Chemistry 271, Section 21xx 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: May 13, 2011 VIEWING: Monday, May 16, 9:30-10:30 a.m., Chem 1112 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, Section 21xx, Final Exam, 5/13/2011 2/10 Score for the page 1. (24 pts) Multiple choice: Circle the single best answer for each question (a; 4 pts) Stratification in sediments or the water column comes about because (1) the best available terminal electron acceptor is used by the local flora and fauna until it’s gone. (2) microorganisms have variable densities so they settle to different levels. (3) the organisms that can grow in the water vary with temperature. (4) None of the above. (b; 4 pts) DNA hybridization (formation of double strand) has (1) ∆H° < 0 and ∆G° < 0 at any temperature. (2) ∆H° < 0 and ∆S° > 0. (3) ∆H° < 0 and ∆S° < 0. (4) ∆H° > 0 and ∆G° > 0 above Tm. (c; 4 pts) A DNA microarray is used to (1) measure Tm on small samples. (2) perform multiple simultaneous hybridization experiments. (3) clone a variety of plasmids in cells that float across the array. (4) None of the above. (d; 4 pts) Overpotential is (1) the reason car batteries can be recharged effectively. (2) frequently observed for electrode reactions where a state change (i.e. aqueous → gas) occurs. (3) the requirement that a voltage greater than Ecell must actually be applied to convince a redox reaction to proceed. (4) All of the above. (e; 4 pts) The rate law for a process aA + bB -> cC + dD (1) is given by Rate = k[A]a[B]b. (2) can be correctly written down by inspection if and only if the reaction is elementary. (3) can never depend on [C] and [D] because they are products. (4) None of the above. (f; 4 pts) The equilibrium constants for electrochemical reactions (1) are easily measured by measuring the final concentrations of reactants and products. (2) are usually independent of pH. (3) are unmeasurable because we use E instead. (4) None of the above.Chemistry 271, Section 21xx, Final Exam, 5/13/2011 3/10 Score for the page 2. (55 pts) Kinetics, Rate Laws, and Arrhenius (a; 12 pts) Ethane, C2H6, dissociates into methyl radicals at 700 °C with a rate constant k = 5.5 × 10–4 s–1. The activation energy is 384 kJ/mole. Determine the rate constant at 800 °C. (b; 12 pts) The rate constant for the second-order reaction 2 HI (g) → H2 (g) + I2 (g) is 2.4 × 10–6 M–1s–1 at 575 K and it is 6.0 × 10–5 M–1s–1 at 630 K. Calculate the activation energy.Chemistry 271, Section 21xx, Final Exam, 5/13/2011 4/10 Score for the page The experimental rate law for the reaction (CH3)3CBr + HO– → (CH3)3COH + Br– (above, top) in an organic solvent is Rate ≡ –d[(CH3)3CBr]/dt = k[(CH3)3CBr]. (c; 2 pts) What is the observed order of reaction with respect to HO–? (d; 6 pts) The SN1 mechanism below has been proposed for the reaction. Which step must be rate limiting for the mechanism to agree with the experiment? Briefly explain your reasoning. (CH3)3CBr k1⎯ →⎯ (CH3)3C+ + Br– (CH3)3C+ + HO– k2⎯ →⎯ (CH3)3COH (e; 9 pts) If the concentration of HO– is decreased to a small enough value, the rate law changes: why must this be so? Explain why the steady-state approximation does not apply to anything in the reaction under these conditions.Chemistry 271, Section 21xx, Final Exam, 5/13/2011 5/10 Score for the page (f; 14 pts) However, the change in the rate law at low [HO–] might be hard to observe because the E1 (Elimination 1) product can also appear, according to the mechanism below (CH3)3CBr k1⎯ →⎯ (CH3)3C+ + Br– (CH3)3C+ k3⎯ →⎯ (CH3)C=CH2 + H+ At low T the substitution is favored, but the E1 reaction is found to dominate more and more over the SN1 reaction as the temperature is increased (assume [HO–] is constant). What does this suggest about the values of the k2 and k3 rate constants for the second steps of the SN1 and E1 reactions respectively, and the activation energies for the k2 and k3 steps? Sketch an Arrhenius plot to illustrate your answer.Chemistry 271, Section 21xx, Final Exam, 5/13/2011 6/10 Score for the page 3. (45 pts) Electrochemistry of Life and the Biosphere The aerobic nitrification of ammonia is carried out by soil bacteria (unfortunately… excess ammonia applied as fertilizer is converted to nitrate, which is more readily washed into the Chesapeake Bay). The overall reaction at pH 7 is NH4+ + 2 O2 → NO3– + H2O + 2 H+ The first step in nitrification is the oxidation of ammonia to give hydroxylamine, NH2OH, which is then converted to nitrite, NO2–, by the enzyme hydroxylamine oxidase (HAO). A different bug then oxidizes nitrite to nitrate. (a; 6 pts) Balance the half-reaction for hydroxylamine oxidation to nitrite in acidic solution: Start with: NH2OH → NO2– (b; 4 pts) Add the appropriate multiple of the molecular oxygen reduction