EXAM 3 PRACTICE Leaders: Deborah Course: CHEM 178 Instructor: Miller Date: 1. A voltaic cell is formed at room temperature (25 C) by a silver strip immersed into an aqueous 1.0 M Ag + solution (E o red = 0.80 V) and an iron strip is immersed into an aqueous 1.0 M Fe 2+ solution (E o red = -0.44 V). a. Write the balanced equation for the reaction that occurs in this cell. b. Calculate E cell (see table on the next page). c. Calculate ΔG. d. Is this reaction spontaneous? 2. The second law of thermodynamics states that there is a direction to the way events occur in nature. In any spontaneous process the entropy of the universe increases. Circle the correct answer: a. For a reversible/irreversible process, ΔSuniv = ΔSsystem + ΔSsurrpundings = 0. b. For a reversible/irreversible process, ΔSuniv = ΔSsystem + ΔSsurrpundings > 0. 3. The hypothetical substance AB has a melting point of 18 C and a boiling point of 75 C. a. For the process AB (g) AB (l) at 75 C and 1.00 atm, Ssystem = -98.7 J/K. What is the value of Ssurroundings? b. For the process AB (s) AB (l) at 19 C and 1.00 atm, Ssurroundings = -104 J/K. What is the value of Ssystem? 4. Molten NaCl(aq) is being electrolyzed. a. What can be oxidized in the solution?
b. What can be reduced in the solution? c. What is the minimum voltage necessary to yield Na(s) and Cl2(g)? 5. Consider the reaction 2 Al 3+ (aq) + 3 Sn (s) 2 Al (s) + 3 Sn 2+ (aq) E o cell = -1.539 V a. How many moles of electrons are transferred for 6 moles of Al 3+ (aq)? b. Which component is the strongest reducing agent? 6. From the list below, identify the which species is the strongest reducing agent and which species is the strongest oxidizing agent: a. Fe 3+ b. Cu
c. Zn d. Al 3+ e. Zn 2+ f. Fe 2+ g. Al h. Cu 2+ 7. A Tl + (aq) solution is electrolyzed with a current of 10.9 A. What mass of Tl(s) is produced after 2 days? (1 mol Tl = 204.4 g) a. What amperage is required to plate out 0.50 moles Tl(s) from a Tl + (aq) solution in a period of 5 hours? 8. Construct a galvanic cell consisting of Cu(s)/Cu 2+ (aq) (E red = +0.337 V) and Ag(s)/Ag + (aq) (E red = -0.7994 V) half-cells under standard conditions at 25ºC and a KNO3(aq) salt bridge. Be sure to label: the solutions, the balanced half-reactions in each half-cell, the direction of electron flow, and the direction of salt ion flow. a. Write the balanced redox reaction for this galvanic cell. b. Calculate ΔE. 9. As E red values decrease/become more negative: the reducing agent strength, the oxidizing agent strength, and the potential energy of electrons. 10. A spontaneous REDOX reaction occurs between Pt 2+ (aq) (E o red = 1.2 V) and Br2(l) (E o red = 1.066 V) at 25 C. a. Calculate ΔG.
b. Find the K of the reaction. 11. a. For chromium: will you use molten CrCl2 or aqueous CrCl2? b. For palladium: will you use molten PdCl2 or aqueous PdCl2? 12. What is the difference between a state versus a path function? a. What does a positive q value mean for the system? b. What does a negative w value mean for the system? c. What does a positive ΔE mean for the system? 13. Given the following information: Fe 2+ + 2 e - Fe E red = -0.44 V Sn 2+ + 2 e - Sn E red = -0.14 V Write the reaction that is spontaneous at standard conditions. 14. The half-reaction for the formation of magnesium metal upon electrolysis of molten MgCl2 is Mg 2+ + 2 e - Mg (s). Calculate the current required to plate 30.2 g of magnesium over a period of 4.00 x 10 3 s (Mg = 24.3 g/mol). 15. Indicate whether the following balanced equations involve oxidation-reduction. If they do, identify the elements that undergo changes in oxidation number. a. PBr3 (l) + 3 H2O (l) H3PO3(aq) + 3 HBr (aq) b. NaI (aq) + 3 HOCl (aq) NaIO (aq) + 3 HCl (aq) c. 3 SO2 (g) + 2 HNO3 (aq) + 2 H2O (l) 3 H2SO4 (aq) + 2 NO (g) d. 2 H2SO4 (aq) + 2 NaBr (s) Br2 (l) + SO2 (g) + Na2SO4 (aq) + 2 H2O (l)
16. What is the value of Ecell at equilibrium? 17. If the equilibrium constant for a two-electron redox reaction at 298 K is 1.5 x 10-4, calculate the corresponding ΔG and E red. a. Is this reaction spontaneous? 18. T/F: A reductant, or reducing agent, is reduced in a chemical reaction. 19. Which values of ΔG, Ecell, and Keq are consistent with spontaneous processes? (The values in an answer are not necessarily consistent with each other, and you are not being asked to assess whether they are consistent with each other.) a. ΔG = -42 kj; Ecell = +0.42 V; Keq = 0.42 b. ΔG = +42 kj; Ecell = -0.42 V; Keq = -0.42 c. ΔG = -42 kj; Ecell = +0.42 V; Keq = 4.2 d. ΔG = +42 kj; Ecell = +0.42 V; Keq = 4.2