18.4
Potentiels d'oxydoréduction
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Q1: Why can't individual electrode potentials be measured directly?
Individual electrode potentials cannot be measured directly because electron transfer requires both a donor (reductant) and recipient (oxidant). Only the potential difference between two half-cells can be measured. To determine individual potentials, one electrode is assigned zero potential as a reference, allowing other electrodes to be measured relative to it.
Q2: What are the operating conditions of a standard hydrogen electrode?
The standard hydrogen electrode (SHE) operates at 25°C with an inert platinum electrode partially submerged in 1 molar hydrochloric acid and exposed to hydrogen gas at 1 atmosphere pressure. The SHE serves as the universal reference electrode with a potential of 0 V, allowing all other electrode potentials to be measured relative to it under standard-state conditions.
Q3: How do standard electrode potentials predict redox reaction direction?
Standard electrode potentials indicate whether reduction or oxidation will occur. More positive potentials show greater tendency for reduction. When two half-cells form a galvanic cell, if the cathode potential exceeds the anode potential, the cell potential is positive and the reaction is spontaneous. This allows prediction of electron flow direction and reaction spontaneity.
Q4: What does a positive standard electrode potential indicate about a species?
A positive standard electrode potential indicates that a species has a greater tendency to be reduced under standard conditions. Species with higher positive potentials are stronger oxidizing agents, meaning they more readily accept electrons. Conversely, negative potentials indicate greater oxidative potential, meaning the species is more easily oxidized.
Q5: Why does zinc dissolve in hydrochloric acid but copper does not?
Zinc dissolves in hydrochloric acid because its standard electrode potential (−0.76 V) is lower than hydrogen's potential, making the reaction spontaneous. Copper does not react because its standard electrode potential (+0.34 V) is higher than hydrogen's, making the reaction nonspontaneous. This demonstrates how electrode potentials predict metal dissolution in mineral acids.
Q6: How is the standard cell potential calculated from individual electrode potentials?
The standard cell potential is calculated by subtracting the anode potential from the cathode potential: E°cell = E°cathode − E°anode. A positive result indicates a spontaneous reaction. This calculation uses the individual standard electrode potentials measured relative to the standard hydrogen electrode reference.
Q7: Is standard electrode potential affected by changes in half-reaction stoichiometry?
No, standard electrode potential is an intrinsic property of a half-reaction and remains unaffected by changes in stoichiometry. Whether a half-reaction is multiplied by a coefficient or reversed, its standard electrode potential value stays constant. This makes electrode potentials reliable for predicting reaction spontaneity regardless of how reactions are balanced.