Sample Exam #4 - Personal.kent.edu

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Which of the following reactions is a redox reaction? a. Cu + S CuS b. Pb+ 2Br– PbBr2. c. K2CrO4 + BaCl2 BaCrO4 + 2KCl d. HCl + NaOH H2O + NaCl.
CHEM 10062 Exam #4 Name__________________ Date___________________

Equations: ΔG = ΔH - TΔS, ΔG = -nFE, Ecell = Eox. + Ered., E = E° − [pic]ln Q, E = E°− [pic]log Q, ln K = [pic], log K = [pic], C = amps x sec, S = k ln W, ΔG = ΔH - TΔS, ΔS° = Σ nS°(products) -  Σ nS°(reactants), ΔG = ΔG° + RT ln Q, ΔG° = -RT ln K, K = e(-ΔG°/RT), ΔGf° = Σ nGf°(products) - Σ nGf°(reactants),

Constants: R = 0.0821 L-atm/mol-K = 8.314 J/K-mol, 1 V = 1J/C, 1 F = = 96,500 C/mol e− = 96,500 J/V-mol e−

Multiple Choice (3 pts. ea.)

____ 1. Use the following standard reduction potentials in V: I2 → 2I– , +0.54; Br2 ( 2Br–, +1.07; 2H+ → H2, 0.00; Cu2+ → Cu, +0.34; Ni2+ → Ni, –0.28. Which one of the following species could be used to oxidize I– to I2?

a. Br2 b. H+ c. Cu2+ d. Ni2+

____ 2. Which reaction below should have ΔS° > 0?

a. 2H2(g) + O2(g) ( 2H2O(g) b. 2NO2(g) ( N2O4(g) c. H+(aq) + F−(aq) ( HF(aq) d. BaF2(s) ( Ba+2(aq) + 2F2(aq)

____ 3. How many grams of copper metal can be obtained by passing a current of 12 amps through a solution of CuSO4 for 15 minutes?

a. 3.6 g b. 5.4 g c. 7.1 g d. 14 g

____ 4. Which one of the following has the greatest entropy?

a. H2O(s) b. H2O (l) c. H2O (g) d. these are all the same

____ 5. The electrode at which oxidation occurs is called the:

a. oxidizing agent b. cathode c. reducing agent d. anode

____ 6. Assuming ΔH and ΔS do not vary with temperature, a reaction that is not spontaneous at low temperature can become spontaneous at high temperature if ΔH is ________ and ΔS is ________.

a. +, + b. −, − c. +, − d. −, + ____ 7. The standard reduction potentials of several common metal ions are listed: Mg2+, –2.38 V; Li+, –3.04 V; Ni2+, –0.28 V; Mn2+, –1.18 V. Considering that E° for: 2H2O(l) + 2e– → H2(g) + 2OH–(aq) is –0.83 V, which of the following metals may be produced via electrolysis of an aqueous solution of its nitrate?

a. magnesium b. lithium c. nickel d. manganese

____ 8. Which of the following reactions is a redox reaction? a. Cu + S → CuS b. Pb+ 2Br– → PbBr2 c. K2CrO4 + BaCl2 → BaCrO4 + 2KCl d. HCl + NaOH → H2O + NaCl

____ 9. Ammonium chloride readily dissolves into water but causes the water temperature to decrease. For this process:

a. (H = −, (S = −, (G = − b. (H = +, (S = +, (G = − c. (H = −, (S = −, (G = + d. (H = +, (S = +, (G = +

____ 10. Calculate the value of (G° (in kJ) for this reaction at 25°C. (R = 8.314 J/mol•K) N2(g) + 3H2(g) ( 2NH3(g) K = 5.0 x 108

a. 38 kJ b. –4.2 kJ c. –45 kJ d. –50 kJ

____ 11. The two electrodes Cr(s)/Cr3+(aq) and Sn(s)/Sn2+(aq) are combined to afford a spontaneous electrochemical reaction. E°cell for this reaction is:

a. +0.88 v b. –0.88 v c. +0.60 v d. –0.60 v

____ 12. A spontaneous electrochemical reaction has:

a. ΔG° = 0, E° = 0, and K > 1 b. ΔG° < 0, E° > 0, and K > 1 c. ΔG° > 0, E° < 0, and K < 1 d. ΔG° < 0, E° = 0, and K > 1

____ 13. The gain of electrons is called:

a. reduction b. oxidation c. disproportionation d. fractionation

____ 14. Consider the following reaction at 25°C. What is the value of (S° for this reaction at 25°C? C(s) + H2O(g) ( CO(g) + H2(g) (G° = 91.2 kJ (H° = 131.4 kJ

a. –135 J/K b. 1.6 J/K c. -1.6 J/K d. 135 J/K

____ 15. One of the differences between a voltaic cell and an electrolytic cell is that in an electrolytic cell:

a. an electric current is produced by a chemical reaction. b. electrons flow toward the anode. c. a non-spontaneous reaction is forced to occur. d. O2 gas is produced at the cathode.

____ 16. Which substance is the oxidizing agent in the following reaction? Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O

a. Pb b. H2SO4 c. PbO2 d. PbSO4

____ 17. The standard reduction potentials in V for Ag+ to Agº and Fe3+ to Fe2+ are +0.80 and +0.77, respectively. Calculate K, the equilibrium constant, for the following reaction at 25°C. Ag+(aq) + Fe2+(aq) → Ag(s) + Fe3+(aq)

a. 10 b. 2.0 c. 3.2 d. 1.0

____ 18. Assuming ΔH and ΔS do not vary with temperature, at what temperature will the reaction shown below become spontaneous? C(s) + H2O(g) → CO(g) + H2(g) ΔH = 131.3 kJ ΔS = 133.6 J/K

a. 325ºC b. 710ºC c. 273ºC d. 552ºC

____ 19. Which transformation could take place at the anode of an electrochemical cell?

a. Cr3+ to Cr2O72– b. F2 to F– c. O2 to H2O d. HAsO2 to As

____ 20. E° for the following reaction is 0.13 V. What is the value of ΔG° (in kJ) for the reaction? Pb(s) + 2H+(aq) → Pb2+(aq) + H2(g)

a. 12 kJ b. 25 kJ c. –12 kJ d. –25 kJ

Solve the following problems showing all work. Use complete sentences for essay questions. Remember to use proper units, significant figures and rounding.

1. a. Draw a schematic diagram of a Ag//Zn galvanic cell. Be certain to label all parts of the cell and show the directions of electron and ion flow. (10 pts.)















b. Calculate the EMF for the Ag//Zn galvanic cell above when [Ag+] = 0.00350 M and [Zn2+] = 0.125 M at 25°C. (5 pts.)





2. Complete and balance the following equations using the half-reaction method (5 pts ea.)

a. H2O2(aq) + BrO2(aq) → BrO2−(aq) + O2(g) (basic solution)















b. MnO4-(aq) + Cl−(aq) → Mn2+(aq) + Cl2(aq) (acidic solution)











3. Calculate E°, ΔG°, and Kc for a Br2//Cu galvanic cell (10 pts.)













4. Use the attached table of thermodynamic data to calculate ΔS for the reaction below, at 25°C. 4Al(s) + 3O2(g) ( 2Al2O3(s)



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