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Electrochemical Cells

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Electrochemical Cells

Objective: The purpose of this lab is to construct various electrochemical cells and to measure to voltage generated.

Observations:

Part 1 | Voltage | Anode | Cathode | Zn vs. Ag | 1.370 | Zn | Ag | Zn vs. Cu | .983 | Zn | Cu | Zn vs. Fe | .525 | Zn | Fe | Zn vs. Mg | .617 | Mg | Zn | Zn vs. Pb | .470 | Zn | Pb |

Anode | Cathode | Equation | Predicted V (E0) | Measured V (E0 Zn) | E0Zn - E0 | Fe | Ag | Fe + 2Ag+ ↔ Fe2+ + 2Ag | 2.04 | 1.7 | 0.34 | Fe | Pb | Fe + Pb2+ ↔ Fe2+ + Pb | .31 | .41 | 0.10 | Mg | Ag | Mg + 2Ag+ ↔ Mg2+ + 2Ag | 3.97 | 3.35 | 0.62 | Mg | Zn | Mg + Zn2+ ↔ Mg2+ + Zn | 1.61 | 1.68 | 0.07 | Mg | Pb | Mg + Pb2+ ↔ Mg2+ + Pb | 2.24 | 2.4 | 0.16 | Mg | Cu | Mg + Cu2+ ↔ Mg2+ + Cu | 2.71 | 2.95 | 0.24 |

Part 2 | Voltage | Anode | Cathode | Zn (s)|Zn2+ (1.0 M) || Cu2+ (0.0010 M)|Cu(s) | 0.909 | Zn | Cu |

Equation for Cell Reaction | Predicted V | Measured V | Zn(s) + Cu2+(aq) ↔ Zn2+(aq) + Cu(s) | 1.19 | 0.909 |

E = 1.1 V – (0.0592 V) / (2 mol) log (1.0 * 0.001)
E = 1.1 + .0888
E = 1.19 V

Part 3 | Voltage | Anode | Cathode | Zn (s)|Zn2+ (1.0 M) || Ag+ (unknown M)|Ag(s) | 1.743 | Zn | Ag |

Equation for Cell Reaction | Calc [Ag+] | Calc Ksp AgCl | Ksp AgCl | Zn(s) + 2Ag+(aq) ↔ 2Ag(s) + Zn2+(aq) | 8.11 x 10-4 | 6.57 x 10-7 | 1.6 X 10-10 |

E = E0 – (0.0592)/n log (Q)
1.743 V = 1.56 V – (0.0592 V) / (2 mol) log ([Zn][Ag+]2)
(0.0592 V) / (2 mol) log ([Zn ][Ag+]2) = 1.56 V - 1.743 V log ([Zn ][Ag+]2) = -6.18
[Zn ][Ag+]2 = 6.57 x 10-7 = Ksp
[Ag+] = 8.11 x 10-4 M

Post-Lab/Conclusion: 1. An electrode potential is the voltage generated when a half-cell is connected to a standard cell, usually the hydrogen electrode, which is assigned a value of zero. 2. Rankings agreed with published values. 3. In this experiment zinc was assigned a value of zero and voltages measured should differ from the published hydrogen electrode values by the value of zinc half-life potential in the standard voltage table, 0.76 V. Most differed by 0.6 V, except for Mg which differed by 1.76 v. 4. The system used in this experiment probably has a larger resistance and thus a lower voltage than standard systems. 5. A negative value means that the reduction reaction occurs less readily than that of the standard zinc ions. 6. Charges in concentration: The voltage for zinc vs. copper cell was 9.8 v when solutions were 1 M.
Zn(s) + Cu 2+(aq) → Zn2+(aq) + Cu(s)
A decrease in Cu2+ ion (to 0.0010 M) would shift the reaction left, according to Le Chatelier’s Principle. This would have the effect of lowering the voltage. In the cell with the 0.0010 M, the measured voltage was 0.89 as predicted. 7. Solubility Product Determination. A 1.0 M solution of chloride ion was obtained and a trace of silver was added. Almost the entire silver ion precipitated. A half-cell was constructed with silver ions and silver metal acting as on half-cell, and the zinc half-cell as the other. The voltage was measured and the Nerst Equation was used to calculate the silver ion concentration. The chloride ion concentration was assumed to be unchanged at 1.0 M, as very little precipitated. These values were substituted into the solubility product expression to calculate Ksp.

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