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For homework problem 14.1, it asks you to write the oxidation and reduction half reactions for the reaction: H+(aq) + Cr2O7^2- (aq) + C2H5OH (aq)---->Cr^3+(aq) C2H4OH (aq) + H2O (l). According to the solutions manual, the balanced reduction half reaction is Cr2O7^2- (aq) + 14H+(aq) + 6e- ----> 2Cr^3+ (aq) + 7H2O (l). Since only Cr is being reduced, why is H+ and H2O included in the reduction half reaction? how does one know when to include other compounds in an oxidation or reduction half reaction?
In a half reaction, by just writing out the oxidation that occurs, it may not be balanced. Hydrogen ions are used to balance any excess hydrogens on the other side of the reaction, while adding water molecules does the same for balancing oxygens. So we don't know for sure from the beginning that any oxidation or reduction reactions will contain hydrogen and/or water. We must write out the half reaction and use them to balance.
How I usually do it is that I use the simple reduction and oxidation reactions and once I have those down into one overall reaction, I balance the charges adding H protons, and balance the H and Os adding water.
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