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Temperature Dependence of EMF

Temperature Dependence of EMF. Where thermodyamics and electrochemistry meet. Objective: Determine the ΔG, ΔH and ΔS of the oxidation- reduction reaction for the cell: Ag(s) | AgCl(s) || Hg 2 Cl 2 (s) | Hg(l) This cell has the half-reactions:.

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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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