Balancing Redox Reactions

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)

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

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

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:

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:

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

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

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

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

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

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)

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