Ettevalmistus Keemiaolümpiaadiks/Elektrokeemia
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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