Department of Chemistry, Geosciences and Environmental Sciences

 

Exam 3-A

Chemisty 1084: Section 010 & 030 Spring 2008

 

Name:________________________________________________________

 

Read all directions and questions carefully!! This exam consists of two parts. The first part consists of 10 multiple choice questions worth four points each for a total of 40 points. The second part consists of five numerical problems worth either 10 or 15 points per question for a total of 60 points. Show all your work necessary for the numerical problems as partial credit will be given for those problems.

 

Possibly Useful Constants and Equations

 

G = ∆H T∆S G = ∆H T∆S

G = RTlnK

Nernst equation: G = nFE

G = nFE

Gas constant: R = 8.314 J/molK Faraday constant: F = 96485 C/mol

 

 

Score

 

Part 1 (40 points):_____________________

 

 

Part 2 (60 points):_____________________

 

 

Total (100 points):_____________________

 

 

Dont forget to put your name on this test!

 

Good Luck!!


Part 1

 

Multiple Choice

 

Please indicate the answer to each question by putting your choice in the space provided. There is only one correct answer for each question. There will be 10 multiple choice questions worth 4 points each.

 

1. Most of the Earths weather occurs in which layer of the atmosphere?

(a) stratosphere (c) thermosphere

(b) troposphere (d) mesosphere

 

Answer: B

 

2. Rainwater is naturally acidic due to the presence of which gas in the atmosphere?

(a) O3 (c) NO

(b) CH4 (d) CO2

 

Answer: D

 

3. Which one of the following processes would be accompanied by an increase in entropy?

(a) The freezing of water to make ice.

(b) Dissolving a solid solute in water.

(c) Increasing the pressure of a gas.

(d) Decreasing the temperature of a substance.

 

Answer: B

 

4. The Second Law of Thermodynamics states that the total entropy of the universe is:

(a) constant (c) equal to zero

(b) always increasing (d) always decreasing

 

Answer: B

 

5. An exothermic reaction with an increase in entropy will:

(a) be spontaneous at all temperatures.

(b) be spontaneous only at lower temperatures.

(c) be spontaneous only at higher temperatures.

(d) not be spontaneous at any temperature.

 

Answer: A

 

6. Given the table of standard reduction potentials on the back page, which of the following species is the strongest oxidizing agent?

(a) O2(g) under acidic conditions (c) Sn2+(aq)

(b) Fe2+(aq) (d) O2(g) under basic conditions

 

Answer: A

 

 

7. A spontaneous redox reaction has a _________ value for E and a __________ value for DG.

(a) positive, positive (c) positive, negative

(b) negative, positive (d) negative, negative

 

Answer: C

 

8. Given the table of standard reduction potentials on the back page, what is the value of E for the following redox reaction:

3 Mg(s) + 2 Al3+(aq) 3 Mg2+(aq) + 2 Al(s)

(a) 0.710 V (c) 3.792 V

(b) 3.792 V (d) 0.710 V

 

Answer: D

 

9. Which element is reduced in the reaction below?

NaI(aq) + 3 HOCl(aq) NaIO3(aq) + 3 HCl

(a) Na (c) H

(b) I (d) O

(e) Cl

 

Answer: E

 

10. The radionuclide, 75Ge, decays by beta emission. What is the daughter nucleus for this decay?

(a) 75Ga (c) 75As

(b) 71Zn (d) 76Ge

(e) 74Ge

 

Answer: C

(Not covered yet in Spring 2009)

 

 

 


Part 2

 

Numerical Problems

 

Solve the following problems, keeping track of significant figures where applicable. Please show all the work necessary to obtain your answer in order to receive partial credit for possibly wrong answers. Generally, full credit will not be given for the correct answer without any of the work performed to obtain the answer being shown on the paper. Each question is worth either 10 or 15 points.

 

 

Table of Thermodynamic Values at 298.15 K

 

Substance

DHf (kJ/mol)

S (J/Kmol)

Substance

DHf (kJ/mol)

S (J/Kmol)

C2H2(g)

+227.0

200.8

CO2(g)

393.5

213.8

O2(g)

0.0

205.2

H2O(g)

241.8

188.8

 

11. (10 points) Given the table of thermodynamic values at 298.15 K shown above, calculate DH, DS, and DG for the reaction:

2 C2H2(g) + 5 O2(g) 2 CO2(g) + 2 H2O(g)

 

DH = (2 mol CO2)(393.5 kJ/mol) + (2 mol H2O)(241.8 kJ/mol) (2 mole C2H2)(227.0 kJ/mol)

= 787.0 kJ 483.6 kJ 454.0 kJ = 1724.6 kJ

DS = (2 mol CO2)(213.8 J/Kmol) + (2 mol H2O)(188.8 J/Kmol) (2 mole C2H2)(200.8 J/Kmol) (5 mol O2)(188.8 J/Kmol)

= 427.6 J/K + 377.6 J/K 401.6 J/K 944.0 J/K = 540.4 J/K

 


12. (15 points) A voltaic (or Galvanic) electrochemical cell is constructed using the following two half-reactions:

Ni2+(aq) + 2e Ni(s) E = 0.257 V

Mo3+(aq) + 3e Mo(s) E = 0.200 V

(a) (2 points) Which half-reaction occurs at the anode? (Write the reaction in the direction that it occurs at the anode)

The half-reaction with the more negative reduction potential: Ni(s) Ni2+(aq) + 2 e

 

(b) (2 points) Which half- reaction occurs at the cathode? (Write the reaction in the direction that it occurs at the cathode)

The half-reaction with the more positive reduction potential: Mo3+(aq) + 3 e Mo(s)

 

(c) (3 points) Write the balanced redox reaction that is associated with this cell:

The balance redox reaction is the sum of the nickel half-reaction times 3 plus the molybdenum half-reaction times 2:

3 Ni(s) + 2 Mo3+(aq) 3 Ni2+(aq) + 2 Mo(s)

 

(d) (4 points) What is the value of E for this electrochemical cell?

(e) (4 points) What is the value of the equilibrium constant, K, at 298 K, for this redox reaction?

 


13. (10 points) A certain reaction has an equilibrium constant, K, equal to 3.68 103 at 167C. What is the value of DG, in units of kJ/mol, for this reaction?

T = 167C + 273 = 440 K

 

14. (10 points) A certain reaction has DH = 41.4 kJ and DS = 50.2 J/K at a temperature of 623C. What is the value of DG, in units of kJ, at this temperature? Is the reaction spontaneous at this temperature?

T = 623C + 273 = 896 K

Since DG is positive, this reaction is not spontaneous.


15. (15 points) Balance the following redox reactions:

 

(a) NO3(aq) + Cu(s) NO2(g) + Cu2+(aq) (acidic solution)

The skeletal half-reactions are:

NO3 NO2

Cu Cu2+

Balancing the oxygens:

NO3 NO2 + H2O

Cu Cu2+

Balancing the hydrogens:

NO3 + 2 H+ NO2 + H2O

Cu Cu2+

Balancing the charge:

NO3 + 2 H+ + e NO2 + H2O

Cu Cu2+ + 2 e

Multiply the 1st half-reaction by 2 and then add:

2 NO3 + 4 H+ + 2 e 2 NO2 + 2 H2O

Cu Cu2+ + 2 e

2 NO3 + Cu + 4 H+ 2 NO2 + Cu2+ + 2 H2O

(b) MnO4(aq) + C2O42-(aq) MnO2(s) + CO2(aq) (basic solution)

The skeletal half-reactions are:

MnO4 MnO2

C2O42- CO2

Balance the carbon atoms in the 2nd half-reaction:

MnO4 MnO2

C2O42- 2 CO2

Balance the oxygen atoms:

MnO4 MnO2 + 2 H2O

C2O42- 2 CO2

Balance the hydrogen atoms:

MnO4 + 4 H+ MnO2 + 2 H2O

C2O42- 2 CO2

Balance the charge:

MnO4 + 4 H+ + 3 e MnO2 + 2 H2O

C2O42- 2 CO2 + 2 e

Multiply the first half-reaction by 2 and the second by 3 and then add:

2 MnO4 + 8 H+ + 6 e 2 MnO2 + 4 H2O

3 C2O42- 6 CO2 + 6 e

2 MnO4 + 3 C2O42- + 8 H+ 2 MnO2 + 6 CO2 + 4 H2O

Convert the basic by adding 8 OH to both sides:

2 MnO4 + 3 C2O42- + 8 H2O 2 MnO2 + 6 CO2 + 4 H2O + 8 OH

Cancel out 4 H2O molecules on both sides:

2 MnO4 + 3 C2O42- + 4 H2O 2 MnO2 + 6 CO2 + 8 OH

 

(c) I(aq) + HSO4(aq) I2(aq) + SO2(g) (acidic solution)

The skeletal half-reactions are:

I I2

HSO4 SO2

Balance the I atoms in the 1st half-reaction:

2 I I2

HSO4 SO2

Balance oxygen atoms:

2 I I2

HSO4 SO2 + 2 H2O

Balance hydrogen atoms:

2 I I2

HSO4 + 3 H+ SO2 + 2 H2O

Balance charge:

2 I I2 + 2 e

HSO4 + 3 H+ + 2 e SO2 + 2 H2O

Add the two half-reactions:

2 I + HSO4 + 3 H+ I2 + SO2 + 2 H2O

 

 

 


For 10 points extra credit:

Define 2 of the following 3 terms. Please remember to choose only 2 of the choices, if you answer all 3 with no indication of which 2 you want graded, only the first two definitions will be graded. Each complete definition is worth 5 points. No partial credit will be given.

Second Law of Thermodynamics: any spontaneous change is accompanied by an increase in the entropy of the universe.

Anode: the electrode at which oxidation takes place

Reversible process: a process in equilibrium.


Table of Standard Reduction Potentials at 298.15 K

Half-reaction

E (V)

Half-reaction

E(V)

Li+(aq) + e Li(s)

3.040

Sn4+(aq) + 2e Sn2+(aq)

0.151

Mg2+(aq) + 2e Mg(s)

2.372

Cu2+(aq) + e Cu+(aq)

0.153

Al3+(aq) + 3e Al(s)

1.662

Bi3+(aq) + 3e Bi(s)

0.308

Mn2+(aq) + 2e Mn(s)

1.185

O2(g) + 2 H2O(l) + 4e 4 OH(aq)

0.400

2 H2O(l) + 2e H2(g) + 2 OH(aq)

0.828

I2(s) + 2e 2 I(aq)

0.536

Fe2+(aq) + 2e Fe(s)

0.447

Fe3+(aq) + e Fe2+(aq)

0.771

Tl+(aq) + e Tl(s)

0.336

O2(g) + 4 H+(aq) + 4e 2 H2O(l)

1.229

Sn2+(aq) + 2e Sn(s)

0.138

Cl2(g) + 2e 2 Cl(aq)

1.358

2 H+(aq) + 2e H2(g)

0.000

F2(g) + 2e 2 F

2.866