CHEMISTRY 112 EXAM 3 JUNE 17, 2011 FORM A 1. Consider the following reaction: A + B C + D What are the signs of ΔH and ΔS for the reaction to always be spontaneous? ΔH ΔS A. + + B. C. + D. + 2. What is the maximum concentration of Cd 2+ that can exist in a solution in which the S 2 concentration is 2.5 10 6 M? K sp (CdS) = 8.0 10 27. A. 2.0 10 32 M B. 2.2 10 19 M C. 3.2 10 21 M D. 8.9 10 14 M E. 8.0 10 27 M 3. What is the reducing agent in the reaction below? A. Cr 2 O 7 2 B. S 2 O 3 2 C. H + D. Cr 3+ E. S 4 O 6 2 Cr 2 O 7 2 + 6S 2 O 3 2 + 14H + 2Cr 3+ + 3S 4 O 6 2 + 7H 2 O
4. Calculate the molar concentration of bromide ions in a saturated solution of mercury(ii) bromide, K sp = 8.0 10 20. A. 1.4 10 10 M B. 2.0 10 20 M C. 5.4 10 7 M D. 2.7 10 7 M E. 7.0 10 11 M 5. What is the oxidation state of carbon in the carbonate ion? A. 2 B. +4 C. +2 D. 0 E. +6 6. Estimate the boiling point of CCl 4 in C. ΔH f CCl 4 (l) = 139.3 kj/mol ΔH f CCl 4 (g) = 106.7 kj/mol S CCl 4 (l) = +214.4 J/mol S CCl 4 (g) = +309.4 J/mol Assume that enthalpy and entropy values are temperature independent. A. 443 C B. 70.0 C C. 0.34 C D. 468 C E. 195 C
7. Calculate the ΔG for the following reaction at 500K. Cu (s) + H 2 O (g) CuO (s) + H 2 (g) ΔH f (kj/mol) S (J/K) Cu (s) 0 33.3 H 2 O (g) 241.8 188.7 CuO (s) 155.2 43.5 H 2 (g) 0 130.6 A. +110.6 kj B. 86.6 kj C. +23.9 kj D. 62.6 kj E. +301 kj 8. The purpose of the salt bridge in an electrochemical cell is to. A. maintain electrical neutrality in the half-cells via migration of ions. B. provide a source of ions to react at the anode and cathode. C. provide oxygen to facilitate oxidation at the anode. D. provide a means for electrons to travel from the anode to the cathode. E. provide a means for electrons to travel from the cathode to the anode.
9. The dissociation constant for water at 25 C is 1 10 14. What is the standard free energy change for the reaction? 2 H 2 O(l) H 3 O + (aq) + OH (aq) A. 7.99 10 4 kj B. 8.0 10 3 J C. 80 kj D. 39.9 kj E. 40 kj 10. Sodium carbonate, Na 2 CO 3, is sometimes added to water supplies to reduce "hardness." The net reaction that occurs is: CO 3 2 (aq) + Ca 2+ (aq) CaCO 3 (s) When this reaction comes to equilibrium, which of the following is true? A. All of the Ca 2+ is precipitated. B. All of the Ca 2+ is present in the form of suspended solids. C. The rate of dissolution of CaCO 3 equals its rate of precipitation. D. The rates of dissolution and precipitation of CaCO 3 are zero. E. CaCO 3 precipitates faster than it re-dissolves. 11. When balanced, what is the co-efficient for MnO 4 in the following reaction? A. 1 B. 2 C. 3 D. 4 E. 5 NO + MnO 4 + H + NO 3 + Mn 2+ + H 2 O (acid solution) Go on to the next page
12. A solution is prepared by mixing 25 ml of 3.41 10 8 M AgNO 3 solution with 25 ml of 4.62 10 5 M NaBr solution? Which of these statements is/are true? (K sp for AgBr = 4.1 10 13 )? I II. III. The solution will be a saturated solution of AgBr. The solution will be unsaturated. A precipitate will form. A. I only B. II only C. III only D. II and III E. I and III 13. A voltaic cell is constructed from Ni and Zn electrodes, immersed respectively in 1.0 M Ni(NO 3 ) 2 and 1.0 M Zn(NO 3 ) 2 solutions. The two solutions are connected by a salt bridge. What is the potential difference between the two electrodes and which direction do the electrons flow in the external circuit? A. 0.48 V, from Zn to Ni B. 0.48 V, from Ni to Zn C. 1.04 V, from Zn to Ni D. 2.08 V, from Zn to Ni E. 0.96 V, from Zn to Ni 14. Calculate ΔG for the following reaction. A. 2.89 kj B. 151.5 kj C. 151.5 kj D. 2.89 kj E. 5.79 kj Ag + (aq) + Fe 2+ (aq) Ag(s) + Fe 3+ (aq) Go on to the next page
15. When AQUEOUS NaCl is electrolyzed, the products of the reaction are: A. H 2 and O 2 B. Na and Cl 2 C. NaOH, H 2, and Cl 2 D. Na and O 2 E. Na, Cl 2, and NaOH 16. A metal can protect iron from corrosion by acting as a sacrificial anode if: A. the metal is more electronegative than iron. B. the salt of the metal is more soluble than the corresponding iron salt. C. the metal is a better reducing agent than iron. D. the metal's standard half cell reaction is more positive than that of iron. E. the salt of the metal is a better oxidizing agent than the corresponding iron salt. 17. How many Faradays are required to convert one mole of Mo 2 Cl 8 4 (aq) to two moles of MoO 4 2 (aq)? A. 2 B. 4 C. 5 D. 8 E. 10 18. Dry cells are batteries that power flashlights, portable CD players, etc. In the dry cell, Zn metal is oxidized. How many hours can a battery containing 16.40 g Zn run at a current of 40.60 ma? A. 1.19 10 6 hours B. 1.19 10 9 hours C. 3.31 10 2 hours D. 2.20 10 4 hours E. 1.66 10 2 hours Go on to the next page
19. What is the concentration of Zn 2+ when the following cell has a potential of 1.30 V at 25 C. Zn(s) + Cu 2+ (1.0 10 5 M) Zn 2+ (x M) + Cu(s) A. 1.0 M B. 1.7 10 12 M C. 4.4 10 8 M D. 4.1 10 9 M E. 1.7 10 7 M 20. Which of the following decreases when solid Ba(OH) 2 is added to a saturated solution of BaCO 3 (K sp = 5.1 10 9 )? A. [Ba 2+ ] B. [CO 3 2 ] C. [OH ] D. ph E. None of the above 21. A voltaic cell utilizes the following reaction: Al(s) + 3Ag + (aq) Al 3+ (aq) + 3Ag(s) Which of the following changes will decrease the cell emf? A. Removing the Al 3+ from the cell. B. Increase the size of the aluminum electrode. C. Add AgNO 3 solution to the cathode compartment, increasing the quantity of Ag + but not changing its concentration. D. Add HCl to the AgNO 3 solution, precipitating some of the Ag + as AgCl. E. Add water to the anode compartment. Go on to the last page
22. Which is the best oxidizing agent in the following set? A. Fe 3+ B. Fe 2+ C. Cu 2+ D. Ag + E. H + 23. Which statement about the hydration of sulfur trioxide is correct? SO 2 (g) + H 2 O (g) H 2 SO 4 ΔH = 227 kj/mol, ΔS = 309 J/mol K A. At 1,000 C, ΔG = 1.98 10 5 J/mol. B. The reaction would be at equilibrium at about 735 K. C. The reaction is spontaneous at 1,400 K. D. ΔG can be negative only at high temperatures. E. ΔG can never be negative 24. A saturated suspension of cobalt carbonate is treated with EDTA 4 (aq). If the equilibrium concentration of EDTA 4 (aq) is 1.0 10 6 M, how much CoCO 3 will dissolve? The K sp for CoCO 3 is 1.0 10 10, and the K f for Co(EDTA) 2 is 2.0 10 16. Hint: CoCO 3 will react with EDTA 4 (aq) to form a complex as shown below: CoCO 3 (s) + EDTA 4 (aq) Co(EDTA) 2 (aq) + CO 3 2 (aq) A. 1.41 M B. 0.35 M C. 0.71 M D. 4.0 M E. 2.0 10 6 M END OF EXAM