Balancing Redox Reactions

1. Balancing Redox Reactions

2. Redox Reactions • Electrochemistry is the branch of chemistry that studies the interconversion between electrical energy and chemical energy • An electrochemical process is a redox reaction in which either • the energy released by a spontaneous reaction is converted into electricity • electrical energy is used to force a non- spontaneous reaction to occur • A redox reaction implies the transfer of electrons from one substance to another • Mg is oxidized (loses electrons) • H is reduced (gains electrons)

3. Balancing Redox Reactions • For redox reactions, it is often very difficult to balance the chemical equation without a systematic approach • Systematic methods exist to balance complex redox reactions • In the ion-electron method: • The overall reaction is divided into two half-reactions (an oxidation and a reduction) • Each half-reaction is balanced • The two balanced half-reactions are added together to give the overall balanced equation

4. The Ion-Electron Method • Use the following reaction as an example in an acidic medium. According to the oxidation states, Cu is oxidized and N is reduced. • Step (1): Write the unbalanced equation for the reaction in its ionic form • Step (2): Separate the equation into two half reactions

5. The Ion-Electron Method • Step (3): Balance the number of atoms other than O and H in each of the half-reactions • This is already the case in our case, so we have nothing to do • Step (4): For reactions in acidic media, add H2O molecules to balance the number of O atoms, and then add H+ ions to balance the number of H atoms • There is nothing to do for the oxidation • For the reduction:

6. The Ion-Electron Method • Step (5a): Add electrons to one side of each half-reaction to balance the charges oxidation: reduction: • Step (5b): If necessary, make the number of electrons in the two half- reactions equal by multiplying one or both half-reactions by appropriate coefficients oxidation: reduction:

7. The Ion-Electron Method • Step (6): Add the two half reactions together and balance the final equation by simplifying it (electrons on both sides must be eliminated) • Step (7): Verify that the charges and the number of atoms of each element are balanced

8. The Ion-Electron Method • When the reaction occurs in a basic medium, step (4) is modified in the following way: • For each H+, add OH- on each side • When there are H+ and OH- on the same side, combine the two ions to make H2O • eg.; Balance the following redox reaction in a basic solution

9. The Ion-Electron Method • Step (1): • Step (2): • oxidation: • reduction: • Step (3): Not necessary

10. The Ion-Electron Method • Step (4): • oxidation: • reduction:

11. The Ion-Electron Method • Step (5a): • oxidation: • reduction: • Step (5b): • oxidation: • reduction:

12. The Ion-Electron Method • Step (6): • Step (7): The charges are balanced and so are the atoms of each element (O, H, C, N, Mn)

13. The Ion-Electron Method • Balance the following reaction in basic medium : Cr2O72-(aq) + C3H8O(aq)  Cr3+(aq) + C3H5O2-(aq)