Oxidation Reduction (redox) Balancing

  1. When balancing oxidation reduction reactions involving acids or bases, what (in the case of acids) allows you to add H+ ions to one side of the reaction and H2O to the other, or in the case of bases, OH- and H2O to balance out hydrogen/oxgen? Normally you are not allowed to just add random compounds to either side of the equation.

    I suspect this has something to do with the ability of H+ to grab oxygen atoms to form H2O, as similarly with 2OH- to supply Oxygen atoms and give H2O, as demonstrated by these two reactions:
    [itex]2H^{+}\; +\; O^{2-}\; -->\; H_{2}O[/itex]

    [itex] 2OH^{-}\; -->\; H_{2}O\; +\; O^{2-} [/itex]

    However, is this the real reason? And instead of using H+ ions for the acids, wouldn't it be more appropriate to use the hydronium ion, H3O+?

    In case the question is not clear, here is an example reaction.

    [itex]MnO_{4}^{-}\; +\; I^{-}\; -->\; I_{2}\; +\; Mn^{2+}[/itex]

    One of the half reactions would be:

    [itex]5e^{-}\; +\; MnO_{4}^{-}\; +\; 8H^{+}\; ->\; Mn^{2+}\; +\; 4H_{2}O[/itex]

    However, in this half reaction what allows you to add H+ to one side and H2O to the other?
  2. jcsd
  3. Borek

    Staff: Mentor

    These reactions usually take place in water, and typically in either low or high pH - so you can safely assume there is plenty of H2O and H+/OH- around. As there is plenty of them, they can be between reactants and products and you won't even notice changes in their concentrations.
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