Chem 112, Fall 05 (Garman/Weis) Final Exam A, 12/20/2005 (Print Clearly) +2 points

Transcription

1 +2 points Before you begin, make sure that your exam has all 10 pages. There are 24 required problems worth 4 points apiece, unless otherwise noted, and two 4-point extra credit problems. Stay focused, stay calm. Work steadily through your exam. YOU MUST: Put your name and student ID on the bubble sheet correctly. Put all your answers on the bubble sheet; nothing on this exam will be used for grading. Sign the statement on the last page of the exam. Turn in both the exam and bubble sheet when you are done. Good Luck! TABLES of DATA All the Necessary Data are Presented with the Problem page 1

2 Physical Constants & Conversions: Room Temperature = 25 C = 298K T ( C) = T (K) 1 atm = 760 mm Hg R = kj/(k mol) = J/(K mol) = (L atm)/(k mol) N = 6.02 x K bp for water: C/m K fp for water: C/m H vap = 40.7 kj/mol for water F = kj/(mol ev) = 96,485 C/(mol e) K W = at 25 C 1 C = 1 coulomb Useful Equations: PV = nrt T = Kmi Π = crt P A = X A P A o k = Ae -E a /RT ln(k 2 /k 1 ) = (E a /R)(1/T 1-1/T 2 ) ln(p 2 /P 1 ) = (ΔH/R)(1/T 1-1/T 2 ) A 0 - A = kt ln(a/a 0 ) = -kt t 1/2 = 0.693/k 1/A - 1/A 0 = kt K p = K c (RT) Δn aa + bb cc + dd Q = ([C] c [D] d )/([A] a [B] b ) K = ([C] eq c [D] eq d )/([A] eq a [B] eq b ) ph = -log[h 3 O + ] poh = -log[oh - ] pk A = -logk A pk B = -logk B K W = [H 3 O + ][OH - ] K W = K A K B HA(aq) + H 2 O(l) A - (aq) + H 3 O + (l) K A = [H 3 O + ][A - ]/[HA] B(aq) + H 2 O(l) B + H(aq) + OH - (l) K B = [OH - ][B + H]/[B] ph = pk A + log([conjugate base]/[acid])] ΔS = q rev /T q rev = ΔH sys (for a constant pressure process) w max = -ΔG sys ΔS sys = Σ S(final state) Σ S(initial state) ΔS univ = ΔS surr + ΔS sys ΔG sys = ΔH sys TΔS sys ΔG o rxn = Σ ΔG o f (products) Σ ΔG o f (reactants) ΔG = ΔG o + (RT)lnQ ΔG o = -(RT)lnK E o cell = E o cathode E o anode (1/2 reactions are expressed as reductions) E o cell = E o (reduction ½ rxn) + E o (oxidation ½ rxn) E = E o (RT/nF)lnQ E = E o (( V)/n)lnQ (at T = 298 K) E o = (RT/nF)lnK w max = nfe ΔG = -nfe Current (amperes, A) = [electric charge (coulombs, C)]/[time (seconds, s)] Electric charge (coulombs, C) = [n (moles of electrons transferred)] x [F (coulombs/mole e - )] page 2

3 1. If we look at the halogens in their elemental form, F 2 and Cl 2 are gases, Br 2 is a liquid, and I 2 is a solid at 298K. What is responsible for this phenomenon? A. dipole-dipole interactions increase with molecular size. B. induced dipole-induced dipole interactions increase with molecular size. C. dipole-induced dipole interactions increase with molecular size. D. polarity increases with molecular size. E. both C and D above 2. Determine the heat of vaporization of a pure liquid from the following experiment: A fixed amount of the liquid is placed in a sealed container and allowed to equilibrate at 250K, where the vapor pressure is measured to be 249 mm Hg. The system is then heated to 500K and allowed to re-equilibrate, when the vapor pressure of the system is measured as 657 mm Hg. A kj/mol B kj/mol C kj/mol D kj/mol E. None of the above 3. Which of the following has the lowest boiling point? A. H 2 O B. H 2 S C. H 2 Se D. H 2 Te 4. Sodium Chloride (NaCl) is commonly used to melt ice this time of year. If 100 g of NaCl is added to 3.00 kg of water to a final volume of 3.08 L, what will the melting point of the solution be? The K mp for water is C/m. A C B C C C D C E C page 3

4 5. An unknown amount of glycerol (a non volatile, non-ionic compound with a molecular weight of 92.1 g/mole) is mixed with water to a final volume of 2.00 L. If the osmotic pressure of the solution at room temperature is measured as 3.00 atm, what is the number of moles of glycerol in the solution? A moles B moles C moles D moles E moles 6. Ethylene glycol, used to prevent automobile radiators from boiling over in the summer, is a non-volatile, non-ionic compound with a molecular weight 62.1 g/mole. If 651 g of ethylene glycol is added to 1.50 kg of water, what is the vapor pressure of the solution at 80 C if the vapor pressure of pure water is mm Hg at 80 C? A mm Hg B. 107 mm Hg C. 248 mm Hg D. 315 mm Hg E. None of these 7. How long does it take to completely oxidize 6 beers (2.0 moles of ethanol) if the zero-order rate constant for the oxidation of ethanol in the liver is M/hour. Assume the volume of fluids in a typical person is 40 L. A. 5.9 hours B. 8.5 hours C. 12 hours D. 14 hours E. None of these page 4

5 8. For the reaction A + 2B + 3C X + 2Y, 8 experiments were performed with different starting concentrations of A, B, and C. The rate of appearance of X is shown: Experiment [A] [B] [C] rate of appearance of X M M M 1.72 x 10-4 M/sec M M M 3.44 x 10-4 M/sec M M M 6.88 x 10-4 M/sec M M M 1.38 x 10-3 M/sec M M M 1.72 x 10-4 M/sec M M M 3.44 x 10-4 M/sec M M M 6.88 x 10-4 M/sec M M M 1.38 x 10-3 M/sec What is the rate equation for the reaction? A. rate = k[a][b][c] B. rate = k[a][b] 2 [C] 3 C. rate = k[b] 2 [C] D. rate = k[b][c] 2 E. None of these 9. Radiocarbon dating is based upon the first-order decay of the 14 C isotope, which has a rate constant of year -1. How long will it take for g of 14 C to convert to g? A years B years C years D years E. None of these 10. Ammonia is made industrially by combining elemental nitrogen and hydrogen according to the following reaction at 450 C: N 2 (g) + 3H 2 (g) 2 NH 3 (g) If a reaction container contains 2.00 M N 2, 5.00 M H 2 and 10.0 M NH 3, which way will the reaction shift to reach equilibrium if the K c for the reaction is 0.16? A. The reaction will make more NH 3 because Q > K. B. The reaction will consume NH 3 because Q > K. C. The reaction will make more NH 3 because Q < K. D. The reaction will consume NH 3 because Q < K. E. [NH 3 ] will remain the same because Q = K. page 5

6 11. What is the correct form of the equilibrium constant expression for the reaction PbI 2 (s) Pb 2+ (aq) + 2 I - (aq) A. K c = [Pb 2+ ][I - ] B. K c = [Pb 2+ ][I - ] 2 C. K c = [PbI 2 ] / [Pb 2+ ][I - ] 2 D. K c = [Pb 2+ ][I - ] 2 / [PbI 2 ] E. None of these is correct 12. (5 pts) Nitrogen and oxygen combine at high temperatures to make the pollutant nitrogen monoxide according to the following reaction: N 2 (g) + O 2 (g) 2 NO(g) A reaction vessel initially contains 4.00 M of N 2 and 1.00 M of O 2. The reaction then proceeds to equilibrium, where the concentration of NO is recorded as M. What is the equilibrium constant K c for this reaction? A x 10-4 B x 10-3 C x 10-3 D x 10-3 E x The conjugate acid of HSO 4 - is. A. OH - B. SO 4 2- C. H 2 SO 4 D. H 2 O E. H 3 O Which one of the following aqueous solutions will have a ph of at 25 C? A. 0.1 M HCl B M NaNO 3 C M HBr D M NaOH E. 1.0 M KOH 15. Which of the following weak acids has the strongest conjugate base in an aqueous solution? A. acetic acid, K a = B. hydrogen sulfite ion, K a = C. phosphoric acid, K a = D. formic acid, K a = E. nitrous acid, K a = page 6

7 16. Which of the following combinations would make the best buffer at ph 5.0? - A. H 3 PO 4 and H 2 PO x B. HNO 2 and NO x C. CH 3 CO 2 H and CH 3 COO x D. H 2 PO and HPO x 10-7 E. NH + 4 and NH x 10-5 K A K B 17. What is the ph of a buffer that results when 0.50 mole of Na 3 PO 4 is mixed with 0.75 mole of HCl and diluted with water to 1.00 L? (The acid dissociation constants for phosphoric acid are K a1 = , K a2 = , and K a3 = ) A B C D E The K sp of PbBr 2 is at 25 C. What is the concentration of Br - in a saturated solution of PbBr 2 (aq)? A M B M C M D M E M 19. The second law of thermodynamics states that A. spontaneous reactions are always exothermic. B. energy is conserved in a chemical reaction. C. the Gibbs energy is a function of both enthalpy and entropy. D. ΔS = - ΔH for any chemical reaction. E. in a spontaneous process, the entropy of the universe increases. 20. If a process is exothermic and spontaneous, which of the following must be true? A. ΔG < 0 and ΔH < 0 B. ΔG > 0 and ΔH < 0 C. ΔG < 0 and ΔS > 0 D. ΔH < 0 and ΔS > 0 E. ΔH > 0 and ΔS < 0 page 7

8 21. DNA forms a double helix by specific base-pairing interactions. Some short DNA segments (oligonucleotides) can self-dimerize. For example, GAAGCTTC forms dimers reversibly according to the process: 2 GAAGCTTC GAAGCTTC CTTCGAAG The equilibrium constant (K eq ) and standard state enthalpy (ΔH o rxn) of dimerization were measured at 37 o C and found to be 7.33 x 10 4 and -184 kj/mol, respectively. What is ΔS o rxn? A J/K B J/K C J/K D J/K E. 524 J/K 22. Write a balanced chemical equation for the following reaction in an acidic solution. Cr 2 O 7 2- (aq) + Br - (aq) Cr 3+ (aq) + Br 2 (aq) A. Cr 2 O 7 2- (aq) + 2 Br - (aq) 2 Cr 3+ (aq) + Br 2 (aq) B. Cr 2 O 7 2- (aq) + 2 Br - (aq) + 14 H + (aq) 2 Cr 3+ (aq) + Br 2 (aq) + 7 H 2 O(l) C. Cr 2 O 7 2- (aq) + 6 Br - (aq) + 14 H + (aq) 3 Br 2 (aq) + 2 Cr 3+ (aq) + 7 H 2 O(l) D. Cr 2 O 7 2- (aq) + 6 Br - (aq) + 7 H + (aq) 2 Cr 3+ (aq) + 3 Br 2 (aq) + 7 OH - (aq) 23. Consider the following half-reactions: Cu 2+ (aq) + 2 e - Cu(s) E = V Sn 2+ (aq) + 2 e - Sn(s) E = V Fe 2+ (aq) + 2 e - Fe(s) E = V Al 3+ (aq) + 3 e - Al(s) E = V Mg 2+ (aq) + 2 e - Mg(s) E = V Which of the above metals or metal ions will reduce Fe 2+ (aq)? A. Cu(s) and Sn(s) B. Cu 2+ (aq) and Sn 2+ (aq) C. Al 3+ (aq) and Mg 2+ (aq) D. Al(s) and Mg(s) E. Sn(s) and Al 3+ (aq) page 8

9 24. (5 pts) During the process of biological electron transfer, riboflavin can undergo a two electron oxidation-reduction. When riboflavin (Rib) is reduced from its oxidized form (RibO) by the (two electron) oxidation of acetaldehyde (CH 3 CHO), the balanced reaction is RibO(aq) + CH 3 CHO(aq) Rib(aq) + CH 3 COOH(aq) The half cell reduction potentials of RibO and acetic acid (CH 3 COOH) are RibO(aq) + 2H + (aq) + 2e - Rib(aq) + H 2 O(l) E o = V CH 3 COOH(aq) + 2H + (aq) + 2e - CH 3 CHO(aq) + H 2 O(l) E o = V What is the equilibrium constant for the balanced reaction at 25 o C? A B x C D x 10-7 E x EXTRA CREDIT 25. (4 pts.) Calculate! G o for the reaction below at 125 C. C 2 H 5 OH(g) + 3 O 2 (g) 2 CO 2 (g) + 3 H 2 O(g) Species! H o f (kj/mol) S o f (J/K mol) C 2 H 5 OH(g) O 2 (g) CO 2 (g) H 2 O(g) A kj B kj C kj D kj E kj page 9

10 26. (4 pts.) 50 ml of an Arsenic acid (H 3 AsO 4 ) solution is titrated with 0.1 M NaOH. After 25 ml of the NaOH is added, the resulting solution is ½ the way to first equivalence point. K a1 = 2.5x10-4 K a2 = 5.6x10-8 K a3 = 3.0x10-13 What is the concentration of AsO 4 3- in this solution? A. 2.3 x M B M C M D M E M Please sign the following statement at the completion of the exam. I, (print name) pledge not to discuss this test with anyone who has not it the tests until are graded and returned. page 10