山东大学：《物理化学》课程教学资源（讲义资料）7.7 Electromotive force-for students

87.7 Thermodynamics of reversible cell Out-class extensive reading Ira N. Levine, pp. 294-310 Section 10.10 standard-state thermodynamic properties of solution components pp.426 Section 14.6 thermodynamics of galvanic cells

§7.7 Thermodynamics of reversible cell Out-class extensive reading: Ira N. Levine, pp. 294-310 Section 10.10 standard-state thermodynamic properties of solution components pp. 426 Section 14.6 thermodynamics of galvanic cells

87. 7 Thermodynamics of reversible cell Goals of this class (1)Understand the principle for determining EMF: high input voltmeter and compensation 2)Demonstrate the component of Weston standard cell (3 )Nernst equation: theoretical deduction and application (4) Apply the principle for determining thermodynamic quantities from electrochemical measurements (5)Explain the principles of relative standard

Goals of this class: (1) Understand the principle for determining EMF: high input voltmeter and compensation. (2) Demonstrate the component of Weston standard cell; (3) Nernst equation: theoretical deduction and application; (4) Apply the principle for determining thermodynamic quantities from electrochemical measurements; (5) Explain the principles of relative standard; §7.7 Thermodynamics of reversible cell

7.7 Thermodynamics of reversible cell 7.7.1 Measurement of Electromotive forces(emfs) What is electromotive forces? Can voltameter be used to measure electromotive force? U E=(R+R)= R r+ E R 1+ Discussion E R R High-impedance input voltameter

7.7.1 Measurement of Electromotive forces (emf's) Can voltameter be used to measure electromotive force? E R R I o i = ( + ) Ro U I = U R R R E o o + i = o i R R U E = 1+ V E Ri Ro U Discussion What is electromotive forces? High-impedance input voltameter §7.7 Thermodynamics of reversible cell

7.7 Thermodynamics of reversible cell 7.7.1. Measurement of Electromotive forces(emfs) (1)Poggendorff's compensation method i=0, thermodynamic reversibility. E E --------A 一==== B A 1111 E E E w: working ce E E、: test cell Principle of potentiometer E: standard celll

(1) Poggendorff’s compensation method i = 0, thermodynamic reversibility. EW: working cell Ex : test cell Es : standard cell A Es Ex Principle of potentiometer G Ew Ex Es K A B C1 C2 7.7.1. Measurement of Electromotive forces (emf's) §7.7 Thermodynamics of reversible cell

87. 7 Thermodynamics of reversible cell 7.7.1. Measurement of Electromotive forces(emfs) (2) Weston standard cell The Weston cell, is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters. Invented by Edward Weston in 1893,it was adopted as the international Standard for emf between 1911 and 1990

(2) Weston standard cell The Weston cell, is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters. Invented by Edward Weston in 1893, it was adopted as the International Standard for EMF between 1911 and 1990. 7.7.1. Measurement of Electromotive forces (emf's) §7.7 Thermodynamics of reversible cell

7.7 Thermodynamics of reversible cell 7.7.1. Measurement of Electromotive forces(emfs) (2)Weston standard cell Cork sealed with paraffin or wax Saturated Cdso solution CdSO4·-H,O PPT Hg2S0 t Qcd(Hgx Commercial weston standard cell -Cd(5%12%)(Hg) Cdso. oH,O(s)CdSo,(sat Cdso,.H,O(S)HgSO (sHg(+

Hg Hg2SO4 4 2 8 CdSO H O 3  Saturated CdSO4 solution Cd(Hg)x + -- Cork sealed with paraffin or wax Commercial Weston Standard cell 4 2 4 4 2 4 8 8 Cd(5% 12%)(Hg) CdSO H O(s) CdSO (sat) CdSO H O(s) HgSO (s) Hg(l)+ 3 3 −         x (2) Weston standard cell 7.7.1. Measurement of Electromotive forces (emf's) §7.7 Thermodynamics of reversible cell

87. 7 Thermodynamics of reversible cell 7.7.1. Measurement of Electromotive forces(emfs (2)Weston standard cell Temperature-dependence of emf The original design was a saturated cadmium cell producing a convenient 1.018638 Volt reference and had the E(T)N=1.01845-405×103(7/K advantage of having a lower temperature 293.15)-95×107(TK-293.15)2+1 coefficient than the previously used 108(7/K-29315) Clark cell

E(T) /V = 1.01845 – 4.05 10-5 (T/K – 293.15) – 9.5 10-7 (T/K –293.15)2 + 1 10-8 (T/K –293.15)3 Temperature-dependence of emf The original design was a saturated cadmium cell producing a convenient 1.018638 Volt reference and had the advantage of having a lower temperature coefficient than the previously used Clark cell (2) Weston standard cell 7.7.1. Measurement of Electromotive forces (emf's) §7.7 Thermodynamics of reversible cell

87. 7 Thermodynamics of reversible cell 7.7.1. Measurement of Electromotive forces(emfs OPle Vout Counter ectrode Reference Voltage Electrode 运算放大器: Operational Amplifier

7.7.1. Measurement of Electromotive forces (emf's) §7.7 Thermodynamics of reversible cell 运算放大器： Operational Amplifier

87. 7 Thermodynamics of reversible cell 7.7.2 Nernst equation and standard eMF of cell 1889, Nernst empirical equation cC+dD=gG+hH e-fe rt aGaH F Walther H. Nernst What is the physical meaning of ee? 1920 Noble Prize. germany 186406/25~1941/11/18 Studies on thermodynamics

7.7.2 Nernst equation and standard EMF of cell 1889, Nernst empirical equation G H C D ln r h c d RT a a E E nF a a = − cC + dD = gG + hH Walther H. Nernst 1920 Noble Prize, Germany 1864/06/25~1941/11/18 Studies on thermodynamics What is the physical meaning of E ? §7.7 Thermodynamics of reversible cell

87. 7 Thermodynamics of reversible cell 7.7.2 Nernst equation and standard EMF of cell 7.7.3. Standard electromotive forces Theoretical deduction of Nernst equation For a general electrochemical reaction: Ee equals e when the activity of any cC+d-G+hh ya d cap For cell ag Van't Orff equation △G=△G+RTln24 Pb(s)-Pbo(s)oH(cHgo(s)-Hg( △G.=-HFE rite out the cell reaction and nernst nFE equation RT E=E

For a general electrochemical reaction: cC + dD = gG + hH G H C D g h a c d a a K a a = Van’t Horff equation G H r m r m C D Δ Δ ln g h c d a a G G RT a a = + Δr m G nFE = − Δr m G nFE = − G H C D ln r h c d RT a a E E nF a a = − Theoretical deduction of Nernst Equation: 7.7.2 Nernst equation and standard EMF of cell EӨ equals E when the activity of any chemical species is unit. For cell: Pb(s)-PbO(s)|OH– (c)|HgO(s)-Hg(l) Write out the cell reaction and Nernst equation. 7.7.3. Standard electromotive forces §7.7 Thermodynamics of reversible cell