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For example, you have two salts in a solution. Say one of the species half reaction requires 1V (standard electrode potential), and another requires 3V, and you have a current running through the solution at 4v.

Here are the possibilities I thought of, I don't know which or if any, are right:

  1. Both species react simultaneously at the same rate.
  2. Both species react simultaneously, but the one that requires 1V reacts faster.
  3. The species that requires 1V reacts first, and once it is completely reacted, the species that requires 3V reacts after.

Am I completely off or what is the right answer?

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If you applied 4V right away, both species would begin reacting. Generally speaking, the higher you are above the half-reaction's themodynamic potential ("overpotential"), the faster the reaction goes, so the 1V would react faster. The rates are not determined solely by the overpotential, though, and at some point you'll hit the rate-limiting current. At this point, increasing the potential doesn't increase the rate any more because you can't supply reactants to the electrode surface fast enough from the solution bulk, so you could wind up with the 1V species reacting slower.

A more interesting experiment is cyclic voltammetry, where you slowly increase the potential from open-circuit up to 4 volts. Measuring the current as a function of potential tells you at what potential the reactions happen and something about their character. In this case you'd see the first species oxidized slightly above 1V and species 2 oxidize just above 3V. By the time you hit 4V there would not be much current (assuming the reaction is fast compared to the sweep rate), since many of the reactants have been consumed.

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