87.11 Polarization of electrode 7.11.1. Decomposition voltage and overvoltage Decomposition voltage the minimum potential difference which 170v must be applied between electrodes before 1.229V decomposition occurs and a current flows Onset potential 0.0---1.0 2.0E/V The reversible electromotive force of the cell (Theoretical decomposition voltage)is 1. 229 V. The effective decomposition voltage is 1.70 V. A discrepancy of ca. 0.5V, which is named as overvoltage exist
The reversible electromotive force of the cell (Theoretical decomposition voltage) is 1.229 V. The effective decomposition voltage is 1.70 V. A discrepancy of ca. 0.5 V, which is named as overvoltage, exist. Decomposition voltage: the minimum potential difference which must be applied between electrodes before decomposition occurs and a current flows. 1.70 V 1.229 V 0.0 1.0 2.0 E / V I / A Onset potential 7.11.1. Decomposition voltage and overvoltage §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11. 2 Thermodynamics of irreversible cell For reversible cell: Wre= nFEre For irreversible cell: w?=nFE For electrolytic cell For galvanic cell E<Er;△E=En-E>0 Ere >E; Ae= ere-E>0 △E=(q1i9i)-(9,r-.r) △E=(9,rq,r)(q,r-qmr (Pa ir -P. re)+(. re -. ir (p. re-p ir)+(pa. ir-pare) (Pa, ir"Pa, re)=na (Pe, re-Pe, ir )=ne (o re - ir)=n (a ir-a re)=n △E=7+T △E=m+ma cre air are +7a cIr cre aIr
7.11. 2 Thermodynamics of irreversible cell For reversible cell: Wre = nFEre For irreversible cell: Wir = nFEir For electrolytic cell: Ere < Eir ; E = Eir - Ere > 0 E = (a, ir-c, ir) - (a, re - c, re) = (a, ir - a, re) + (c, re - c, ir) (a, ir − a, re ) = a E = c + a (c, re − c, ir ) = c c,ir c,re c = − a,ir a,re a = + For galvanic cell: Ere > Eir; E = Ere − Eir > 0 E = (c, re− a, re)−( c, ir − a, ir) = (c, re−c, ir) + (a, ir−a,re) E = c + a (c, re − c, ir ) = c (a, ir − a, re ) = a c,ir c,re c = − a,ir a,re a = + §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11.2 Thermodynamics of irreversible cell Galvanic cell Electrolytic cell c. re re a,ir a, re a,Ir a, re Under irreversible conditions, electrode potential differs from its reversible value this phenomenon is defined as polarization The discrepancy between reversible potential and irreversible potential is termed as overpotential(n). By definition, overpotential always has positive value
Galvanic cell Electrolytic cell c, ir = c, re − c a, ir = a, re + a c, ir = c, re − c a, ir = a, re + a Under irreversible conditions, electrode potential differs from its reversible value, this phenomenon is defined as polarization. The discrepancy between reversible potential and irreversible potential is termed as overpotential (). By definition, overpotential always has positive value. 7.11.2 Thermodynamics of irreversible cell §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11.2 Thermodynamics of irreversible cell Galvanic cell Electrolytic cell q,i=卯,re-ne re Pa ir=Pa re t na a. re 几m T ne a E a Pc ir ir Pa Pa C.re g,re驴 The irreversible potential and the irreversible electromotive force of cell depend on the current density imposed. Polarization cause decrease in electromotive force of galvanic cell and increase in decomposition voltage of electrolytic cell
The irreversible potential and the irreversible electromotive force of cell depend on the current density imposed. Polarization cause decrease in electromotive force of galvanic cell and increase in decomposition voltage of electrolytic cell. Galvanic cell Electrolytic cell c, ir = c, re − c a, ir = a, re + a c, ir = c, re − c a, ir = a, re + a 7.11.2 Thermodynamics of irreversible cell §7.11 Polarization of electrode