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Experiment 9: Gibbs Free Energy Chemistry 203 – General Chemistry II Dept

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Experiment 9: Gibbs Free Energy Chemistry 203 – General Chemistry II Dept. of Physical Sciences & Engineering Wilbur Wright College Experiment 9: Gibbs Free Energy (??) Galvanic (or voltaic) cells convert chemical energy to electrical energy. The amount of energy (??) available to do electrical work (???? ), depends on both the cell potential (????? ) and the total number of electrons (?) that are transferred from the reductant to the oxidant during the course of the redox reaction. In the relationships that follow, ? is Faraday’s constant, which has a value of 96,485 J/(V-mol e-). ???? = −??????? (1) ?? = −??????? (2) If all the reactants and products are in their standard states (1 atm and 1 M at 298 K), this becomes ° ?? ° = −??????? (3) Recall that the free-energy change for a reaction under nonstandard conditions, ??, is given as follows: ?? = ?? ° + ?? ln ? (4) Equation 5 is derived from equations 2-4. The Nernst Equation (eq. 5) describes the effects of concentrations and partial pressures on the maximum voltage produced by a redox reaction by relating them to the standard cell ° potential (????? ). ? ln ? ° ????? = ????? − (5) ?? At 298 K, the Nernst Equation simplifies to ° ????? = ????? − 0.0592 ln ? ? (6) Changes in reaction conditions (i.e. concentrations and/or partial pressures) change the magnitude of the reaction quotient, ?, which in turn impacts the magnitude of the cell potential (????? ). Since ????? is a factor in determining the maximum electrical work for a galvanic cell, the selection of the anode and cathode half-reactions as well as the reaction conditions are important considerations in cell construction. The impact of changing an ion’s concentration or a gas’s partial pressure can be predicted by LeChatlier’s Principle and verified by ????? measurements. At equilibrium, ?? = ????? = 0 and ° ????? = 0.0592 ln ? ? (7) Using equation 7, a redox reaction’s equilibrium constant, ?, can be calculated from the standard cell potential ° (????? ). IMPORTANT – READ BEFORE STARTING THE EXPERIMENT 1. Each student will submit individual lab report. 2. The answers to all parts of the lab report can be typed or inserted into this Word document. 3. When completed, save as PDF file, and submit to Brightspace > Assignments > Lab 9, by indicated due date. 4. File name should include student’s name and report submitted. Example “R. Todorovic-Lab 9”. 1 Experiment 9: Gibbs Free Energy Chemistry 203 – General Chemistry II Dept. of Physical Sciences & Engineering Wilbur Wright College PRE-LAB QUESTIONS 1) Using the line notation and the half-reaction information provided, answer the following questions. Zn (s) | Zn2+ (aq) | Cu2+ (aq) | Cu (s) Zn2+ (aq) + 2e- → Zn (s) ° ???? = - 0.763 V Cu2+ (aq) + 2e- → Cu (s) ° ???? = 0.339 V a) Write the oxidation half-reaction. b) Write the reduction half-reaction. c) Which metal is the anode? d) Which metal is the cathode? e) Write the balanced redox reaction. ° f) Calculate ????? . (show work) g) Determine the value of ?. h) Write the expression for the reaction quotient, ?. i) Calculate the equilibrium constant, ?. (show work) 2) Consider the Zn/Cu galvanic cell reaction (above) with each set of reaction conditions at 298 K. Without any ° calculations, predict if ????? l will be greater than, less than, or equal to ????? . Zn2+ (M) Cu2+ (M) 0.10 1.0 1.0 0.1 0 1.0 1.0 2 Ecell Experiment 9: Gibbs Free Energy Chemistry 203 – General Chemistry II Dept. of Physical Sciences & Engineering Wilbur Wright College 3) Calculate ????? for the Zn/Cu galvanic cell (given in question 1) with each set of reaction conditions at 298 K. Zn2+ (M) Cu2+ (M) 0.10 0.10 0.10 0.50 0.10 1.0 0.10 1.5 0.10 2.0 0.50 0.10 1.0 0.10 1.5 0.10 2.0 0.10 1.0 1.0 2.0 2.0 Ecell (V) * * * Show the ????? calculation for the data set [ Zn2+]=0.10 M, [Cu2+]=2.0 M. 4) Compare your ????? predictions (question 2) with your ????? calculations for the reaction conditions marked with a (*) above. Were your predictions correct or incorrect? Explain. LAB QUESTIONS 5) Use the link for virtual lab from Experiment 8 (http://web.mst.edu/~gbert/Electro/Electrochem.html) and measure the following cell potentials for Zn/Cu galvanic cell. Zn2+ (M) Cu2+ (M) 0.10 0.10 0.10 0.50 0.10 1.0 0.10 1.5 0.10 2.0 0.50 0.10 1.0 0.10 1.5 0.10 2.0 0.10 1.0 1.0 2.0 2.0 3 Ecell (V) Experiment 9: Gibbs Free Energy Chemistry 203 – General Chemistry II Dept. of Physical Sciences & Engineering Wilbur Wright College 6) Compare your calculated and measured ????? values (from questions 4 and 5, respectively). Are they the same or different? 7) Evaluate your ????? values (above) and compare the reaction conditions in the galvanic cells to identify trends (i.e. Does ????? increase or decrease? Does the reaction become more or less spontaneous?) a) Summarize the outcome when [Cu2+] increases. b) Summarize the outcome when [Zn2+] increases. c) Summarize the outcome when [Cu2+] and [Zn2+] simultaneously increase. 8) According to your data, which galvanic cell’s reaction conditions can convert the most chemical energy into electrical energy? Calculate ?? (??) for this cell. (Show work) 9) Design a nonstandard galvanic cell that coverts more chemical energy into electrical energy than the Zn/Cu combination. a) Write the complete line notation with concentrations in place of (aq) and partial pressures in place of (g). Phase symbols of (s) and (l) should be shown. b) Report the following values for your nonstandard galvanic cell. ° (?) = _______________________________________ (measured value) ????? ????? (?) = _______________________________________ (measured value) ?? (??) = _______________________________________ (calculated value) Show calculation of ?? (??) here. Student Name: Date: 4