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Are the half-reactions used in analyzing redox reactions a real thing or are they a conceptual shortcut?

Is there a way to, however briefly, run a half-reaction by itself without the corresponding other half? For example, reduce metal atoms to metal ions without the corresponding oxidation half-reaction?

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Electrolysis/Galvanic cell is basically redox reaction split in half-reactions, running on different electrodes. For example, hydrogen fuel cell has two electrodes. On one hydrogen donates electrons to electrode:

$H_2 = 2H^+ + 2e^-$

on another, oxygen drains electrons from electrode

$O_2 + 4 e^- + 4 H^+ = 2 H_2O $

The electrons travel from one electron to another via external electricity consumer. If electrons are forced to travel by external force, the reaction may be reversed. This ideas are utilized in electric chemical batteries, accumulators, fuel cells, industrial electrolyzers and so on.

This way, half-reactions are separated between different electrode. Running half-reaction by itself without other half elsewhere is problematic at best, because of electroneitrality violation. You'll need extreme voltage source and even then it will stop quickly.

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  • $\begingroup$ When you say "extreme voltage source" -- aren't the voltages involved rather small (1-3 volts)? I understand that if the electrode becomes surrounded by charges that counteract the half-reaction, it will be squelched quickly. Assuming there was another mechanism to remove the half-reaction products, the half-reaction might proceed indefinitely. $\endgroup$ – thinker Oct 1 '12 at 15:47
  • $\begingroup$ @thinker If half-reaction products will not discharge (wich requires other half-reaction to run), you will simply charge up some capacitor (in electrical engineering meaning). Molar charge of electron is ~96 485 C/mol. So to charge a capacitor with chemically visible amount of electrons (some millimoles) one need to store about 100 culons. Typical capacitances are, to my knowledge, measured in microfarads, so about several MV are required to run measurable amount of half-reaction without product discharge. $\endgroup$ – permeakra Oct 1 '12 at 16:01

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