04 REDOX EQM Standard Hydrogen Electrode and Measurement of Electrode Potentials C. Y. Yeung (CHW, 2009)p.01 How to measure the absolute values of Electrode.

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

p.02 Reference Electrode: S.H.E. standard hydrogen electrode 2H + (aq) + 2e - H 2 (g) platinized (coated with platinium black) connected to the half cell whose E to be measured (w/ salt bridge) compare the tendency of accepting e - with S.H.E. [S.H.E.] E = 0 V H + |H 2 Values of E is measured by potentiometer.

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 = 0V H + |H 2 E 1 < 0E 2 > 0 half cell Xhalf cell Y [anode] [cathode] E cell = E 2 – E 1 If the values of E of 2 half cells (X,Y) are measured: connected as anode

p.04 ref.: p To measure the E value of a half cell … Connecting S.H.E. as the anode. -ve. e.m.f. would be resulted if S.H.E. accepts e -. (lower tendency to be reduced than S.H.E.) +ve. e.m.f. would be resulted if S.H.E. releases e -. (higher tendency to be reduced than S.H.E.) Cell Diagram Pt(s) | H 2 (g,1atm) | 2H + (aq.1M) half cell under investigation (anode)(cathode)

p.05 Electrochemical cells without Salt Bridge (1) …? Zn  Zn e - (anode) (cathode) Cu e -  Cu Zn 2+ SO 4 2- excess –ve. charges excess +ve. charges Cell Diagram : Zn(s) | Zn 2+ (aq) Cu 2+ (aq) | Cu(s) porous pot

p.06 Electrochemical cells without Salt Bridge (2) …? share the same electrolyte Cell Diagram Pt(s) | H 2 (g,1atm) | 2H + (aq.1M) Cl - (aq,1M) | AgCl(s) | Ag(s) Pt(s) | H 2 (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 > energetically feasible  no information on the Activation Energy  reactions may be very slow! (due to high E a … )

p HKAL Paper 2A Q.1(c) [3M] Cr 2 O 7 2- : reduction (cathode), Zn: oxidation (anode) ! E cell = 1.33 – (-0.76) = V > 0 The rxn is favourable because of the large +ve. E value. Will Cr 3+ be reduced by Zn? Cr 3+ : reduction (cathode), Zn: oxidation (anode) ! E cell = – (-0.76) = V > 0 The rxn is favourable because of the +ve. E value. ** Cr 3+ is formed.  The products will be Cr 2+ (aq) and Zn 2+ (aq).

p HKAL Paper 2B Q.6(c) [4M] air sensitive! (easily oxidized by O 2 ) In aqueous solutions, TiO 2+ is colourless. It can be reduced to give a violet solution containing [Ti(H 2 O) 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) E cell = 1.23 – (+0.11) = V > 0 4[Ti(H 2 O) 6 ] 3+ + O 2 + 4H +  4TiO H 2 O + 8H + 4Ti H 2 O + O 2  4TiO H +  In the presence of O 2, [Ti(H 2 O) 6 ] 3+ will be oxidized to TiO 2+, the violet solution will turn colourless.

p.11Summary 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.

p.12Assignment p.212 Check Point 20.5(c),(d) [due date: 13/5 (Wed)] Next …. Nernst Equation, Primary Cells (p ) p.223 Q.7, 9, 10 [due date: 13/5 (Wed)] Worksheet Q.1-3 [due date: 13/5 (Wed)]