Until now it was clear to me what balancing a reaction was. Balancing redox reaction also make sense to me. What does it mean to balance a redox reaction in acidic or basic medium?
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$\begingroup$ Well, some reactions may turn out different ways depending on the medium. $\endgroup$– Ivan NeretinNov 28, 2019 at 16:14
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$\begingroup$ I think you are confused by the word "medium." Most of the reactions are carried out in the liquid phase. When you are able to adjust the pH of a liquid phase, and redox reaction occurs, one can talk about an "acidic or basic" medium. $\endgroup$– AChemNov 28, 2019 at 16:55
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2 Answers
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In acidic medium the equation must mention somewhere that some $\ce{H+}$ ions appear somewhere in the equation. In basic medium, the equation must mention somewhere that some $\ce{OH-}$ ions appear somewhere in the equation,
For example, you may say that permanganate ion reacts in acidic conditions to produce $\ce{Mn^2+}$. In basic conditions, it could not produce $\ce{Mn^2+}$, because this ion will react with $\ce{OH-}$ ions to produce a precipitate of $\ce{Mn(OH)2}$.
As an application, the permanganate ion reacts with oxalic acid $\ce{H2C2O4}$ in acidic conditions according to :
$$\ce{2 MnO4- + 5 H2C2O4 + 6 H+ -> 2 Mn^2+ + 10 CO2 + 8 H2O}$$
Here, one sees that 6 ions $\ce{H+}$ are necessary for the reaction to happen. If no $\ce{H+}$ are present in the mixture permanganate + oxalic acid, this reaction cannot take place. Another reaction is produced without $\ce{H+}$. If you mix permanganate ion with oxalic acid in a basic solution, the first reaction that happen is the neutralization of the acid by the base namely :
$$\ce{H2C2O4 + 2 OH- -> C2O4^2- + 2 H2O}$$
So, here, it is not oxalic acid that reacts with permanganate, but the oxalate ion $\ce{C2O4^2-}$. The reaction will be :
$$\ce{2 MnO4- + 3 C2O4^2- + 4 OH- -> 2 MnO2 + 6 CO3^2- + 2 H2O}$$
Here you see that some $\ce{OH-}$ are needed for the reaction to proceed. It cannot happen in acidic medium.
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$\begingroup$ Protons or hydroxide ions must appear as reagents, if the solution is acidic or basic; not just somewhere. $\endgroup$– Martin - マーチン ♦Nov 28, 2019 at 19:12
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Say we have a given redox reaction: $$\ce{MnO4- + Fe^2+ -> Fe^3+ + Products}$$ This is an unbalanced redox reaction, just depicting how $\ce{Fe^2+}$ is oxidized to $\ce{Fe^3+}$ by $\ce{KMnO4}$. Now, depending on the medium, the following reactions may take place:
In an acidic medium, $\ce{MnO4-}$ is reduced to $\ce{Mn^2+}$ by accepting 5 electrons $$\ce{MnO4- + 5Fe^2+ + 8H+ -> 5Fe^3+ + Mn^2+ + 4H2O}$$
In a neutral/weakly basic medium, $\ce{MnO4-}$ is reduced to $\ce{MnO2}$ by accepting 3 electrons $$\ce{MnO4- + 3Fe^2+ + 2H2O -> 3Fe^3+ + MnO2 + 4OH-}$$
In a strongly basic medium, $\ce{MnO4-}$ is reduced to $\ce{MnO4^2-}$ by accepting 1 electron $$\ce{MnO4- + Fe^2+ -> Fe^3+ + MnO4^2-}$$
This is just one example of how different mediums can affect a given redox process; other redox reactions may also be influenced, depending on whether the medium is acidic or basic.
answered Nov 28, 2019 at 16:59
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$\begingroup$ In basic medium, $\ce{Fe^3+}$ does not exist. It is transformed in Fe(OH)3 $\endgroup$– MauriceNov 28, 2019 at 17:46