Chem 204 Spring 2014 Name PRACTICE EXAM III. There are 11 pages (counting this one) and a periodic table.

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1 Chem 204 Spring 2014 Name TA/Section PRACTICE EXAM III This Examination is worth 100 points. There are 11 pages (counting this one) and a periodic table. No books, notes, cellphones, anything with earbuds/earphones, or laptops; you may only have out pencils or pens and a scientific calculator. Write directly on the exam. You have two hours to complete this exam. You may leave early if you are done early. When you turn your exam in, you must show your university ID card to the proctor/ta. Constants and conversions that you might need are given on the next page. Good luck! 1

2 Constants and Conversions NA = Avogadro's number = x mol -1 h = x J s c = 3.00 x 10 8 m s -1 charge on one electron = e = x C ε0 = x C 2 J -1 m -1 F = 96,485 C/mol me = mass of electron = x kg 1 atomic mass unit = x kg k = k B = Boltzmann s constant = x J K -1 1 cm -1 = x J R = L atm K -1 mol -1 = J K -1 mol -1 1 ev = x J K = C 1 J = 1 kg m 2 s -2 Hz = hertz = s torr = 1 atm = 14.7 psi = bar = mb 1000 L = 1 m 3 I V = 1 J/C Formulas (P + a(n 2 /V 2 )) (V - nb) = nrt ln (P) = - H vap /RT + S vap /R ln(p 2 /P 1 ) = ( H vap /R) [(1/T 1 ) (1/T 2 )] ln(k 2 /K 1 ) = ( H /R) [(1/T 1 ) (1/T 2 )] -b ± b 2-4ac x = for ax 2 + bx + c = 0 2a Equilibrium Constants at 25 C AgCl: Ksp = 1.6 x AgBr: Ksp = 7.7 x AgI: Ksp = 1.5 x H 3 P 4 : Ka (first proton) = 7.52 x10-3 HF: Ka = 6.6 x 10-4 H 2 P 4 - : Ka (first proton) = 6.23 x 10-8 HCl: Ka ~ 10 7 HP 4 2- : Ka = 2.2 x HI: Ka ~10 10 H 2 S: Ka = 9.1 x 10-8 S 2- : Kb = 10 5 NH 3 : Kb = 1.8 x 10-5 CH 3 CH: Ka = 1.8 x 10-5 HN 3 : Ka = 24 K w = 1.0 x H 2 C 3 : Ka = 4.4 x 10-7 HC 3 - : Ka = 4.7 x

3 Standard reduction potentials at 298 K: 2 H 2 + 2e - à H 2 (g) + 2 H - E = V Zn e - ---> Zn(s) Fe e - ---> Fe(s) 2H 3 + (aq) + 2e- ---> H 2 (g) + 2 H 2 (l) (aka 2H + + 2e - à H 2 (g) ) E = V E = V E = V (defined) Al e - ---> Al(s) E = V H 2 + 4e - à 4 H - E = 0.40 V Fe 3+ + e - ---> Fe 2+ E = V Ag + + e - ---> Ag(s) E = V 2 + 4H + + 4e - à 2 H 2 E = 1.23 V Au + + e - ---> Au(s) E = 1.68 V 3

4 I. (50 points). Multiple Choice. Circle the letter of the best answer. 5 points each. No partial credit. 1. The oxidation state of cobalt in [Co(NH 3 ) 4 Cl 2 ]Cl is a. 0 b. 1 c. 2 d. 3 e. 4 f Which molecule or ion is a possible product from the reduction of water? a. elemental hydrogen b. elemental oxygen c. hydroxide ion d. protons e. more than one of the above 3. According to Marcus theory a. the rate of electron transfer is ultrafast (10-15 s time scale). b. the rate of electron transfer is linear with G. c. the rate of electron transfer is inversely parabolic with G. d. more than one of the above. e. none of the above. 4. If an electrochemical cell has an E cell of V, that means a. the overall reaction is spontaneous. b. the overall reaction is not spontaneous. c. you must have hooked the cell up incorrectly. 5. What kind of reaction is this: H H 3 C C H 3 CH 3 CH 3 CH 2 CH C CH2 n H à H 3 C C H 3 CH 3 CH 3 CH 2 CH C CH2 n H H a. oxidation b. reduction c. none of the above 4

5 6. Predict what would happen if you mixed together elemental iron, elemental zinc, and 1.0 M solutions of iron(ii) nitrate and zinc nitrate. a. Nothing will happen. b. Zinc will be oxidized to Zn(II) and Fe(II) will be reduced to iron. c. Iron will be oxidized to Fe(II) and Zn(II) will be reduced to zinc. d. There is no way at all to predict what will happen. 7. Which of the following statements about batteries and fuel cells is FALSE? a. Batteries and fuel cells convert a chemical reaction into electrical work. b. Batteries and fuel cells are galvanic cells. c. Fuel cells do not need an anode and a cathode. d. Lithium ion batteries can provide voltages over 3 V because they use nonaqueous electrolytes. 8. Neurons are the nerve cells in our brains that fire signals based on voltages. Inside the neurons, the potassium ion concentrations are a factor of 25x larger than the potassium ion concentration outside the neuron. What voltage would you expect across the neuron s membrane, based on these potassium ion concentration differences, at body temperature (37 C)? a. Zero. b. 25 V. c. 25 mv. d V e V. f V. g V. h. None of the above. 5

6 9. For this problem only: circle ALL of the statements that are TRUE. No partial credit. a. In this reaction, Ru(II) and Ru(IV) disproportionate into two Ru(III) species. b. The reactants have to collide directly, without solvent, for this reaction to occur, if this chemical drawing is meant to show an elementary step. c. This is an example of a proton-coupled electron transfer reaction. d. This is an example of a hydrogen atom transfer reaction. e. The overall reaction is thermodynamically favorable. 10. If an electrochemical reaction has an overall E cell = V at 298 K, a. S > 0 b. S < 0 c. S = 0 d. K > 1 e. K < 1 f. K = 1 6

7 For the remainder of the exam: you must show clear work in order to earn any partial credit unless otherwise noted. II. (5 points). Balance this redox reaction in basic solution: no partial credit. Zn(s) + N3 - (aq) ----> Zn(H)4 2- (aq) + NH3(aq) 7

8 III. (10 points). Carbon dioxide is placed into a 5.0-L reactor at 380 K. After equilibrium is achieved, 4.95 moles C2, mol C, and mol 2 are present. C(g) + 1/2 2 (g) C2(g) (a) What is being oxidized, and what is being reduced? (b) Calculate E for the reaction, as written, at 380 K. 8

9 IV. (15 points). The direct methanol fuel cell (DMFC) is a promising energy technology, since methanol is a liquid at room temperature and is easier to deal with than the hydrogen gas that is used in other fuel cells. The reaction at the cathode is the reduction of molecular oxygen, with protons, to make water; the anode reaction is oxidation of methanol (CH 3 H) with water to make carbon dioxide (and protons). (a) Write out the cathode half-reaction. (b) Write out the anode half-reaction. (c) For one mole of methanol, what is n? (d) The standard cell potential, in theory, for the DMFC is 1.21 V at 25 C. Usually people run DMFC s at 50 C. Calculate the cell potential at 50 C, stating your assumptions. 9

10 V. (5 points). Bisphenol A (structure below), is commonly abbreviated as BPA and until recently was used in plastics, such as baby bottles. Eventually it was found that BPA is an endocrine disruptor, leading to adverse hormonal effects in humans; BPA is now listed as a toxic substance and partially banned in Europe and in Canada. H H (a) The materials safety data sheet says BPA is incompatible with strong oxidizers. Draw a chemical structure that might result from BPA being oxidized. (b) The materials safety data sheet says BPA is incompatible with strong bases. Draw a chemical structure that might result from BPA being reacted with a strong base. 10

11 VI. (15 points). An electrode is prepared by dipping a piece of silver metal into a solution saturated with silver thiocyanate, AgSCN, and containing M SCN -. This silver electrode is used as the cathode and is wired up to the standard hydrogen electrode as the anode. A voltage of 0.25 V is measured. What is the Ksp of AgSCN? 11

Chem 204 Spring 2014 Name PRACTICE EXAM II. There are 9 pages (counting this one) and a periodic table.

Chem 204 Spring 2014 Name PRACTICE EXAM II. There are 9 pages (counting this one) and a periodic table. Chem 204 Spring 2014 ame TA/Section PRACTICE EXAM II This Examination is worth 100 points. There are 9 pages (counting this one) and a periodic table. o books, notes, cellphones, anything with earbuds/earphones,