Science Concepts and Questions (K to 12): Balancing Oxidation-Reduction Reaction

There are ways in which we can balance chemical equation, simple equations can be balanced using inspection method . Simple oxidation-reduction reaction can also be balanced by inspection method but complicated oxidation-reduction reaction cannot be balanced by this method. In this post I will be discussing two methods of balancing Redox reaction, the oxidation number method and ion-electron method.

Balancing Redox Reaction using Oxidation Number Method

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In balancing redox reaction using oxidation method, there are guidelines to follow:
1. Examine the reactants and products to determine if there is a change in the oxidation number by calculating the oxidation number or by inspection. Use the technique used in my previous blog about how to determine the oxidation number.

a. Write the oxidation number of the element above its symbol. See to it that all elements in the reactant side and in the product side have their oxidation number.

b. Diagram the number of electron lost by the oxidized element and the number of electron gained by the reduced element.

2. Balance each element using a coefficient and remember that the electron lost and gain are equal.

a. Place a coefficient before the formula of the oxidized substance that corresponds to the number of electrons gained by the reduced substance.

b. Place a coefficient before the formula of the reduced substance that corresponds to the number of electrons lost by the oxidized substance.

c. Balance the remaining elements by inspection.

3. After balancing the equation, check if the coefficient of the reactants and products are correct.

a. Place a check above the symbol of the element to indicate that the coefficient are correct. Meaning the number of atoms in the reactant side is equal to the number of atoms in the product side.

b. For ionic equation, make sure also that the charges both in the reactant side and product side are equal.

Let us have an example:

1. Balance using oxidation number method the reaction between Copper and Silver nitrate forming Copper (II) nitrate and solid silver, as shown in the unbalanced equation below:

Step 1. Write the oxidation number of each element:

Step 2. Diagram the number of electrons lost and gained.

Step 3. Place a coefficient before the formula of the oxidized substance that corresponds to the number of electrons gained by the reduced substance.

Based from the figure above Cu loses 2e-, therefore to balance the electrons gain coefficient of two is placed in front of AgNO3 and Ag. Since silver gains only 1 e- therefore only 1 is the coefficient of Cu

Step 4. Place a check above the symbol of the element to indicate that the coefficient are correct. Meaning the number of atoms in the reactant side is equal to the number of atoms in the product side.

Since number of atoms are balanced both in the reactant and product side, therefore the equation is already balanced.

Balancing Equation Using Ion-Electron Method in Acidic and Basic Solution

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Ion-electron method is also called half reaction method, used to balance ionic redox reactions in acidic and basic solution. A half reaction is a part of redox reaction that shows the element that undergoes oxidation and reduction process. The ion-electron method balances the oxidation half-reaction with the reduction half-reaction separately.

There are several steps to be followed in balancing ionic redox equation by ion-electron method.

1. Write the half-reaction both the oxidation and reduction. Write the reactant and the product of the element that undergoes oxidation, Also write the reactant and product of the element that undergoes reduction.

2. Balance the element in each half-reaction by writing coefficient.

a. Balance all elements except oxygen and hydrogen.

b. Balance oxygen using H2O.

c. Balance hydrogen using H+.

Please take note: For reaction in basic solution, add OH- is to neutralize H+.

For example 2OH- neutralizes 2H+ to form 2H2O molecules.

d, Balance the charge by using e-.

3. Multiply each half-reaction by a whole number to balance the number of electrons lost in oxidation process and the number of electrons gained in reduction process.

4. Add the two half-reactions together and cancel similar ions, including electrons on each side of the equation.

5. After balancing, check the number of atoms of all the elements if balanced. Also calculate the net charged if balance both in the reactant side and in the product side.

Let us have an example:

Example 1. Write a balanced equation on the reaction below in acidic solution:

Solution:

Step 1. Write the half-reaction both the oxidation and reduction.

Step 2. Balance all the elements and the charge.

Step 3. Multiply the two half reaction with a whole number to balance the number of e-. Since the first half-reaction has only one e- and the second half reaction has 5e- then the first half-reaction must be multiplied by 5.

Step 4. Add the two half-reactions together and cancel 5e- both in the reactant side and product side,

Step 5. Verify the equation if balance by checking the number of atoms and charges.

Example 2. Write a balance equation of the equation below in basic solution.

Solution:

Step 1. Write the half-reaction the oxidation and reduction.

Step 2. Balance all the elements, balance the O by adding H2O, and to balance H add H+. Balance charges by adding e-. Since this is for basic solution neutralize H+ with OH-.

In basic solution H+ should be neutralize with the same number of OH-.

Step 3. Balance the number of e- by multiplying whole number to the half reaction. Since the first half reaction lost 2e- and the second half-reaction gained 3e-, to balance the lost and gained of electrons multiply the first half-reaction with 3 to make it 6e- and the second half-reaction 2 to balance the e-.