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UMD CHEM 271 - Final Exam

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Chemistry 271, Section 22xx Your Name: Prof. Jason Kahn University of Maryland, College Park Your SID #: General Chemistry and Energetics Final Exam (200 points total) Your Section # or time: December 16, 2011 VIEWING: Monday, May 19, 9:30-10:30 a.m., Chem 1356-1360 You have 120 minutes for this exam. Explanations should be concise and clear. 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: Ka = [H+][A–]/[HA] pH = – log([H+]) Kb = [HA][HO–]/[A–] Kw = [H+][HO–] pH = pKa + log [A–]/[HA] ∆G° = – RTlnKeq R = 0.08206 L·atm/mole K kB = 1.38 x 10–23 J/K lnKeq = –∆H°/(RT) + ∆S°/R ∆S – q/T ≥ 0 R = 8.314 J/mole K = 1.987 cal/mole K = NAkB S = kBlnW ∆G = ∆H – T∆S E = Σ ni εi W = N!/(∏ ni!) ni/n0 = exp[–(εi–ε0)/kT] N = Σ ni Chemical standard state: 1 M solutes, pure liquids, 1 atm gases Biochemical standard state: pH 7, all species in the ionic form found at pH 7 °C = °K – 273.15 P(v)dv = Cv2exp(-mv2/2kT) E = E° – 2.303(RT/nℱ)log10Q 2.303RT/ℱ = 0.0592 Volts at 25 °C ℱ = 96500 C(oulomb)/mole ∆G° = –nℱE°cell ln k = (–Ea/RT) + ln A 1 Volt = 1 Joule/Coulomb [A] = [A]0 – kt ln[A] = ln[A]0 – kt 1/[A] = 1/[A]0 + 2kt Standard hydrogen electrode: 2 H+ (aq, 1 M) + 2 e– → H2 (g) E° = 0.000 V 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.” (+2 pts)Chemistry 271, Final Exam, 12/16/2011 2/10 Score for the page 1. (24 pts) Multiple choice: Circle the single best answer for each question (a; 4 pts) If the rate law for a process A + B → C is observed to be Rate = k[A][B] (1) the reaction must be an elementary reaction. (2) the reaction cannot be an elementary reaction. (3) the reaction might be an elementary reaction. (4) the experiment was done wrong. (b; 4 pts) If the rate law for a process A + B → C + D is observed to be Rate = k[A]2/[C] (1) the reaction must be an elementary reaction. (2) the reaction cannot be an elementary reaction. (3) the reaction might be an elementary reaction. (4) the experiment was done wrong. (c; 4 pts) The melting temperature in DNA hybridization (1) increases with the number of mismatches between target and probe. (2) increases with the length of the complementary region. (3) increases as the concentration of DNA decreases. (4) None of the above. (d; 4 pts) Cyclic reactions A → B → C → A (1) are impossible. (2) can proceed in a net forward direction as drawn only if we heat the reaction to drive it. (3) at equilibrium, must also proceed in reverse at the same rate for each step. (4) None of the above. (e; 4 pts) Nitrogen fixation describes conversion of (1) N2 to NH3. (2) NH3 to NO3–. (3) NO3– to N2. (4) NH3 to NO3–. (f; 4 pts) The lead-acid battery used in cars is so large and heavy because (1) the weight of the battery is needed to adjust the car’s suspension. (2) it needs to store a lot of energy to recharge the alternator while driving. (3) it needs to have 60 internal cells to provide 120 V to the distributor and the car’s electronics. (4) the starter motor requires high power and therefore current to move the pistons in the engine.Chemistry 271, Final Exam, 12/16/2011 3/10 Score for the page 2. (48 pts) Kinetics, Rate Laws, and Arrhenius N2 (g) + O2 (g) → 2NO (g) The rate constant k for this reaction, which occurs during combustion as a precursor to NO2 in smog formation, is 318 (units) at 2000 K and 7396 (units) at 2400 K. (Derived from Gilbert 15.85.) (a; 6 pts) Write down the rate law for the reaction assuming that it is elementary. What are the units for the rate constant? (b; 12 pts) Calculate the activation energy (Ea) and the pre-exponential factor (A) for the reaction. (c; 6 pts) Calculate the rate constant at 300 K. Given that NO is poisonous, it’s a good thing the reaction is slow at anything close to room temperature.Chemistry 271, Final Exam, 12/16/2011 4/10 Score for the page (d; 12 pts) The half-life of 137Cs, one of the main radioactive pollutants from the Fukushima and Chernobyl disasters, is 30.2 years. From the integrated rate equation for a first-order reaction, derive an expression for the half-life in terms of the rate constant, and calculate the rate constant for Cs-137 decay. (e; 12 pts) Explain the functional form of the Maxwell-Boltzmann distribution P(v)dv = Cv2exp(–mv2/2kT). In other words, where do the v2 and exp(–mv2/2kT) factors come from? Explain why doubling the temperature typically causes much more than a factor of 2 increase in the rate of elementary processes.Chemistry 271, Final Exam, 12/16/2011 5/10 Score for the page 3. (40 pts) Electrochemistry of Life and the Biosphere The nitrogen triangle describes the interconversions among different oxidation states of nitrogen in the biosphere. (a; 6 pts) Is conversion of N2 to NH3 and amines a reduction or an oxidation? Is it assimilation or dissimilation? (b; 9 pts) Nitrification refers to the oxidation of ammonia to nitrite or nitrate. Why do prokaryotes do this, assuming that they don’t use nitrate for biomass? Why is this generally an aerobic process? What do we call the oxygen in aerobic nitrification? (c; 9 pts) Denitrification is the reduction of nitrate to N2. What might the reducing agent be, generically? What function is the nitrate serving? Is this assimilation or dissimilation? (d; 6 pts) Eukaryotes like us don’t affect the nitrogen triangle very much (at least insofar as our personal metabolisms as opposed to industrial processes are concerned). Why not? What is the oxidation number of nitrogen in an amino acid, as at the right?Chemistry 271, Final Exam, 12/16/2011 6/10 Score for the page (e; 10 pts) Why did the emergence of life on land have to wait for all the surface iron to be oxidized to rust? 4. (40 pts) Electrochemistry and Technology The sodium-sulfur battery has been proposed as an energy storage component of the “smart grid.” Sodium ions migrate through the beta alumina tube, but anions cannot go through it. Molten Na2S5 and molten S are immiscible liquids. The relevant


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UMD CHEM 271 - Final Exam

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