Name ______

CHEM V01B THERMODYNAMICS/ELECTROCHEMISTRY

Spring 2016 version 1

Answer questions 1-25 on the scantron and question 26 and 27 on this sheet . (Assume R = 8.314 J mol-1K-1, F = 96,485 C/mol)

MULTIPLE CHOICE QUESTIONS (50% of Total Grade)

1. Which of the following statements could be used to define the first law of thermodynamics?

  1. Heat flows from a hot body to a cold body
  2. Energy cannot be created or destroyed but can only change from one form to another
  3. The absolute entropy of a crystal at 0K is zero
  4. In a spontaneous process Suniv < 0

2. Which of the following statements is consistent with the second law of thermodynamics?

  1. Heat flows from a hot body to a cold body
  2. Energy cannot be created or destroyed but can only change from one form to another
  3. The absolute entropy of a crystal at 0K is zero
  4. In a spontaneous process Suniv < 0

3. Which of the following statements is a definition of the third law of thermodynamics?

  1. Heat flows from a hot body to a cold body
  2. Energy cannot be created or destroyed but can only change from one form to another
  3. The absolute entropy of a crystal at 0K is zero
  4. In a spontaneous process Suniv < 0

4. Which of the following statements is FALSE?

(a) Oxidation and reduction half-reactions occur at electrodes in electrochemical cells.

(b) All electrochemical reactions involve the transfer of electrons.

(c) Reduction occurs at the cathode.

(d) Oxidation occurs at the anode.

(e) All voltaic (galvanic) cells involve the use of electricity to initiate nonspontaneous chemical reactions.

5. Are Endothermic reactions spontaneous?

  1. Always
  2. Never
  3. Sometimes

6. Are reactions in which ΔSuniv > 0 spontaneous?

  1. Always
  2. Never
  3. Sometimes

7. Are reactions in which ΔSuniv < 0 spontaneous?

  1. Always
  2. Never
  3. Sometimes

8. Are reactions in which ΔGsys > 0 spontaneous?

  1. Always
  2. Never
  3. Sometimes

9. Are reactions in which ΔGsys < 0 spontaneous?

  1. Always
  2. Never
  3. Sometimes

10. Are reactions in which ΔSsys > 0 spontaneous?

  1. Always
  2. Never
  3. Sometimes

11. Which of the following is the strongest oxidizing agent?

  1. Pb2+
  2. I2
  3. Ag+
  4. Pb
  5. Cu2+

12. The spontaneous melting of ice at room temperature is

  1. An enthalpy driven process
  2. An entropy driven process
  3. Neither a or b

13. The dissolution of NaCl in water NaCl(s)  Na+(aq) + Cl-(aq) is

  1. An enthalpy driven process
  2. An entropy driven process
  3. Neither a or b

14. For the process C6H6(l) C6H6(g)

  1. S < 0
  2. S > 0
  3. S = 0
  4. Depends on the temperature
  5. Impossible to say

15. For the process 4Al(s)+3O2(g)2Al2O3(s)

  1. S < 0
  2. S > 0
  3. S = 0
  4. Depends on the temperature
  5. Impossible to say

16. As the cell given below operates, the strip of silver gains mass (only silver) and the concentration of silver ions in the solution around the silver strip decreases, while the strip of lead loses mass and the concentration of lead increases in the solution around the lead strip. Which of the following represents the reaction that occurs at the negative electrode in the above cell?

Pb |Pb(NO3)2 (1.0 M) || AgNO3 (1.0 M) | Ag

(a) Pb2+ + 2 e-Pb

(b) Pb Pb2+ + 2 e-

(c) Ag+ + e- Ag

(d) Ag Ag+ + e-

(e) none of the above

17. What is ΔS for the reaction below at 25oC

2SO2(g) + O2(g)  2SO3(g)

given

S0 (j/K/mol)
SO2(g) / 248.1
O2(g) / 205.3
SO3(g) / 256.6
  1. -188.3 JK-1mol-1
  2. +188.3 JK-1mol-1
  3. -196.8 JK-1mol-1
  4. +196.8 JK-1mol-1
  5. none of the above

18. Calculate the potential (in volts) for the voltaic (or galvanic) cell indicated at 25oC.

Ga |Ga3+ (10-6 M) || Ag+ (10-4 M) | Ag

(a) 1.29 V

(b) 0.97 V

(c) 1.45 V

(d) 1.21 V

(e) 1.37 V

19. Where does oxidation occur in an electrochemical cell?

(a) at the cathode in both an electrolytic cell and a voltaic cell

(b) at the cathode in an electrolytic cell and at the anode in a voltaic cell

(c) at the anode in both an electrolytic cell and a voltaic cell

(d) at the anode in an electrolytic cell and at the cathode in a voltaic cell

20. Which allotrope of C will have the highest entropy?

a. Diamond

b. Graphite

c. neither since Diamond and Graphite will have the same entropy

21. Consider 1 mol of H2O which will have the greatest entropy

a. Ice

b. liquid water

c. water vapor

d. They are all the same

22. Given the balanced ionic equation representing the reaction in an operating voltaic cell:

Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)

The flow of electrons through the external circuit in this cell is from the

(a) Cu anode to the Zn cathode

(b) Cu cathode to the Zn anode

(c) Zn anode to the Cu cathode

(d) Zn cathode to the Cu anode

23. Which will have the larger entropy?

a. 1 mole of Argon gas (molar mass 39.948 g/mol)

b. 1 mole of NO gas (molar mass 30.006 g/mol)

c. impossible to say

(The following information is needed for questions 24 and 25)

Borax (Na2[B4O5(OH)4].8H2O) is soluble in water

Na2[B4O5(OH)4].8H2O(s) ↔ 2 Na+ (aq) + [B4O5(OH)4]2- (aq) + 8 H2O (l)

The solubility product Ksp = [Na+]2[B4O5(OH)42-] is measured at temperatures ranging from 15 – 55 oC. A graph is made of ln(Ksp) vs. 1/T (see below) it is found to be linear, the equation for this straight line gives

ln(Ksp) = -12406(1/T) + 37.88

Where T is in Kelvin

24. What is the enthalpy Ho for this process?

  1. -12.406 kJ/mol
  2. +12.406 kJ/mol
  3. 103.14 kJ/mol
  4. -103.14 kJ/mol
  5. 37.88 J/mol

25. What is the entropy changeSo for this process

  1. 37.88 J/(mol.K)
  2. -37.88 J/(mol.K)
  3. -12.406 J/(mol.K)
  4. 314.93 J/(mol.K)
  5. -314.93 J/(mol.K)

LONG QUESTIONS

Thermodynamics Question (25% of total Grade)

26. For the reaction

CO(g) + 2 H2(g) ➝ CH3OH(g)

There is the following data

Compound / ΔH°f (kJ/mol) / S°f (J/mol-K)
CO(g) / -110.53 / 197.6
H2(g) / 0 / 130.6
CH3OH(g) / -238.70 / 126.8
  1. What is the value of ΔS° per mole of CH3OH produced in this reaction? (4 marks)

ΔS°
  1. What is ΔH° per mole of CH3OH produced in this reaction at 25°C? (4 marks)

ΔH° =
  1. What is ΔG° per mole of CH3OH produced in this reaction at 25°C? (4 marks)

ΔG° =
  1. Calculate the equilibrium constant for this reaction at 25°C (4 marks)

K =
  1. Does the reaction favor reactants or products at 25oC? (`1 mark)
  1. Calculate the equilibrium constant for this reaction at 325°C (4 marks)

K =
  1. Does the equilibrium at 325oC favor reactants or products?

Electrolysis (25% of total Grade)

  1. In an electrolysis experiment a solution of aqueousCopper (II) Chloride has two carbon graphite electrodes attached to a 6V battery inserted into it. You observe a pinkish deposit at electrode 1, while at electrode 2 the bubbles are seen
  1. Write out all of the possible reactants in this cell (hint some reactants are ions) (3 marks)

First reactant
Second reactant
Third reactant
  1. Write out the two possible oxidation reactions along with their oxidation potentials (4 Marks)

Oxidation ½ reaction / Eoox (V)
  1. Write out the two possible reduction reactions for the cell along with their reduction potentials (4 Marks)

Reduction ½ reaction / Eored (V)
  1. Write out the predicted reaction at the anode (2 Marks)

Predicted Anode Reaction
  1. Write out the predicted reaction at the cathode (2 Marks)

Predicted Cathode Reaction
  1. Is electrode 1 the anode or the cathode? Explain (2 Marks)

Electrode 1 is the the ______because
  1. Is electrode 2 the cathode or the anode? Explain (2 Marks)

Electrode 2 is the ______because
  1. What are the bubbles due to? (2 Marks)

The bubbles at Electrode 2 are due to
  1. What is the pinkish deposit? (2 Marks)

The pink deposit at Electrode 1 is due to
  1. What is the minimum cell voltageEbattery needed for electrolysis to occur? (4 Marks)

Ebattery =

Standard Reduction Potentials

Cathode (Reduction)
Half-Reaction / Standard Potential
E° (Volts)
2H2O(l) + 2e- -> H2(g) + 2OH-(aq) (pH = 7) / -0.414
Zn2+(aq) + 2e- -> Zn(s) / -0.76
2H+(aq) + 2e- -> H2(g) / 0.00
Sn4+(aq) + 2e- -> Sn2+(aq) / 0.15
Cu2+(aq) + 2e- -> Cu(s) / 0.34
I2(s) + 2e- -> 2I-(aq) / 0.54
Cl2(g) + 2e- 2Cl-(aq) / 1.36
Br2(l) + 2e- -> 2Br-(aq) / 1.07
O2(g) + 4H+(aq) + 4e- -> 2H2O(l) (pH = 7) / 0.816
Fe3+(aq) + e-Fe2+(aq) / 0.77
Au3+(aq) + 3e- Au(s) / 1.50
Co2+(aq) + 2e- Co(s) / -0.28
Ag+(aq) + e- Ag(s) / 0.80
Ga3+(aq) + 3e- Ga(s) / -0.53
Pb2+(aq) + 2e-Pb(s) / -0.13

Extra Credit Problem (10%)

Apply REDOX to organic chemistry

You are asked to oxidize benzaldehyde to benzoic acid using Chromic Acid (H2CrO4)

If the Chromic acid is reduced to Cr3+(aq), write out the balanced oxidation and reduction equations for the reaction of chromic acid with benzaldehyde (5%)

What is the balanced redox reaction? (3%)

Benzaldehyde (molar mass 106.121 g/mol), has a density of 1.04 g/mL and you are asked to oxidize it with 6M Chromic acid. You are given 1mL of benzaldehyde. How many moles of benzaldehyde are there in 1 mL? (2%)

Using your balanced redox reaction, how many mL of the 6M chromic acid are needed to oxidize the benzaldehyde? (2%)