each).

1. The combustion of isopropyl alcohol (C₃H₈O) to form CO_2(g) and H₂O_(l) has DH^o = - 4011 kJ mol^-1. Using this fact and the following data, calculate the standard enthalpy of formation of isopropyl alcohol.

2. For the reaction CO_(g) + H₂O_(g) ¾ CO_2(g) + H_2(g), K_p = 23.2 at 600 K. If 0.25 mol of each of CO_(g) and H₂O_(g) are placed in a 10 L container at 600 K what will be the equilibrium concentrations (mol/L) of each of the four species?

3. A molecular orbital diagram for the F₂ molecule is shown below.

(a) What is the bond order of F₂? Show the calculation.

(b) Which, if any, of F₂^-, F₂⁺ and F₂⁺² could exist? Show why or why not.

(c) Which, if any, of F₂, F₂^- and F₂⁺ are paramagnetic? Why?

(d) Compare and explain the relative bond lengths and bond energies of F₂ and F₂^-.

4. Use VSEPR theory to predict the shapes of the following species:

(a) ClO₃^-

(b) ClF₃

(c) PCl₄F

(d) BrICl^-

5. The following (unbalanced) REDOX reaction is used to determine the amount of permanganate (MnO₄^-_(aq)) ions in solution:

MnO₄^-_(aq) + Br^-_(aq) à MnO₂(s) ₊ BrO₃^-_(aq)

(a) balance the reaction in basic solution

(b) calculate what mass (in g) of MnO_2(s) is produced when 4.00 g Br^-_(aq) ions are reacted with an excess of MnO₄^-_(aq) ions in basic solution.

6. What is the total volume, in liters, that will be obtained if 1.28 g N_2(g) is collected over water at 21^oC at a total pressure of 696 mm Hg? The vapor pressure of water at 21^oC is 18.7 mm Hg.

Some Useful Constants

N_AV = 6.02 x 10²³ mol^-1

R = 0.082 L atm K^-1 mol^-1 = 8.314 J K^-1 mol^-1

1 atm = 760 mm Hg