Ettevalmistus Keemiaolümpiaadiks/Elektrokeemia

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Algoritm

  • Tuvasta oksüdeerija ja redutseerija (vt laengud ja oa)
  • Kirjuta välja (pool)reaktsioonid
  • Koosta skeem (kui küsitakse), muidu märka, milline poolreaktsioonidest toimub katoodil ja milline anoodil.
  • Arvuta standartne EMJ
  • Rakenda Nernsti võrrand


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IChO Montréal 1997 PP3

Pure zinc is in contact with well oxygenated (P(O2) = 1.000·105 Pa) aqueous solution concentrations of HCl and ZnCl2 are c(HCl) = 1.000 M and c(ZnCl2) = 1.000 M, respectively, and the temperature of the electrolyte is 25.00 °C. The dissolution of Zn in this solution is represented by the equation given below. A table of standard reduction potentials will be required for this question.

Zn + 2HCl + ½O2 → ZnCl2 + H2O

a) Does Zn dissolve in this solution or not?

b) If Zn does dissolve in this solution, when will the process cease in a spontaneous manner?

Oxidation process (process 1): Zn → Zn2+ + 2e

Reduction process (process 2): 2H+ + ½O2 + 2e → H2O

The reduction potential for the two abowe processes are E1° = −0.762 V and E2° = 1.229 V

Lahendus

a) Determination of the spontaneous direction of the reaction.

Oxidation process (process 1): Zn → Zn2+ + 2e

Reduction process (process 2): 2H+ + ½O2 + 2e → H2O

The reduction potential for the two abowe processes are E1° = −0.762 V and E2° = 1.229 V.

The standard potential of the overall process (the concentrations of Zn2+ and H+ equal unity) is:

E = E2° − E1° = 1.991 V

The Gibbs free energy, ∆G°, of the process equals:

ΔG° = −nFE° = −3.842·105 J mol−1

Because ΔG° is negative, Zn undergoes spontaneous dissolution.

b)

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http://www.pianetachimica.it/olimpiadi/29icho/29_Prep.pdf