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Summary

The main principles and theories of this experiment are the effects of a voltaic cell undergoing a simultaneous oxidation and reduction reaction also known as a redox reaction. In a typical voltaic cell, the redox pair is usually copper and zinc shown in this half cell reaction...

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Experiment #10: Electrochemistry Professor Afshar Chem 1110-08 5 April 2020

Purpose:

The purpose of this experiment is to determine the Faraday constant through the electrolysis of water and measuring any potential differences of the several galvanic cells.

Galvanic Cells Chemical energy ⇔ Electrical Energy Electrolysis Theory and Principles: The main principles and theories of this experiment are the effects of a voltaic cell undergoing a simultaneous oxidation and reduction reaction also known as a redox reaction. In a typical voltaic cell, the redox pair is usually copper and zinc shown in this half cell reaction. Zinc electrode (anode): Zn(s) → Zn2+(aq) + 2 e– Copper electrode (cathode): Cu2+(aq) + 2 e– → Cu(s) The theory behind the half cell reactions of these two elements is the zinc electrode will produce two electrons as it oxidizes which travels through the wire to the copper cathode. (Zn → Zn2+ + 2e). The voltaic cells will be used as a source of electrical power and will produce the direct current. (Cu2+ + 2e- → Cu.)The copper electrode will become larger due to the deposits of Cu being produced. Consequently, the zinc electrode will be used and the metal will shrink. The process of this change is called electrolysis.

Procedures: We followed the procedures from Harold Goldwhite, Wayne Tikkanen. Solubility Product of potassium Hydrogen Tartrate. Electrochemistry, Edition 5; McGraw-Hill Primis Custom

Publishing; 2001.

Data To Be Recorded: Current: 0.108 Amperes Time: 46 Minutes (2,760 Seconds) Volume of H2 (Total) : 41.40 mL Volume of O2 (Total) : 19.40 mL Temperature of Electrolyte = 30.0 ℃ = 303 K Barometric pressure = 760.5 mmHg Vapor pressure = 31.8 mmHg Correct pressure = Barometric Pressure - Vapor Pressure Corrected pressure = 760.5 mmHg - 31.8 mmHg = 728.7 mmHg

Table 1. Data for part B. Solutions are saturated in AgCl. Ecell (V)

Experiment

Solution A

Electrode A

Solution B

Electrode B

-1.106

1

Cu2+

Cu

Zn2+

Zn

0.375

2

Pb2+

Pb

Cu2+

Cu

-0.309

3

Fe2+

Fe

Zn2+

Zn

0.606

4

Sn2+

Sn

Cu2+

Cu

-0.041

5

Cu2+

Cu

0.1 M KCl*

Ag wire

0.451

6

Pb2+

Pb

0.1 M KCl*

Ag wire

0.912

7

Zn2+

Zn

0.1 M KCl*

Ag wire

0.410

8

Sn2+

Sn

0.1 M KCl*

Ag wire

measured

0.682

Fe2+

9

Fe

0.1 M KCl*

Concentrations for CuSO4, ZnSO4 and Pb(NO3)2 solutions are 0.10 M. Concentrations for FeSO4 and SnCl2 solutions are 0.010 M. Calculations for Part A: Anode (+); oxidation : 2H2O → O2 + 4H+ + 4 eCathode (-); reduction : 4H2O + 4 e- → 2H2 + 4OHNet reaction; 6H2O → 2H2 + O2 + 4H+ + 4OHOverall reaction: 2H2O → 2H2 + O2 Moles of H2 = Amps x Time (s )=Coulombs ¿ 0.108 Amps x 2,760 seconds = 298.1 C 298.1 C x

1F 96,485C

3.090 x 10-3 mole e- *

= 3.090 x 10-3 F 1 mole e−¿ 2 mole H 2 = 6.18 x 10-3 mole H2 ¿

Moles of H2 = 6.18 x 10-3 mole H2 Moles of O2 = 7.73 x 10-4 Q= (i)(t) Q = number of coulombs of electricity, i = time, t =current. Q = (2,760)*(0.108) Q = 298.1 Coulombs. 298.1 C x

1F = 3.090 x 10-3 F 96,485C

Calculations for Part B: Redox potential Rt - Redox Potential Lt = Ecell -0.03 V = EAg - ECu

ECu = EAg + 0.03 V

Ag wire

1.07 V = EAg - EZn EZn = EAg - 1.07 V Ecell for Cu|Cu2+||Zn+|Zn Ecell = EZn - ECu = ? Ecell = (EAg - 1.07 V) - (EAg + .03V) Ecell = -1.1 V Table 2: Galvanic cells with Ag wire and their half reactions. Galvanic cell #

Half Reaction

5

Cu2+ + 2e- → Cu

6

Pb2+ +2e- → Pb

7

Zn2+ + 2e- → Zn

8

Sn2+ + 2e- → Sn

9

Fe2+ + 2e- → Fe

Coulombs passed through cell (C) = Current (in amperes) * times (in seconds) = Current (in amperes) * 0.108 * 2,760 = 298.08 Electrons passed through Cell (e-) 298.08 21 −19 = 1.863 x 10 1.6∗1 0 Avogadro’s number (e- divided by Mole e-) 1.863 x 10 21 = 1.21 x 1024 0.001545mole Cu Faraday Constant (C/mol e-) = Coulombs passed through cell divided by Moles of electrons transferred. 298.08 C = 192,932 Coulombs 0.001545 Mole Cu

Percent error: 100% (

96,465−192,932 192,932

) = -50%

Results and Discussion: 1. A. Sodium sulfate was used because pure water doesn’t contain high enough concentrations of ions to produce a current. Adding the ions from an electrolyte lowers the resistance and increases the conductivity. B. Yes, sodium nitrate solution would work with the same purpose as we did using the sodium sulfate to produce an effective current. C. Copper sulfate solution in this electrolysis is not good because the copper deposit in this case is at the cathode and hydrogen gas is not obtained at the cathode. 2. Cd2+ + 2e- → Cd, E° = -0.403 V Cu2+ + 2e- → Cu, E° = +0.34 V Zn2+ + 2e- → Zn, E° = -0.76 V Fe3+ + 3e- → Fe, E° = -0.04 V a. Cd potential more than Zn2+ potential, so Cd cannot replace Zn2+. so it’s not possible. b. Cd can replace Cu2+ because Cu potential is more than Cd. c. Cu cannot replace Fe3+ because Cu potential is more. Only B takes place. 3. Literature value of Avogadro’s number = 6.02 x 1023 mole Percent error = 100% * (Average value of Avogadro’s number - Literature value of Avogadro’s number) divided by Literature value of Avogadro’s number. 100% (

1.21∗1 024−6.02∗10 23 ) = 101% 6.02∗10 23

Conclusion: In conclusion, we’ve successfully calculated the moles of H2 and O2 produced in the electrolysis reaction. We’ve determined the Faraday constant, F, and averaged the actual number of moles of electrons. We also determined the values of potentials and measured the potential of the metal/solutions vs. the silver/silver ion electrode. We were also able to write the half reactions of each galvanic cell with silver wirings. However, calculations were fairly different from our Experimental E° values obtained from a reference. If we were to conduct a similar experiment with Mg, Zn, and Cu, we would be able to repeat the process. References: Harold Goldwhite, Wayne Tikkanen. Electrochemistry. Experiments in General Chemistry, Edition 5; McGraw-Hill Primis Custom Publishing; 2001....