Lecture_4_ppt_3

NERNEST EQUATION NERNEST EQUATION FOR ELECTRODE FOR ELECTRODE POTENTIAL (E):POTENTIAL (E):

Et = E0 + RT/nF ln [MnEt = E0 + RT/nF ln [Mn++] where;] where;

Et = electrode potential at temperature t.Et = electrode potential at temperature t.E0= standard electrode potential (constant E0= standard electrode potential (constant depend on the system)depend on the system)R= gas constant , 8.314 JK-1mol-1R= gas constant , 8.314 JK-1mol-1T = the temperature in KelvinsT = the temperature in Kelvins

F = faraday (96500 Coulombs)F = faraday (96500 Coulombs)ln = (natural logarithm = 2.303log)ln = (natural logarithm = 2.303log)n = number of moles of electrons that n = number of moles of electrons that

appear in the half reaction appear in the half reaction [Mn[Mn++] = molar concentration of metal ions in ] = molar concentration of metal ions in

solution .solution . This formula can be simplified by This formula can be simplified by

introducing known values as follows: introducing known values as follows: E = Eº + 0.0591 / n log [ Mn+]E = Eº + 0.0591 / n log [ Mn+]

E = E0E = E0 if [Mn if [Mn++] equals one , (i.e. molar ] equals one , (i.e. molar solution)solution)

Standard electrode potential Standard electrode potential (E0):(E0):

( Is electrode potential when all reactants ( Is electrode potential when all reactants and products of half-reaction have unit and products of half-reaction have unit activity )activity )

E0 E0 is defined as the electromotive force is defined as the electromotive force (e.m.f) produced when a half cell (e.m.f) produced when a half cell (consisting of the elements immersed in a (consisting of the elements immersed in a molar solution of its ions) is coupled with a molar solution of its ions) is coupled with a standard hydrogen electrode standard hydrogen electrode

(E0 = zero)(E0 = zero)Table 1 lists the E0 values of some Table 1 lists the E0 values of some

common metalscommon metals

Notes:Notes:

The arrangement of metals in order of The arrangement of metals in order of their standard electrode potential their standard electrode potential gives the gives the electrochemical serieselectrochemical series (Table 1).(Table 1).

A metal with a more A metal with a more –ve –ve potential will potential will displace any other below it in the displace any other below it in the series from its salt solution , thus iron series from its salt solution , thus iron will displace copper or mercury from will displace copper or mercury from their salt solutions.their salt solutions.

Table 1 : Standard electrode Table 1 : Standard electrode potential at 250 C :potential at 250 C :

System E0(volts) System E0(volts)System E0(volts) System E0(volts) Li/Li+ Cd/Cd2+ -0.40Li/Li+ Cd/Cd2+ -0.40 K/K+ -2.92 Sn/Sn2+ -0.13K/K+ -2.92 Sn/Sn2+ -0.13 Mg/Mg2+ -2.37 Mg/Mg2+ -2.37 H2(pt)/H+ 0.00H2(pt)/H+ 0.00 Al/Al3+ -1.33 Cu/Cu2+ +0.34Al/Al3+ -1.33 Cu/Cu2+ +0.34 Zn/Zn2+ -0.44 Hg/Hg2+ +0.79Zn/Zn2+ -0.44 Hg/Hg2+ +0.79 Fe/Fe2+ -0.44 Ag/Ag+ +0.80Fe/Fe2+ -0.44 Ag/Ag+ +0.80

Silver treeSilver tree

Standard Oxidation potential Standard Oxidation potential (E0):(E0):

E25E25˚̊C = E0+ 0.0591/n log [M n+]C = E0+ 0.0591/n log [M n+]It is the e.m.f. produced when a half It is the e.m.f. produced when a half

cell consisting of an inert electrode cell consisting of an inert electrode (as platinum) dipped in a solution of (as platinum) dipped in a solution of equal concentration of both the equal concentration of both the oxidized and reduced forms (such as oxidized and reduced forms (such as FeFe3+3+/Fe/Fe2+2+) is connected with a NHF , ) is connected with a NHF , Figure 2.Figure 2.

Figure 2 :Figure 2 : Apparatus for determination Apparatus for determination of the standard oxidation potential of of the standard oxidation potential of the system Fethe system Fe3+3+/Fe/Fe2+2+

1,2: glass vessel,3: electrolyte bridge ,1,2: glass vessel,3: electrolyte bridge ,4: tube for hydrogen gas ,4: tube for hydrogen gas ,5: potentiometer5: potentiometerTable 2: lists the E0 of some common Table 2: lists the E0 of some common

redox couples.redox couples.

Table 2: standard oxidation Table 2: standard oxidation potentials at 25potentials at 25

System E0(volts) System E0(volts) System E0(volts) System E0(volts) MnOMnO44

--/Mn/Mn2+2+ +1.52 AsO +1.52 AsO443-3-/AsO/AsO33

3-3- +0.57 +0.57

CeCe4+4+/Ce/Ce3+3+ +1.45 I +1.45 I22/2I/2I-- +0.54 +0.54ClOClO33

--/Cl/Cl-- +1.45 Fe(CN) +1.45 Fe(CN)663-3-/Fe(CN)/Fe(CN)66

4-4- +0.36 +0.36CrCr22OO77 2-2-/2Cr/2Cr3+3+ +1.33 Cu +1.33 Cu2+2+/Cu/Cu++ +0.16 +0.16IOIO33

--/I/I22 +1.19 +1.19 ClCl22/Cl/Cl- - +1.36V+1.36V S S44OO66

2-2-/2S/2S22OO33 2-2- +0.09 +0.09

FeFe3+3+/Fe/Fe2+2+ +0.77 +0.77 HH22/2H/2H++ 0.00 0.00 FeFe2+2+/Fe -0.44/Fe -0.44

Notes:Notes:The higher E0 , the stronger the oxidizing power The higher E0 , the stronger the oxidizing power

of its oxidant and the weaker the reducing of its oxidant and the weaker the reducing power of its reduced form.power of its reduced form.

The most powerful oxidizing agent are those at The most powerful oxidizing agent are those at the top (highly +ve) and the most powerful the top (highly +ve) and the most powerful reducing agent are at the bottom (highly –reducing agent are at the bottom (highly –ve)ve)

If any two redox systems are combined , the If any two redox systems are combined , the stronger oxidizing agent gains electrons stronger oxidizing agent gains electrons from the stronger reducing agent with the from the stronger reducing agent with the formation of weaker reducing and oxidizing formation of weaker reducing and oxidizing agents.agents.

ClCl22/Cl/Cl--=(E0+1.36V), Fe=(E0+1.36V), Fe3+3+/Fe/Fe2+2+(E0=+0.77V)(E0=+0.77V)ClCl22 + 2 Fe + 2 Fe2+2+ Fe Fe3+3+ + 2 Cl + 2 Cl--

strong Strong strong Strong oxidizing agent reducing agent oxidizing agent reducing agent weak oxid weak reducing weak oxid weak reducing

reducingreducing agent agent agent agent

Nernest equation for oxidation Nernest equation for oxidation potential:potential:

EE25c25c = E0 + 0.0591/n log = E0 + 0.0591/n log [Ox]/[Red][Ox]/[Red]

[Ox] & [red] = molar concentration of [Ox] & [red] = molar concentration of the oxidized and reduced forms,when the oxidized and reduced forms,when

[Ox] = [Red] , [Ox] = [Red] , then E=Ethen E=E00..

Phosphoric acid lowers the potential of Phosphoric acid lowers the potential of FeFe3+3+/Fe/Fe2+2+ system by complex with system by complex with FeFe3+3+ as [Fe(PO as [Fe(PO44))22]]3-3- . .

↓ ↓E= E0 +.0591/1log[FeE= E0 +.0591/1log[Fe3+3+]/[Fe]/[Fe2+2+]↑]↑Sulphuric acid is used for acidification.Sulphuric acid is used for acidification.

Effect of [H+]Effect of [H+]The oxidation potential of an oxidizing agent containing The oxidation potential of an oxidizing agent containing

oxygen increases by increasing acidity and vice versaoxygen increases by increasing acidity and vice versaPotassium permenganate:Potassium permenganate: MnOMnO44

-- + 8H + 8H++ +5e Mn +5e Mn2+2+ + 4H + 4H22OO EMnOEMnO44

--/Mn/Mn2+2+=E0 + =E0 + 0.05910.0591 log [MnO log [MnO44--[H[H++]]88/[Mn/[Mn2+2+] ]

55 Potassium dichromate:Potassium dichromate: CrCr22OO77

2-2- + 14H + 14H++ + 6e 2Cr + 6e 2Cr3+3+ + 7H + 7H22OOE = E0 + 0.0591/6 log [CrE = E0 + 0.0591/6 log [Cr22OO77

2-2-][H+]][H+]1414 / [Cr / [Cr3+3+]]22

(Cr(Cr22OO77/Cr/Cr3+3+))

Arsenate/ Arsenite system:Arsenate/ Arsenite system: HCO3 - HCO3 - AsOAsO44

3-3- + 2I + 2I-- + 2H + 2H + + AsO AsO33 3-3- +I +I22 + H + H22OO H+H+In presence of much acid , arsenate will In presence of much acid , arsenate will

oxidize iodide to iodine. If oxidize iodide to iodine. If EE is reduced by is reduced by decreasing [Hdecreasing [H++] , arsenite is oxidized with ] , arsenite is oxidized with iodine iodine (in presence of sodium bicarbonate)(in presence of sodium bicarbonate)

Effect of complexing agents:Effect of complexing agents:E (IE (I22/21/21--) ) system increases by the addition of HgClsystem increases by the addition of HgCl22

since it complexes with iodide ions .since it complexes with iodide ions .HgHg2+2+ + 4I + 4I-- [HgI [HgI44]]2- 2- low dissociation complexlow dissociation complex EE (Fe(Fe3+3+/Fe/Fe2+2+) ) is reduced by the addition of Fis reduced by the addition of F-- or PO or PO44

3-3- due to the formation of the stable complexes due to the formation of the stable complexes [FeF6][FeF6] 3- 3- and and [Fe(PO[Fe(PO44))22]]3- 3-

respectively.respectively. ThusThus ferric ions in presence of F ferric ions in presence of F-- or PO or PO44

3-3- cannot cannot oxidize iodide although oxidize iodide although

EE00(Fe(Fe3+3+/Fe/Fe2+2+) = 0.77) = 0.77 while while EE00(I(I22/2I/2I--)) = = 0.540.54

(4) (4) Effect of precipitating Effect of precipitating agents:agents:

Addition of ZnAddition of Zn2+2+ salts which precipitates salts which precipitates ferrocyanide increases the oxidation ferrocyanide increases the oxidation potential of ferri/ferrocyanide system to potential of ferri/ferrocyanide system to oxidize iodide to iodine, although the oxidize iodide to iodine, although the oxidation potential of Ioxidation potential of I22/2I- system is /2I- system is higher.higher.

[Fe (CN)[Fe (CN)66]]4-4- +Zn +Zn2+2+ Zn Zn22 [Fe(CN) [Fe(CN)66]↓]↓↑↑EEferri/ferroferri/ferro = =EE00 + 0.0591/1 log[Fe(CN) + 0.0591/1 log[Fe(CN)66]]3-3-/Fe(CN)/Fe(CN)66]]4-4-↓↓