Enthalpy and Entropy Exp S11

Enthalpy and Entropy Exp S11 - Chemistry II Laboratory...

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Chemistry II Laboratory Experiment #1 Enthalpy and Entropy of Zinc with Copper Sulfate {Adapted from CCLI Initiative – Computers Used in Chemistry Laboratory Instruction “Enthalpy & Entropy of Zinc with Copper Sulfate”, Amend, J., Montana State University, Bozeman, MT, 2009.} Objectives : To determine changes in enthalpy and entropy of the reaction of zinc with copper sulfate by two methods: electrochemistry and calorimetry. To compare enthalpy values found by both methods. Introduction: Thermodynamics is the study of energy changes that occur in chemical and physical processes. The enthalpy and entropy changes of a system undergoing such processes are interrelated by the change in free energy, G, according to the equation, G = H - T S ( 1 ) The investigation focuses on the reaction Zn (s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu (s) (2) G will be calculated from the H and S values obtained electrochemically. The validity of the Equation (1) will be tested by comparing the value of H obtained electrochemically with the value of H obtained calorimetrically for the same reaction. The Electrochemical Method: The electrochemical method offers simple and accurate means for the determination of thermodynamic quantities. A simple electrochemical cell can be assembled with use of small beakers and a convenient and easily constructed salt bridge. The galvanic cell is described Cu(s)/CuSO 4 (aq) || Zn(s)/ZnSO 4 (aq) (3) The overall galvanic cell reaction is Zn (s) + Cu +2 (aq) Zn +2 (aq) + Cu (s) (4) and it is essentially the same as that taking place in the calorimeter. The quantity of electrical energy, F, produced or consumed during the electrochemical reaction is a constant measured per mole of electrons, and can be accurately measured. The free energy change, G, of an electrochemical reaction is related to the voltage, E , of the electrochemical cell by the equation
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G = -n F E (5) where n = number of moles of electrons transferred in a redox reaction. and F = Faraday’s constant of 96,500 C/mole of electrons. Combining equations (1) and (5), and dividing both sides by the constant “n”, we obtain a linear relationship between the voltage change, DE, and the enthalpy and entropy changes at different temperatures. E = - H _ + T S (6) n F n F or E = S (T) - H (7) n F n F By measuring voltage E, of our electrochemical cell, at several temperatures, we can obtain a plot of the voltage versus temperature. Assuming that H and S remain constant over a small temperature range, we can calculate the S and H from the slope and the intercept of the straight line respectively. slope = S _ (8) n F and Y- intercept = - H_ (9) n F G can now be calculated by means of Equation (1). We can verify its value by using Equation (5). Please note that in both cases, G must be calculated for the same temperature. If the calculations are done for 298 K (25 ° C), we can also verify the experimental value of
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This note was uploaded on 02/23/2011 for the course CHEM 1200 taught by Professor Mcintosh during the Spring '08 term at Rensselaer Polytechnic Institute.

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Enthalpy and Entropy Exp S11 - Chemistry II Laboratory...

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