1. (15 points) Three 4 10 isomers are shown below, along with their boiling points. ( 3 ) 3 3 2 2 2 3 2 2 3 tert-butyl alcohol n-butyl alcohol methyl propyl ether bp: 82 118 39 (a) Based on their boiling points, which substance has the strongest intermolecular interactions in the liquid state? (b) Which has the weakest? (c) Explain briefly why n-butyl alcohol has a much higher bp than methyl propyl ether. (d) Explain briefly the difference between the first two, i.e., why does tert-butyl alcohol have a lower bp than n-butyl alcohol. 2. (15 points) Early in the semester you did a modeling experiment using yperchem. (a) In one part of the lab, you found that the energy of the optimized complex formed between formaldehyde and formamide (at right) was about 10 kj/mol lower than the energy of the separated molecules. What kind of interaction exists between the molecules? (b) In the space below, sketch the relative energy as a function of the distance between the molecules. Show clearly what happens to the energy when they get closer and when they get farther apart than they are in the drawing above.
(c) In another part of the modelling lab you found that the strength of a similar interaction between two "amide" groups, like the ones shown at right, was about 15 kj/mol. There must be a structural reason that the oxygen of 2 = interacts more strongly than the oxygen of 2 = with that hydrogen. What do you think that reason is? (hint: can you draw resonance structures with negative s for these two molecules? If so, how do these contributions account for the difference?) 3. (10 points) alculate the p of each of the following aqueous solutions. (a) 2.72 x 10 4 M K (b) 3.68 x 10 11 M Br 4. (25 points) The equilibrium constant for the gas-phase reaction below is 6.25 x 10 4 M 1. l 2 + 2 ==== 2 l (a) 0.810 moles of l is added to a 1.00-L flask. What are the concentrations of l 2,, and l after equilibrium is established? (The "5% rule" is not appropriate here; please find the correct concentrations to three significant figures.) page 2
(b) 0.960 mol of l 2 and 0.470 mol of are added to the equilibrium mixture from part a. alculate the concentrations after equilibrium is reestablished. 5. (30 points) hloroacetic acid, l 2 2, has K a = 1.38 x 10 3. (a) alculate the p of a solution made by dissolving 0.0117 moles of chloroacetic acid in 100 ml of water. (b) What is the extent of dissociation (in %) of the chloroacetic acid in part a? (c) When the solution from part a is combined with 100 ml of a solution of aq. 3 having a p of 0.523, what is the p of the solution that results? page 3
(d) What is the extent of dissociation (in %) of the chloroacetic acid in part c? (e) A new solution is prepared as described in part a, and 0.00822 moles of solid K is added. What is the p of the resulting solution? (f) More solid K is then added, so that the total amount of K added is exactly 0.0117 mol (i.e. to reach the equivalence point). What is the p of the solution? 6. (15 points) The pk a s of acetic acid and its chloro-, dichloro-, and trichloro- derivatives are shown below. 3 2 l 2 2 l 2 2 l 3 2 pk a : 4.76 2.86 1.29 0.65 (a) For the most acidic compound in this series, write the acid dissociation reaction. (b) We already know that resonance stabilizes the conjugate bases of all four acids by delocalizing the charge, and this is the reason that carboxylic acids like these are much more acidic than water or alcohols. ow do you account for the effect of the chlorines on the acidities in the series above? page 4
7. (15 points) When 1 pound of each substance below is dissolved in a gallon of water, is the resulting solution acidic, basic, or neutral? (o need to calculate the exact p, of course!) (a) 3 3 l (b) s (c) Kl 4 (d) F 2 2 2 (e) 3 2 2 a Last semester's first exam was later in the term than yours, so there was one more problem on material we haven't covered yet. So expect yours to include an additional problem or two. Detailed solutions to these problems will not be made available, but we will certainly be willing to go through the problems with you in discussion sections or office hours. page 5