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When components in a chemical reaction receive or lose electrons, the oxidation number increases or decreases, resulting in oxidation-reduction reactions. These reactions are balanced using the Half Equation Method.
One or more elements are oxidized and one or more elements are reduced in a redox reaction. The loss of electrons is oxidation, whereas the gain of electrons is reduction.
Consider the charges: when an element gets electrons, its charge is lowered (one acronym to remember the difference is LEO = Lose Electron Oxidation and GER = Gain Electron Reduction). Redox reactions normally take place in either an acidic or basic environment. Understanding oxidation states is required to balance redox equations.
When balancing redox reactions, keep the following in mind:
The equation is broken down into two halves, one for oxidation and the other for reduction.
Adjusting coefficients and adding H2O, H+, and e– in this order balances the equation:
Aside from O and H, balance the atoms in the equation.
Add the necessary number of water (H2O) molecules to the other side to balance the Oxygen atoms.
Add H+ ions to balance the Hydrogen atoms (including those added in step 2).
Total the charges on both sides. The more positive side must be brought equal by adding enough electrons (e-).
Each side’s e- must be made equal; if they aren’t, they must be multiplied by appropriate integers to make them equal.
To construct one balanced equation, the half-equations are combined together, canceling out the electrons. As much as feasible, cancel out.
(In a basic solution, the required amount of OH- must be added to convert the remaining H+ into water molecules.)
Now you can check to see if the equation is balanced.
Example 1A: Aqueous Acidic Solution
This reaction is balanced.
MnO4_ +I−⟶I2+Mn2+
Solution
Balance steps include:
Step 1: Separate the oxidation and reduction halves of the process.
Oxidation:
2I–⟶I2
Because the oxidation state moves from -1 on the left to 0 on the right, this is the oxidation half. This shows an electron gain.
Reduction:
MnO4–⟶Mn2+
Because the oxidation state shifts from +7 on the left to +2 on the right, this is the reduction half. This shows an electron decrease.
Step 2: To balance this half reaction, we must first balance all atoms except those that are Hydrogen or Oxygen.
Oxidation:2I–⟶I2
To balance the oxidation part of the reaction, place a 2 in front of the I on the left hand side so that both sides have the same number of atoms.
Reduction:
MnO4–⟶Mn2+
Because there is one manganese atom on both sides of the half reaction, all atoms other than Hydrogen and Oxygen are already balanced for the reduction half of the reaction.
Step 3: Add H2O to the side of the equation that requires oxygen to balance the oxygen atoms. After you’ve finished this step, add H+ to the side of the equation that doesn’t have enough H atoms to be balanced.
Oxidation:
2I−⟶I2
There is no need for any balance in this half of the process because there are no Oxygen or Hydrogen atoms.
Reduction:
MnO4–⟶Mn2++4H2O
To balance this reaction using step 3, add 4 H2O atoms to balance the Oxygen atoms with the 4 on the other side of MnO4-.
Step 4: Now that the two parts of the reaction have been correctly balanced, it’s time to balance the charges in each half reaction so that both the reduction and oxidation portions consume the same number of electrons.
Step 5: Multiply both sides of both reactions by the least common multiple that will result in the half-reactions having the same number of electrons and canceling each other out.
Step 6: By canceling out the electrons and any H2O and H+ ions on both sides of the equation, add the two half reactions to get the total equation.
Overall:
10I–+16H++2MnO4–⟶5I2+2Mn2++8H2O
Except for the electrons, there is nothing on both sides of the equation that can be canceled out in this case. Finally, double-check your work to ensure that both the mass and charge are equal. To double-check this equation, notice that both sides of the equation have an overall charge of +4.
Conclusion
When components in a chemical reaction receive or lose electrons, the oxidation number increases or decreases, resulting in oxidation-reduction reactions. These reactions are balanced using the Half Equation Method.
One or more elements are oxidized and one or more elements are reduced in a redox reaction. The loss of electrons is oxidation, whereas the gain of electrons is reduction.
Consider the charges: when an element gets electrons, its charge is lowered (one acronym to remember the difference is LEO = Lose Electron Oxidation and GER = Gain Electron Reduction). Redox reactions normally take place in either an acidic or basic environment. Understanding oxidation states is required to balance redox equations.
