Spontaneity of Redox Reactions △G=W.lecricalrk Zn(s)+Cu"(c)Cu(s)+Zn(c) Welcdrical worktotal charg of(ptential diferen) =-n.eNEcel eW4=(1.602×10-19C)×(6.022×1023/mole) =96485C/mol e=F Faraday Constant △G=W elecmrical work =neNA Ecell =-nFE cell △G=-n:F.Eel E>0,spontaneous reaction; E<0,non-spontaneous reaction
Spontaneity of Redox Reactions W electrical work (total charg of e) (potent = × Q E ial difference) A cell = − ⋅ ⋅ n eN E 19 23 (1.602 10 ) (6.022 10 / ) A eN C mol e − − = × × × 2 2 2 2 ( ) ( ) ( ) ( ) Cu Zn Zn s Cu c Cu s Zn c + + + + ∆ = G Welectrical work + → + - (1.602 10 ) (6.022 10 / ) 96485 / A eN F C mol e C mol e = × × × = = = ∆ = − ⋅ ⋅ = − G W n eN E nFE electrical work A cell cell = G cell ∆ − ⋅ n⋅F E ,spontaneous reaction; , non-spontaneous reaction. 0 0 cell cell E E > < Faraday Constant
Exercise:Given the following redox reaction,(a)Is it spontaneous under standard condition?(b)What is the change in Gibbs free energy for this reaction?Fe"(aq)+Ag(s)=Fe(s)+Ag'(aq) Solution: Fe2(aq)+2Ag(s)=Fe(s)+2Ag(aq) 1V= Reduction:Fe(aq)+2e=Fe(s)Er-0.44V Double? Oxidation:2Ag(s)=2Ag"(aq)+2eE=0.80V =0.44-0.80=-1.24V Spontaneity？ Non-spontaneous! =-nFE =-2molx96500C/mol×(-1.24V) edox =+239320C.V=+239320J=239.32kJ Standard condition! △G, =-nFE redox
Oxidation: Reduction: Exercise: Given the following redox reaction, (a) Is it spontaneous under standard condition? (b) What is the change in Gibbs free energy for this reaction? 2 Fe aq Ag s Fe s Ag aq ( ) 2 ( ) ( ) 2 ( ) + + Solution: + + 2 ( ) 2 ( ) 2 Ag s Ag aq e + − + 2 Fe aq e Fe s ( ) 2 ( ) + − + / 0.80 Ag Ag E V + ° = 2 / 0.44 Fe Fe E V + ° = − 1 J 1 V 1 C = 2 Fe aq Ag s Fe s Ag aq ( ) ( ) ( ) ( ) + + + + Double? G n redox FEredox ° ° ∆ = − 2 / / redox Fe Fe Ag Ag E E E + + ° ° ° = − = − − = − 0.44 0.80 1.24V 2 96500 / ( 1.24 ) 239320 239320 239.32 C mol C mol V V J kJ = − × × − = + = + = ⋅ Non-spontaneous! Spontaneity ? G n redox FEredox ° ° ∆ = − Standard condition!