北京化工大学：《物理化学》课程教学资源（双语实践教学）electrochemical determination of GHS.pdf_大学文库

Experiment Electrochemical Determination of AG,AH,and AS Objectives .To determine thermodynamic properties using electrochemical techniques Introduction odynamics is with the energy changes that ccur in chemical and physica processes.Several fundamental laws summarize these energy changes The first law of thermodynamics states that energy is conserved.that is.it can not be created or destroyed.Energy can be converted from one form to another,but the total remains constant.In chemical reactions,rearrangements of atoms and electrons occur,leading to new which interact with their su roundings This changes the chemical potentil of the system and thus hea △H=Heu-H The second law of thermodynamies relates to the spontaneous nature of a chemical process. It has been established through observation that proces sses that are s可 ontaneous in one direc ion are no sponta direction.For insta nce.if you drop a b hook on the floor it does no spontaneously move back into your hand.This fact applies to chemical as well as physical systems Spontaneity is associated with an increase in randomness or disorder of a system. Thermodynamically,this is expressed by a quantity called entropy.S.the change in entropy of a system is expressed as △S=Saa-Sna The free energy,G,interrelates the enthalpy and entropy of a system.For a process occurring at constant temperature and pressure,the change in free energy of a system is expressed by △G=△H-T△S The sign of AG relates to the spontaneity of the reaction 1.If AG is negative,the reaction is spontaneous in the forward direction. 2.If AG is zero,the reaction is at equilibrium. 3.If AG is positive.the reaction is spontaneous in the reverse direction. The quantitiesG.AH and S depend ony on the initial and final states of a sys and ot on the pathway of the change.called state are said tob path independent The investigation of electrochemical reactions offers a simple and direct method for measuring thermodynamic values,because the quantity of electrical energy produced or consumed during an electrochemical reaction can be measured very accurately.The free energy change,AG △G=-nFE Where nis the number of moles of electrons transferred in the reaction per mole of reactant. and Fis Faraday's constant. PDF文件使用"pdfFactory Pro”试用版本创建wnw,fineprint,com,cn

Experiment Electrochemical Determination of ΔG, ΔH, and ΔS Objectives • To determine thermodynamic properties using electrochemical techniques. Introduction Thermodynamics is concerned with the energy changes that occur in chemical and physical processes. Several fundamental laws summarize these energy changes. The first law of thermodynamics states that energy is conserved, that is, it can not be created or destroyed. Energy can be converted from one form to another, but the total remains constant. In chemical reactions, rearrangements of atoms and electrons occur, leading to new species, which interact with their surroundings. This changes the chemical potential of the system, and thus heat is evolved or absorbed. This heat energy is usually expressed as enthalpy, DH H H final initial = - The second law of thermodynamics relates to the spontaneous nature of a chemical process. It has been established through observation that processes that are spontaneous in one direction are not spontaneous in reverse direction. For instance, if you drop a book on the floor, it does not spontaneously move back into your hand. This fact applies to chemical as well as physical systems. Spontaneity is associated with an increase in randomness or disorder of a system. Thermodynamically, this is expressed by a quantity called entropy, S. the change in entropy of a system is expressed as final initial DS = - S S The free energy, G, interrelates the enthalpy and entropy of a system. For a process occurring at constant temperature and pressure, the change in free energy of a system is expressed by DG = DH - DT S (1) The sign of DG relates to the spontaneity of the reaction 1. If DG is negative, the reaction is spontaneous in the forward direction. 2. If DG is zero, the reaction is at equilibrium. 3. If DG is positive, the reaction is spontaneous in the reverse direction. The quantitiesDG , DH and DS depend only on the initial and final states of a system and not on the pathway of the change. Such quantities are called state functions and are said to be path independent. The investigation of electrochemical reactions offers a simple and direct method for measuring thermodynamic values, because the quantity of electrical energy produced or consumed during an electrochemical reaction can be measured very accurately. The free energy change, DG , of an electrochemical reaction is related to the voltage, E , of the electrochemical cell: DG = -nFE (2) Where n is the number of moles of electrons transferred in the reaction per mole of reactant, and F is Faraday’s constant. PDF 文件使用 "pdfFactory Pro" 试用版本创建 www.fineprint.com.cn

If voltage measurements of the cell are made at different temperatures, the change in entropy, DS , of the system can be expressed at constant pressure as d G ( ) S dT D -D = (3) Substitution of Equation 2 into Equation 1 gives d( ) nFE S dT - -D = or dE S nF dT -D = (4) Faraday’s constant (F) is the electrical charge on 1 mol of electrons, 96,500 J V-1mol. E is expresses in volts and the temperature in K. DS can be expressed as J V-1mol-1 or cal K-1mol-1 (1 calorie = 4.184J). Note that the quantity dE / dT in Equation 4 is a slope－the change in the electrochemical voltage with respect to temperature. Thus, a plot of E (y axis) versus T (x axis) will have an instantaneous slope (tangent) of dE / dT at any temperature T. In this experiment, the electrochemical cell will consist of the Zn/Zn2+ and Cu/Cu2+ half-cells, connected by a salt bridge. The half-cell reactions are 2+ Zn Zn 2e- + (oxidation at the anode) 2+ Cu +2e Cu - (reduction at the cathode) __________________________________________________ 2+ 2+ Zn+Cu Zn +Cu (overall cell reaction) Tables of half-cell potentials can be found in Appendix. It should be noted that the potentials given in these tables are of o E , that is, the standard potentials. Real potentials, E, depend on temperature, concentration of irons in solution, etc., and are obtained from the Nernst equation: products reac tan ts ln o RT a E E nF a = - In most cases, concentration can be used in place of activity without incurring too large of an error, the various constants are collected, and the expression is evaluated at 25℃, resulting in a simplified version of the Nernst equation: 0.059 [products] ln [reac tan ts] o E E n = - When the concentrations of the Zn2+ and Cu2+ solutions are equal, the log part of the Nernst equation reduces to zero. Under these conditions, the standard voltage, o E , and will be 1.100V for the cell described. (Do you experimentally obtain this value for E?). The free energy, ΔG, may be obtained from the voltage by the equation DG = -nFE (5) In this experiment, the cell voltage will be measured at three or more temperatures. If several temperatures are used, a plot of E versus T yields the value of dE/dT from the instantaneous slope of the line, as previously discussed. Substitution of this slope into Equation 4 gives ΔS. The value PDF 文件使用 "pdfFactory Pro" 试用版本创建 www.fineprint.com.cn

Once the cell is constructed, allow the system to reach thermal equilibrium (15 minutes). Measure the voltage of the cell on the voltmeter and also the room temperature. Record these values on the data sheet. Figure 2 Electrochemical cell Cool the cell assembly in an ice bath. After the temperature has stabilized (about 15 minutes), measure and record the voltage and temperature. Now, place the cell in a previously warmed water or sand bath at approximately 50℃. Allow the system to come to thermal equilibrium (about 15 minutes). Record the voltage and temperature. If further readings are desired, the cell may be placed in a series of baths set at selected temperatures. It is important to remember to allow the system to come to thermal equilibrium before readings are obtained. Calculation of Thermodynamic Values Construct a plot of E(y axis) versus T(x axis). The instantaneous slope of this plot at any temperature T gives the value of dE/dT at that temperature. Over the temperature range in this experiment, the value of dE/dT is reasonably constant, and the plot of E versus T should give a straight line. Substitution into Equation 4 gives ΔS. The value of E measured at room temperature is used to determine ΔG of the reaction. E° may be determined by plotting E versus T and obtaining the value at 25℃. The literature value is 1.100V for the cell used in this experiment. ΔG and ΔH for the reaction can then be calculated, as can ΔG° and ΔH°. Note: It is important to realize that the voltage measurements at each temperature should not be taken until the system is at thermal equilibrium. Usually, a period of at least 15 minutes is sufficient for these conditions to be met. No reaction occurs until all connections are made. Connect the wires to the meter last. If a negative deflection occurs on the meter, reverse the wires to the meter. Disconnect one wire during the thermal equilibration so that the concentrations don’t change. PDF 文件使用 "pdfFactory Pro" 试用版本创建 ÿwww.fineprint.com.cn