Name Date Chemistry 201: General Chemistry II - Lecture Short-Answer Exam #3, 70 Points Total Form: A Read all directions carefully. Answers not conforming to the directions will be marked as incorrect! Section 1: Matching (10 points) Match the following concepts with the appropriate definitions or examples listed below. Make selections such that each term has its best possible match. Indicate your choices by marking your Scantron sheet (1-10) with the appropriate two-letter code. complex ion S G f high entropy state thermodynamics electrical current emf G redox reaction plate out CE. describes solid forming on an electrode BC. generally increases as substance goes from s l g BD. standard free energy of formation AC. gives indication of direction and extent of reaction CD. results from charge in motion AE. available energy for work AB. comprised of a central metal bound to 1 or more ligands BE. must be balanced w/ respect to mass and charge DE. cell potential AD. coincides with highest probable state 1
Section 2: Review Topics Multiple Choice (10 points) Choose the term or phrase that best completes the sentence or answers the question. Indicate your selection by marking the appropriate letter on your Scantron sheet. 11. In which compound does vanadium have the lowest oxidation state? V 2 O 5 V 2 O 3 VO 2 VO 12. When a material in the liquid state is vaporized and then condensed to a liquid, the steps in the process are, respectively,. exothermic and exothermic exothermic and endothermic endothermic and exothermic endothermic and endothermic 13. If 5.00 g of gold has C s = 0.128 J/g C, then 10.00 g of gold has C s =. 0.128 J/g C 1.28 J/ C 0.356 J/g C None of these 14. The oxidation state of C in C 2 O 2 4 is. 3 + + 3 + 4 + 6 2
15. A reaction is exothermic if the energy of bond making is the energy of bond breaking. the absolute value of equal to less than greater than 16. The H rxn for Zn (s) + 2HCl (aq) ZnCl 2(aq) + H 2(g) was measured at. 1 atm and 273.15 K 1 atm and 25 C 1 atm and 0 C unknown conditions 17. The standard heat of formation for N 2(g) is. 28 J/mole 7 J/mole 0 J/mole Not enough information to answer 18. Which change requires an oxidizing agent to produce the indicated product? S 2 O 2 3 S 4 O 2 6 Zn 2+ Zn ClO Cl SO 3 SO 2 4 19. The combustion of ammonia is represented by the equation 4NH 3(g) + 5O 2(g) 4NO (g) + 6H 2 O (g) with H rxn = -904.8 kj. What is the enthalpy of formation of NH 3(g) if the enthalpies of formation of NO (g) and H 2 O (g) are +90.4 kj/mole and -241.8 kj/mole, respectively? -449 kj/mole -46.1 kj/mole -184 kj/mole 227 kj/mole 3
20. is applied when a reaction under study is broken up into 4 smaller reactions. Valence Bond Theory the Born-Haber cycle VSEPR Theory Hess s Law Section 3: Multiple Choice (40 points) Choose the term or phrase that best completes the sentence or answers the question. Indicate your selection by marking the appropriate letter on your Scantron sheet. 21. Which change is likely to be accompanied by an increase in entropy? N 2(g) + 3H 2(g) 2NH 3(g) at 25 C Ag + (aq) + Cl (aq) AgCl (s) at 25 C CO 2(s) CO 2(g) at 70 C H 2 O (g) H 2 O (l) at 100 C 22. Which of the following is the criterion for spontaneity? S sys > 0 S surr < 0 S univ < 0 S univ > 0 23. For the process O 2(g) 2O (g), H = 498 kj. What would be predicted for the sign of S rxn and the conditions under which this reaction would be spontaneous? S rxn positive, spontaneous at low temperatures only S rxn positive, spontaneous at high temperatures only S rxn negative, spontaneous at high temperatures only S rxn negative, spontaneous at low temperatures only 4
24. An oxidation-reduction reaction in which 3 electrons are transferred has G = 18.55 kj. What is the value of E? Note that 1 F = 96,485 C. 0.192 V 0.064 V 0.192 V 0.577 V 25. Which of the following quantities can be evaluated to determine spontaneity? G E cell K All of these 26. In which case would the LEAST number of faradays of electricity (1 F = 96,485 C/mole e ) be required for the liberation of 1.0 g of free metal? K from molten KOH Na from molent NaCl Cu from aqueous CuSO 4 Ag from aqueous AgNO 3 27. Lewis bases typically the solubility of transition metal precipitates. increase decrease do not change buffer 28. The reaction 6I (aq) + Cr 2O 2 7(aq) + 14H+ (aq) 3I 2(aq) + 2Cr 3+ (aq) + 7H 2O (l) has Ecell = 0.79 V. What is the standard potential for I 2(aq) being reduced to I (aq) given that the standard reduction potential for Cr 2 O 2 7(aq) changing to Cr3+ (aq) is 1.33 V? 0.54 V 0.54 V 0.18 V 0.18 V 5
29. Fluoride ions in aqueous solution are difficult to oxidize at the anode of an electrolytic cell because. the aqueous solutions of fluoride are nonconducting it is impossible to find the proper material from which to build the electrodes the fluorides are not very soluble oxygen is released from water in preference to fluorine 30. When the oxidation-reduction equation?cr 2 O 2 7(aq) +?H 2S (aq)?cr 3+ (aq) +?S (s) is balanced in acidic solution, using only whole number coefficients, what is the coefficient for S (s)? 4 3 2 1 31. In the overall reaction represented by Sn (s) Sn 2+ (aq) NO (g) NO 3(aq), H+ (aq) Pt (s), the oxidizing agent is. Sn (s) NO (g) NO 3(aq) Pt (s) 32. Using the provided table of reduction potentials, which of the following is the strongest oxidizing agent? H + (aq) Cl 2(g) F (aq) Na (s) 6
33. A quantity of a powdered mixture of zinc and iron is added to a solution containing Fe 2+ and Zn 2+ ions, each at 1.0 M. What reaction will occur? Zinc ions will oxidize Fe to Fe 2+. Fe 2+ ions will be oxidized to Fe 3+ ions. Zinc ions will be reduced to zinc metal. Zinc metal will reduce Fe 2+ ions. 34. A current of 5.00 A is passed through an aqueous solution of chromium(iii) nitrate for 30.0 min. How many grams of chromium metal will be deposited at the cathode? 0.027 g 1.62 g 4.85 g 6.33 g 35. Which of the following can reduce Fe 3+ (aq) to Fe2+ (aq)? F 2(g) Br (aq) Zn 2+ (aq) Sn 2+ (aq) 36. In which process is entropy DECREASED? dissolving sugar in water expanding a gas evaporating a liquid freezing water 37. Which of the following has the greatest S? NaCl (aq) AlCl 3(aq) KNO 3(aq) CaCl 2(aq) 7
38. The cell potential for SHE IO 3(aq), H+ (aq), I 2(aq) Pt (s) is. 0.24 V 0.60 V 1.20 V 2.40 V 39. Endothermic reactions. are never spontaneous always decrease the entropy of the system increase the entropy of the universe None of these 40. In a complex ion, the transition metal acts as a. Lewis acid Lewis base Arrhenius acid ligand 8
Section 4: Short Answer (5 points each) 41. Explain why a salt bridge is necessary for a galvanic cell. 42. If G is a large negative number, can we expect the corresponding process to occur quickly? Why or why not? 9
Answer Key for Exam A Section 1: Matching (10 points) AB BC BD AD AC CD DE AE BE CD complex ion S G f high entropy state thermodynamics electrical current emf G redox reaction plate out CE. describes solid forming on an electrode BC. generally increases as substance goes from s l g BD. standard free energy of formation AC. gives indication of direction and extent of reaction CD. results from charge in motion AE. available energy for work AB. comprised of a central metal bound to 1 or more ligands BE. must be balanced w/ respect to mass and charge DE. cell potential AD. coincides with highest probable state 1
Section 2: Review Topics Multiple Choice (10 points) 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. Section 3: Multiple Choice (40 points) 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 2
Section 4: Short Answer (5 points each) 41. Answer: In a galvanic cell, oxidation occurs at the anode and reduction occurs at the cathode. Since oxidation is the loss of electrodes, the anode compartment is losing negative charge (or gaining positive charge). Since reduction is the gain of electrodes, the cathode compartment is gaining negative charge (or losing positive charge). Therefore, the redox reaction creates a buildup of positive charge on the anode side and a buildup of negative charge on the cathode side. Solutions must remain electrically neutral, so a salt bridge is introduced. Anions flow from the salt bridge to the anode compartment, and cations flow from the salt bridge to the cathode department in order to maintain electroneutrality. 42. Answer: No, we cannot expect the process to happen quickly. DeltaG is a thermodynamic quantity that tells us direction or favorability or urgency of a process, but not the speed. How fast a process occurs is determined by kinetics. Thus, a highly favorable process could proceed slowly. 3