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Friday, May 29, 2009

How to Balance Redox Reactions - Balancing Redox Reactions

Balancing Redox Reactions 1 of 6- Half-Reaction Method

To balance redox reactions, assign oxidation numbers to the reactants and products to determine how many moles of each species are needed to conserve mass and charge. First, separate the equation into two half-reactions, the oxidation portion and the reduction portion. This is called the half-reaction method of balancing redox reactions or the ion-electron method. Each half-reaction is balanced separately and then the equations are added together to give a balanced overall reaction. We want the net charge and number of ions to be equal on both sides of the final balanced equation.

For this example, let's consider a redox reaction between KMnO4and HI in an acidic solution:

MnO4- + I- → I2 + Mn2+

Balancing Redox Reactions 2 of 6- Separate the Reactions

Separate the two half reactions:

I- → I2

MnO4- → Mn2+

Balancing Redox 3 of 6Reactions - Balance the Atoms

To balance the atoms of each half-reaction, first balance all of the atoms except H and O. For an acidic solution, next add H2O to balance the O atoms and H+ to balance the H atoms. In a basic solution, we would use OH- and H2O to balance the O and H.

Balance the iodine atoms:

2 I- → I2

The Mn in the permanganate reaction is already balanced, so let's balance the oxygen:

MnO4- → Mn2+ + 4 H2O

Add H+ to balance the 4 waters molecules:

MnO4- + 8 H+ → Mn2+ + 4 H2O

The two half-reactions are now balanced for atoms:

MnO4- + 8 H+ → Mn2+ + 4 H2O

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