REDOX EQM

1
C. Y. Yeung (CHW, 2009)
p.01
04
REDOX EQM Standard Hydrogen Electrode and
Measurement of Electrode Potentials
How to measure the absolute values of Electrode
Potentials?
We cannot do it.
Relative values are measured with respect to a
reference electrode.
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p.02
Reference Electrode S.H.E.
3
p.03

• Compare the tendency of accepting e- with
  S.H.E.the tendency to undergo reduction

Ecell E2 E1
E1 lt 0
E2 gt 0
half cell X
half cell Y
anode
cathode
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p.04
5
p.05
Electrochemical cells without Salt Bridge (1) ?
Zn ? Zn2 2e-
Cu2 2e- ? Cu
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p.06
Electrochemical cells without Salt Bridge (2) ?
Pt(s) H2(g,1atm) HCl (aq,1M) AgCl(s)
Ag(s)
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p.07
Another Application of Electrode Potentials ?
? Predict the Energetic Feasibility of Redox
Reactions

• E value gt 0 ----- energetically feasible

• no information on the Activation Energy

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

8
p.08
1992 HKAL Paper 2A Q.1(c) 3M
Cr2O72- reduction (cathode), Zn oxidation
(anode) !
Will Cr3 be reduced by Zn?
Cr3 reduction (cathode), Zn oxidation (anode) !
? The products will be Cr2(aq) and Zn2(aq).
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p.09
1995 HKAL Paper 2B Q.6(c) 4M
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p.10
(anode)
(cathode)
4Ti(H2O)63 O2 4H ? 4TiO2 22H2O 8H
4Ti3 2H2O O2 ? 4TiO2 4H
? In the presence of O2, Ti(H2O)63 will be
oxidized to TiO2, the violet solution will turn
colourless.
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p.11
Summary
Drawing the set-up of S.H.E.
Meaning of ve./-ve. cell e.m.f.
Electrochemical cells without salt bridge
Writing Cell Diagrams of different cominations of
half cells.
Predict the energetic feasibility of redox rxns
basing on cell e.m.f.
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p.12
Assignment
p.212 Check Point 20.5(c),(d) due date 13/5
(Wed)
p.223 Q.7, 9, 10 due date 13/5 (Wed)
Worksheet Q.1-3 due date 13/5 (Wed)