0303REDOX EQMREDOX EQM Half Cells with Inert Electrode

C. Y. Yeung (CHW, 2009)p.01

Inert Electrode :Inert Electrode :

In the metal-metal ion system, In the metal-metal ion system, metal is the electrode, and metal is the electrode, and metal ion is the electrolyte. metal ion is the electrolyte.

MMn+n+

MM

However, if the system does not invHowever, if the system does not involve metal, and “olve metal, and “inert electrodeinert electrode” is r” is required. (e.g. equired. (e.g. platiniumplatinium / / graphitegraphite))

p.02

PtPt

FeFe2+2+FeFe3+3+

- e-

+ e-

(1) Metal Ion – Metal Ion Sytem(1) Metal Ion – Metal Ion Sytem

FeSOFeSO44(aq) + (aq) +

FeFe22(SO(SO44))33(aq)(aq)

EE FeFe3+3+,Fe,Fe2+2+| Pt| Pt = +0.77 V = +0.77 V

as cathode:as cathode: FeFe3+3+(aq) , Fe(aq) , Fe2+2+(aq) | Pt(s)(aq) | Pt(s)

Cell DiagramCell Diagram

as anode:as anode: Pt(s) | FePt(s) | Fe2+2+(aq) , Fe(aq) , Fe3+3+(aq)(aq)

Half Cells using Inert Electrode :Half Cells using Inert Electrode :

EE MnOMnO44--,Mn,Mn2+2+| Pt| Pt = +1.52 V = +1.52 V

MnOMnO44--(aq) + 8H(aq) + 8H++ + 5e + 5e-- Mn Mn2+2+(aq) + 4H(aq) + 4H22O(l)O(l)

<Example <Example 22>>

FeFe3+3+(aq) + e(aq) + e-- Fe Fe2+2+(aq)(aq)

<Example <Example 11>>

Cell DiagramCell Diagramas cathode:as cathode: [MnO[MnO44

--(aq) + 8H(aq) + 8H++(aq)] , [Mn(aq)] , [Mn2+2+(aq) + 4H(aq) + 4H22O(l)] | Pt(s)O(l)] | Pt(s)

as anode:as anode: Pt(s) | [MnPt(s) | [Mn2+2+(aq) + 4H(aq) + 4H22O(l)] , [MnOO(l)] , [MnO44--(aq) + 8H(aq) + 8H++(aq)] (aq)]

p.03

ExExerciseercise

Calculate the cell e.m.f. and write the cell diagram of the electrochemical cell shown below in accordance with the IUPAC convention. (ref.: p. 207)

EE FeFe3+3+,Fe,Fe2+2+| Pt| Pt = +0.77 V = +0.77 V

EE MnOMnO44--,Mn,Mn2+2+| Pt| Pt = +1.52 V = +1.52 V

cathodecathodeanodeanode

EEcellcell = +1.52 – (+0.77) = +1.52 – (+0.77)

= +0.75V= +0.75V

Cell DiagramCell Diagram

C(graphite) | FeC(graphite) | Fe2+2+(aq) , Fe(aq) , Fe3+3+(aq) (aq) [MnO[MnO44

--(aq) + 8H(aq) + 8H++(aq)] , [Mn(aq)] , [Mn2+2+(aq) + 4H(aq) + 4H22O(l)] | C(graphite)O(l)] | C(graphite)

[Ref.: p. 210 Example 20-5C]

Oxidation

Reduction

p.04

PtPt

II--II22

- e-

+ e-

(2) Non-metal Ion – Non-metal Ion Sytem(2) Non-metal Ion – Non-metal Ion Sytem

II22(aq) + I(aq) + I--(aq)(aq)

EE II22, I, I- - | Pt| Pt = +0.54 V = +0.54 V

as cathode:as cathode: II22(aq) , 2I(aq) , 2I--(aq) | Pt(s)(aq) | Pt(s)

Cell DiagramCell Diagram

as anode:as anode: Pt(s) | 2IPt(s) | 2I--(aq) , I(aq) , I22(aq)(aq)

II22(aq) + 2e(aq) + 2e-- 2 I 2 I--(aq)(aq)

<Example><Example>

ExExerciseercise If iron(III) / iron(II) system acts as the cathode, while iodine / iodide acts as the anode, with using platinium as electrodes. Calculate the cell e.m.f. and write the cell diagram. (ref.: p.207)

EEcellcell = +0.77 – (+0.54) = +0.77 – (+0.54)

= +0.23V= +0.23V

Cell Diagram:Cell Diagram: Pt(s) | 2IPt(s) | 2I--(aq) , I(aq) , I22(aq) Fe(aq) Fe3+3+(aq) , Fe(aq) , Fe2+2+(aq) | Pt(s)(aq) | Pt(s)

p.05

PtPt

(3) Metal – Sparingly Soluble Salt Sytem(3) Metal – Sparingly Soluble Salt Sytem

HCl(aq)HCl(aq)

Platinium Black Platinium Black coated with Ag coated with Ag and AgCland AgClEE AgCl | AgAgCl | Ag = +0.54 V = +0.54 V

AgCl(s) + eAgCl(s) + e-- Ag(s) + Cl Ag(s) + Cl--(aq)(aq)

<Example <Example 11>>

+ e-

- e-AgAg

AgClAgCl

as cathode:as cathode: ClCl--(aq) | AgCl(s) | Ag(s)(aq) | AgCl(s) | Ag(s)

Cell DiagramCell Diagram

as anode:as anode: Ag(s) | AgCl(s) | ClAg(s) | AgCl(s) | Cl--(aq)(aq)

electrolyte!

EE PbSOPbSO44 | Pb | Pb = -0.35 V = -0.35 V

PbSOPbSO44(s) + 2e(s) + 2e-- Pb(s) + SO Pb(s) + SO442-2-(aq)(aq) <Example <Example 22>>

as cathode:as cathode: SOSO442-2-(aq) | PbSO(aq) | PbSO44(s) | Pb(s)(s) | Pb(s)

Cell DiagramCell Diagram

as anode:as anode: Pb(s) | PbSOPb(s) | PbSO44(s) | SO(s) | SO442-2-(aq)(aq)

i.e. i.e. HH22SOSO44 is used is used

as electrolyte.as electrolyte.

p.06

AssignmentAssignment

p.202 Check Point 20.3 p.202 Check Point 20.3 [due date: 11/5 (Mon)] [due date: 11/5 (Mon)]

Next ….Next ….Standard Hydrogen Electrode (S.H.E.), Standard Hydrogen Electrode (S.H.E.), Measurement of Standard Electrode Measurement of Standard Electrode Potentials (p. 197-212)Potentials (p. 197-212)

p.20.5(b) p.20.5(b) [due date: 11/5 (Mon)] [due date: 11/5 (Mon)]

p.202 Table 20.1…?p.202 Table 20.1…?