Chemistry 65.100 A and V
Final Exam
April, 1999

Part A. Answer all questions (5 marks each).

  1. Define all terms in the equation E = Eo - (RT/nF) ln(Q)
  2. Will the solubility of SrCl2(s) in water increase or decrease upon addition of NaCl(s)? Why?
  3. Why does graphite conduct electricity and why is diamond an insulator?
  4. If the activation energy for a reaction is positive, how does its rate constant vary with temperature? How do you know?
  5. What will be the effect on the equilibrium constant of a reaction if we add some reactant?
  6. Name five functional groups in organic chemistry containing the carbonyl (C=O) group.
  7. Why is the second ionization constant of a polyprotic acid always smaller than the first?
  8. Why do atomic radii decrease going from left to right across a period?

 Part B. Answer all three questions (20 marks each).

1.    The colorless gas N2O4 decomposes to the brown gas NO2 in a first order reaction:

N2O4(g) ® 2 NO2(g)

The rate constant k = 4.5 x 103 s-1 at 274 K and 1.00 x 104 s-1 at 283 K.

    1. Calculate the activation energy in kJ/mol
    2. Calculate the value of k at 300 K.
    3. Calculate the half-life of the reaction (in seconds) at 274 K.
    4. If the initial concentration of N2O4(g) is 1.00 mol L-1, calculate its concentration after 2 x 10-4 s at 274 K.

2.    The Dead Sea Scrolls, Hebrew manuscripts of the books of the Old Testament, were discovered in 1947. The activity of 14C in the linen wrappings of the books is 11 min-1 g-1. The activity of 14C in living material is 14 min-1 g-1. Calculate the age of the linen (in years). The half-life of 14C is 5730 years.

3.    (i) Name the following compounds:

(a) (b) (c)

 

(ii) Draw or give correct names for the products of the following reactions:

(a)

(b)

(c)    Acetic acid + ethyl alcohol ®

 Part C. Answer any four of the eight questions. If you answer more than four, the best four will be used to calculate your mark (20 marks each).

4.    An electrochemical cell is constructed which uses the following (unbalanced) reaction:

Fe+3(aq) + I-(aq) à Fe+2(aq) + I2(s)

 

    1. Balance the REDOX reaction
    2. Calculate the standard free energy change for the reaction (kJ mol-1). Is the reaction spontaneous at 25oC?
    3. Calculate the cell potential at 50oC when [Fe+2(aq)] = 0.01 M, [I-(aq)] = 0.01 M and [Fe+3(aq)] = 0.01 M

5.    Gold crystallizes in a face centred cubic unit cell. The density of the solid is 19.32 g cm-3. Calculate the radius of a gold atom (pm).

6.    Your body's pH is controlled primarily by three buffer systems, one of which is the phosphate system (H2PO4- / HPO4-2). Ka for H2PO4-(aq) is 6.31 x 10-8. If the total phosphate concentration ([H2PO4-] + [HPO4-2]) is 0.0200 M, calculate the concentrations of each of these two species required to maintain the pH at 7.40.

7.    A solution prepared by dissolving 1.25 g of methyl salicylate in 99.0 g of benzene has a boiling point of 80.31oC. Calculate the molar mass of methyl salicylate. Pure benzene boils at 80.10oC, Kb for benzene is 2.53 oC kg mol-1, and its density is 0.88 g/mL.

8.    At 450oC, 3.60 moles of ammonia are placed in a 2.00 L vessel and allowed to equilibrate according to the reaction:

2 NH3(g) ¾ N2(g) + 3 H2(g)

The value of Kc at 450oC is 6.3.

    1. Calculate the concentration of each species at equilibrium (mol L-1).
    2. Calculate the total pressure in the vessel at equilibrium assuming ideal gases (atm).

9.    Calculate the heat of vaporization of diethyl ether (kJ mol-1) using the fact that its vapor pressure is 400 mm Hg at 18oC and that it boils at 35oC.

10.    Calculate the solubility (in mol L-1) of Fe(OH)3(s) in:

    1. Pure water (assume pH = 7.00)
    2. A solution buffered to pH = 5.00
    3. A solution buffered to pH = 11.00

The value of Ksp for Fe(OH)3(s) is 2.6 x 10-39

11.    The reaction 2 NO(g) + Cl2(g) ® 2 NOCl(g) was studied at -10oC. The following results were obtained:

[NO(g)], mol L-1

[Cl2(g)], mol L-1

rate, mol L-1 min-1

0.10

0.10

0.18

0.10

0.20

0.35

0.20

0.20

1.45

where rate = -d[Cl2(g)]/dt

(a) Find the differential rate law for this reaction.
(b) Calculate the value and units of the rate constant.
(c) Does the reaction occur as written or is it the sum of more than one step? How do you know?