Score for the page Chemistry 271, Section 21xx Your Name: University of Maryland, College Park Your SID #: General Chemistry and Energetics Prof. Jason Kahn Final Exam (200 points total) May 14, 2007 You have 120 minutes for this exam. Exams written in pencil or erasable ink will not be re-graded under any circumstances. Explanations should be concise and clear. I have given you more space than you should need. There is extra space on the last page if you need it. You will need a calculator for this exam. No other study aids or materials are permitted. Generous partial credit will be given, i.e., if you don’t know, guess. Useful equations and schemes are given on a tear-off sheet at the end. Honor Pledge: At the end of the examination time , please write out the following sentence and sign it, or talk to me about it: “I pledge on my honor that I have not given or received any unauthorized assistance on this examination.” 1. Thermodynamics (50 pts) (a; 12 pts) In the table below, fill in the signs of ∆G° and whether the reaction is spontaneous at all T, spontaneous at no T, spontaneous at low T, or spontaneous at high T. (Don’t be insulted, some of the questions below are harder.) Sign of ∆H° Sign of ∆S° Sign of ∆G° Spontaneity: circle none, one, or both + + ______at low T ______at high T low T high T + – ______at low T ______at high T low T high T – + ______at low T ______at high T low T high T – – ______at low T ______at high T low T high TChemistry 271, section 21xx Final Exam, 5/14/07 2/15 Score for the page (b; 4 pts) Give an example of an exothermic ordering reaction. (c; 4 pts) Give an example of an endothermic disordering reaction. (d; 8 pts) What are the signs of ∆S° and ∆H° for the reaction at the right, and why? (e; 2 pts) What does part (d) tell us about the kinetics of the octanitrocubane decomposition reaction? (A one-word answer will do.)Chemistry 271, section 21xx Final Exam, 5/14/07 3/15 Score for the page Consider the reaction A <-> B -> C, where (1) the overall reaction A -> C is strongly downhill, (2) the reaction is a rapid pre-equilibrium followed by a rate-limiting step, (3) the equilibrium A <-> B is exothermic, and (d) the activation free energy for the B -> C step is temperature-independent. (f; 8 pts) In the space below, draw a free energy reaction coordinate diagram for the reaction, at a temperature at which Keq = 1 for A <-> B. (g; 6 pts) On the same diagram, draw the curve observed at a higher temperature; be sure to label your curves. Assume that the free energy of the reactant A is constant (in practice, it doesn’t matter -- we adopt the free energy of A as the reference state). (h; 6 pts) With reference to your diagram, explain why we might observe an apparent negative activation free energy for the overall reaction A -> C using the standard Eyring equation to measure ∆G‡.Chemistry 271, section 21xx Final Exam, 5/14/07 4/15 Score for the page 2. Acetoacetate (95 pts) This is an extended question covering many aspects of the chemistry and biochemistry of acetoacetate. You can answer many of these questions even if you have missed those that came before. This is my idea of an integrated question, one that demonstrates that a competent chemist picks up whatever theoretical or experimental tool is appropriate to the problem at hand. Acetoacetate is an intermediate in amino acid catabolism, and it is produced from fat in the bodies of people who are starved for glucose, or whose bodies think they are starved (diabetics). Its decarboxylation is responsible for the smell of acetone on the breath of patients with uncontrolled diabetes. Acetoacetate is oxidized to provide energy in the brains of starving people. We are concerned first with the mechanism of acetoacetate decarboxylation, both for the uncatalyzed reaction shown along the top of the scheme below and also the reaction catalyzed by primary amines along the bottom three sides of the square. The pH is roughly 7. For your reference, the same scheme is reprinted on the last page of the exam. (a; 6 pts) Is there net oxidation or reduction of carbon atoms in this reaction? What is the formal oxidation state of each of the four carbons in acetone and CO2?Chemistry 271, section 21xx Final Exam, 5/14/07 5/15 Score for the page (b; 12 pts) On the free energy reaction coordinate diagram below for the catalyzed reaction, identify the rate-limiting step, and sketch in a graphical representation of ∆G‡ for the forward reaction, ∆G‡ for the reverse reaction and the thermodynamic ∆G for the reaction. (c; 9 pts) The rate-limiting step of the uncatalyzed reaction is the same chemical transformation as for the catalyzed reaction, at least around pH 7. Given this fact, on the diagram below sketch in the remainder of the free energy reaction coordinate diagram for the uncatalyzed reaction. Briefly explain your reasoning.Chemistry 271, section 21xx Final Exam, 5/14/07 6/15 Score for the page The catalyzed mechanism shown above resembles that of the enzyme acetoacetate decarboxylase, which is produced by the obligate anaerobic bacterium Clostridium acetobutylicum shown at the right. This bug produces acetoacetate as one product of carbohydrate metabolism, and then degrades it using acetoacetate decarboxylase in order to raise its intracellular pH. There is renewed interest in this “solventogenic” fermentation as a means of industrial production of acetone. Accompanying reactions give butanol and ethanol, so this is called ABE fermentation. Cars can run on “biobutanol.” (d; 3 pts) How does the overall reaction function to neutralize the cytoplasm of Clostridium? (e; 4 pts) Would the free energy change for the reaction become more negative or more positive as pH decreases? Why? (f; 8 pts) Fermentation was referred to in class as a means of living via rearranging the deck chairs. What did this mean in terms of redox chemistry? Clostridium is poisoned by oxygen, but even if it weren’t it couldn’t compete with aerobic bacteria living on carbohydrates. Why not?Chemistry 271, section 21xx Final Exam, 5/14/07 7/15 Score for the page (g; 6 pts) Briefly, what is the thermodynamic origin of the stratification of nitrate reducers and sulfate reducers in the Black Sea? (h; 8 pts) With reference to the global carbon cycle, explain why