0404REDOX EQMREDOX EQM Standard Hydrogen Electrodeand Measurement of Electrode Potentials

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

How to measure the How to measure the absoluteabsolute values values of Electrode Potentials?of Electrode Potentials?

We cannot do it.We cannot do it.

Relative values are measured with respect to Relative values are measured with respect to a “a “reference electrodereference electrode”.”.

p.02

Reference Electrode:Reference Electrode: S.H.E.S.H.E.standard hydrogen electrode

2H2H++(aq) + 2e(aq) + 2e-- H H22(g)(g)

platinized platinized (coated with pla(coated with pla

tinium black)tinium black)

connected to the half connected to the half cell whose E to be cell whose E to be measured measured (w/ salt (w/ salt bridge)bridge)

compare the tendency of compare the tendency of accepting eaccepting e-- with S.H.E. with S.H.E.

[S.H.E.]

E = 0 VH+|H2

Values of E is measured Values of E is measured by by potentiometerpotentiometer..

p.03 Compare the tendency of accepting e- with S.H.E.

[the tendency to undergo reduction]

if > S.H.E., +ve. E value [O.A.] (e.g. p.205)

if < S.H.E., -ve. E value [R.A.] (e.g. p.204)

[S.H.E.]

E = 0VH+|H2

E1 < 0 E2 > 0

half cell X half cell Y

[anode] [cathode]

Ecell = E2 – E1

If the values of E of 2 half cells (X,Y) are measured:

connected as anode

p.04

ref.: p. 204-205

To measure the E value of a half cell …To measure the E value of a half cell …

Connecting Connecting S.H.E. as the anodeS.H.E. as the anode..

-ve. e.m.f.-ve. e.m.f. would be resulted would be resulted if S.H.E. accepts eif S.H.E. accepts e--. . (lower tendency to be reduced than S.H.E.)(lower tendency to be reduced than S.H.E.)

+ve. e.m.f.+ve. e.m.f. would be resulted would be resulted if S.H.E. releases eif S.H.E. releases e--.. (higher tendency to be reduced than S.H.E.) (higher tendency to be reduced than S.H.E.)

Cell DiagramCell Diagram

Pt(s) | HPt(s) | H22(g,1atm) | 2H(g,1atm) | 2H++(aq.1M) half cell under investigation(aq.1M) half cell under investigation(anode) (cathode)

p.05

Electrochemical cells Electrochemical cells without Salt Bridgewithout Salt Bridge (1) …? (1) …?

ZnZn Zn Zn2+2+ + 2e + 2e--

(anode) (cathode)

CuCu2+2+ + 2e + 2e-- Cu Cu

ZnZn2+2+

SOSO442-2-

excess –ve. cexcess –ve. chargeshargesexcess +ve. cexcess +ve. c

hargesharges

Cell Diagram :Cell Diagram : Zn(s) | ZnZn(s) | Zn2+2+(aq) Cu(aq) Cu2+2+(aq) | Cu(s) (aq) | Cu(s)

porous potporous pot

p.06

Electrochemical cells Electrochemical cells without Salt Bridgewithout Salt Bridge (2) …? (2) …?

share the same share the same electrolyteelectrolyte

Cell DiagramCell Diagram

Pt(s) | HPt(s) | H22(g,1atm) | 2H(g,1atm) | 2H++(aq.1M) Cl(aq.1M) Cl--(aq,1M) | AgCl(s) | Ag(s) (aq,1M) | AgCl(s) | Ag(s)

Pt(s) | HPt(s) | H22(g,1atm) | HCl (aq,1M) | AgCl(s) | Ag(s) (g,1atm) | HCl (aq,1M) | AgCl(s) | Ag(s)

p.07

Another Application of Electrode Potentials … ?

Predict the Energetic Feasibility of Redox Reactions

E value > 0 ----- energetically feasible

no information on the Activation Energy

reactions may be very slow! (due to high Ea …)

p.08 1992 HKAL Paper 2A Q.1(c) [3M]1992 HKAL Paper 2A Q.1(c) [3M]

Cr2O72-: reduction (cathode), Zn: oxidation (anode) !

Ecell = 1.33 – (-0.76) = +2.09 V > 0

The rxn is favourable because of the large +ve. E value.

Will Cr3+ be reduced by Zn?

Cr3+: reduction (cathode), Zn: oxidation (anode) !

Ecell = -0.41 – (-0.76) = +0.35 V > 0

The rxn is favourable because of the +ve. E value.

** Cr3+ is formed.

The products will be Cr2+(aq) and Zn2+(aq).

p.09 1995 HKAL Paper 2B Q.6(c) [4M]1995 HKAL Paper 2B Q.6(c) [4M]

air sensitive! air sensitive! (easily oxidized by O(easily oxidized by O22))

In aqueous solutions, TiO2+ is colourless. It can be reduced to give a violet solution containing [Ti(H2O)6]3+. The violet solution formed should be kept in a sealed vessel or handled in an inert atmosphere. Using the following data, predict, giving a balanced equation, what will happen to the violet solution if it is not kept in a sealed vessel or not handles in an inert atmosphere.

p.10

(cathode)

(anode)

Ecell = 1.23 – (+0.11) = +1.12 V > 0

4[Ti(H2O)6]3+ + O2 + 4H+ 4TiO2+ + 22H2O + 8H+

4Ti3+ + 2H2O + O2 4TiO2+ + 4H+

In the presence of O2, [Ti(H2O)6]3+ will be oxidized to TiO2+, the violet solution will turn colourless.

p.11

SummarySummary

Drawing the set-up of S.H.E.Drawing the set-up of S.H.E.

Meaning of +ve./-ve. cell e.m.f.Meaning of +ve./-ve. cell e.m.f.

Electrochemical cells without salt bridgeElectrochemical cells without salt bridge

Writing Cell Diagrams of different cominations Writing Cell Diagrams of different cominations of half cells.of half cells.

Predict the energetic feasibility of redox rxns bPredict the energetic feasibility of redox rxns basing on cell e.m.f.asing on cell e.m.f.

p.12

AssignmentAssignment

p.212 Check Point 20.5(c),(d) p.212 Check Point 20.5(c),(d) [due date: 13/5 (Wed)] [due date: 13/5 (Wed)]

Next ….Next ….Nernst Equation, Primary Cells Nernst Equation, Primary Cells (p. 213-214) (p. 213-214)

p.223 Q.7, 9, 10 p.223 Q.7, 9, 10 [due date: 13/5 (Wed)] [due date: 13/5 (Wed)]

Worksheet Q.1-3 Worksheet Q.1-3 [due date: 13/5 (Wed)] [due date: 13/5 (Wed)]