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Temperature Dependence of EMF

Explore the thermodynamics and electrochemistry by determining ΔG, ΔH, and ΔS of a specific oxidation-reduction reaction cell. Learn the physical principles and experiment procedures required for accurate measurements. Conduct calculations to analyze the data obtained from measurements and plot the cell EMF against temperature for further analysis.

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Temperature Dependence of EMF

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  1. Temperature Dependence of EMF Where thermodyamics and electrochemistry meet.

  2. Objective: Determine the ΔG, ΔH and ΔS of the oxidation- reduction reaction for the cell: Ag(s) | AgCl(s) || Hg2Cl2(s) | Hg(l) This cell has the half-reactions: Anode (oxidation): 2Ag(s) = 2AgCl(s) + 2e- Cathode (reduction): 2e- + Hg2Cl2(s) = 2Hg(l) Net: 2Ag(s) + Hg2Cl2(s) = 2AgCl(s) + 2Hg(s) All at atmospheric pressure and some temperature T.

  3. Physical Principles For a reversibly operating electrochemical cell operating at constant temperature and pressure and doing no other work: ΔG = - NF E Where N is the number of Faradays passing through the cell, F is the Faraday constant and E is the emf of the cell. From the Gibbs-Helmholz Equation:

  4. Substituting for ΔG gives: With both ΔG and ΔS determined, ΔH can then be found: ΔH = ΔG + TΔS

  5. Experiment • EMF is measured by a high-impedance, solid-state voltmeter • The AgCl is deposited onto a silver strip • Clean Ag strip with sandpaper and rinsed with deionized water • Strip is place in beaker of 1M HCl and AgCl is electrolytically deposited ( use 1.5V dry cell) • Stop when shiny strip is darkened • Let sit for 5 min, then rinse and place in H-cell as quickly as possible (avoid drying of the coating) • Hg2Cl2 is “floated” on a pool of Hg

  6. Cell must be clean. An obviously dirty cell will not give acceptable results. • KCl solution in cell must be fresh • Water to make solution must be boiled to remove dissolved oxygen • Be sure connectors such as alligator clips are not corroded. If corrosion is present then “polish” using sandpaper • Be sure wires are firmly connect to clips

  7. Place cell in bath and wait usually 10-15 minutes to reach equilibrium. • Measure emf (voltage) until there is no drift in value and it is essentially constant • Measure emf of cell for at least four different temperatures in the range of 0o-35o Celsius. • Start at low temperature, go to high, then go from high to low. • Use the same voltmeter to make the measurements

  8. Calculations • Plot the cell emf against the absolute temperature. • Using a linear-least squares plot determine the slope (E / T )p and the best value of E . • Use E to calculate ΔG • Use the slope, (E / T )p , to calculate ΔS • Use the computed values of ΔG and ΔS to calculate ΔH

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