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REDOX EQM Standard Hydrogen Electrode and Measurement of Electrode Potentials

p.01. C. Y. Yeung (CHW, 2009). 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 ”. p.02.

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REDOX EQM Standard Hydrogen Electrode and Measurement of Electrode Potentials

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  1. p.01 C. Y. Yeung (CHW, 2009) 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”.

  2. p.02 standard hydrogen electrode 2H+(aq) + 2e- H2(g) platinized (coated with platinium black) connected to the half cell whose E to be measured (w/ salt bridge) E = 0 V H+|H2 [S.H.E.] compare the tendency of accepting e- with S.H.E. Values of E is measured by potentiometer. Reference Electrode:S.H.E.

  3. p.03 connected as anode • 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) If the values of E of 2 half cells (X,Y) are measured: E = 0V H+|H2 [S.H.E.] • Compare the tendency of accepting e- with S.H.E.[the tendency to undergo reduction] Ecell = E2– E1 E1 < 0 E2 > 0 half cell X half cell Y [anode] [cathode]

  4. p.04 ref.: p. 204-205 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 (anode) (cathode) Pt(s) | H2(g,1atm) | 2H+(aq.1M) half cell under investigation

  5. p.05 Zn2+ excess –ve. charges excess +ve. charges SO42- (anode) (cathode) porous pot Cell Diagram : Zn(s) | Zn2+(aq) Cu2+(aq) | Cu(s) Electrochemical cells without Salt Bridge (1) …? Zn Zn2+ + 2e- Cu2++ 2e- Cu

  6. p.06 share the same electrolyte Cell Diagram Pt(s) | H2(g,1atm) | 2H+(aq.1M) Cl-(aq,1M) | AgCl(s) | Ag(s) Electrochemical cells without Salt Bridge (2) …? Pt(s) | H2(g,1atm) | HCl (aq,1M) | AgCl(s) | Ag(s)

  7. 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…)

  8. p.08 Ecell = 1.33 – (-0.76) = +2.09 V > 0 Ecell = -0.41 – (-0.76) = +0.35 V > 0 The rxn is favourable because of the large +ve. E value. ** Cr3+ is formed. The rxn is favourable because of the +ve. E value. 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).

  9. p.09 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. air sensitive! (easily oxidized by O2) 1995 HKAL Paper 2B Q.6(c) [4M]

  10. p.10 Ecell = 1.23 – (+0.11) = +1.12 V > 0 (anode) (cathode) 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.

  11. 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.

  12. p.12 Next …. Nernst Equation, Primary Cells (p. 213-214) 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)]

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