SCH4U - Chemistry 12 (2024-25) - A

Balancing Chemical Equations

The following presentation explores techniques for balancing chemical equations.   When balancing redox reactions you must first follow the steps outlined in the learning activities in the rest of the lesson.

When there are unequal numbers of atoms of elements between the products and reactants, the equation is “unbalanced”. For example, an unbalanced reaction between hydrogen and nitrogen gas to produce ammonia could look like this:

N₂ + H₂ --> NH₃

On the reactants side, there are 2 N and 2 H. However on the products side, there is 1 N and 3 H. Therefore the equation is unbalanced.

A balanced chemical equation will have the same total number of atoms of each element in the reactants as in the products.

Unbalanced: N₂ + H₂ --> NH₃

When a reaction has been successfully balanced, there will be an equal number of each type of atom in the reactants and products.

Balanced: N₂ + 3H₂ --> 2NH₃

Balancing chemical equations is a process involving adjusting the coefficients in front of different molecules and atoms. It often takes going back and forth, adjusting the coefficients until a fully balanced equation is achieved.

The step-by-step process is as follows:

1. For each element, count the number of atoms on each side of the arrow.

2. Identify any unbalanced elements.

3. Multiply the chemical formulas by appropriate coefficients. Keep checking and adjusting the coefficients until all of the elements are balanced. This may take some trial and error. Note: You cannot adjust the subscripts.

4. Check to make sure the equation is balanced.

5. Write the final chemical equation.

Balance the following equation: N₂ + H₂ --> NH₃

1. For each element, count the number of atoms on each side of the arrow.

Reactants: • 2 N • 2 H

Products: • 1 N • 3 H

2. Identify any unbalanced elements. Both the N and the H are unbalanced as there are unequal numbers of both on either side of the arrow. 

3. Multiply the chemical formulas by appropriate coefficients. Keep checking and adjusting the coefficients until all of the elements are balanced. This may take some trial and error. Since we have 2 N on the left, we need 2 N on the right. If we add the coefficient 2 in front of NH₃, this will give us 2 N on the product side. 

N₂ + H₂ --> 2NH₃

Reactants: • 2 N • 2 H

Products: • 2 N (2 x 1 = 2) • 6 H (2 x 3 = 6)

The nitrogen atoms (N) are now balanced. However, the equation is still not balanced, because the hydrogen (H) atoms are unbalanced. Therefore, we must adjust the coefficients again. By multiplying the number of H2 molecules on the left by 3 this will balance out the H atoms. 

N₂ + 3H₂ --> 2NH₃

Reactants: • 2 N • 6 H (3 x 2 = 6)

Products: • 2 N • 6 H

There are equal numbers of nitrogen atoms and hydrogen atoms on each side of the equation. Therefore, the balanced chemical equation for the production of ammonia from hydrogen and nitrogen is:

N₂ + 3H₂ --> 2NH₃

Test your understanding.

Balance the following equation which shows the combustion of methane gas. CH₄ + O₂ --> CO₂ + H₂O

Reactants: • 1 C • 4 H • 2 O

Products: • 1 C • 2 H • 3 O

There are 4 H on the left and 2 H on the right. Therefore if a coefficient of 2 is added to the H₂O molecule, the H is balanced. 

CH₄ + O₂ --> CO₂ + 2H₂O

Reactants: • 1 C • 4 H • 2 O

Products: • 1 C • 4 H • 4 O

The C and H are now balanced, but the O is unbalanced. If a coefficient of 2 is added to the O₂ molecule on the left, the O will balance out

CH₄ + 2O₂ --> CO₂ + 2H₂O

Reactants: • 1 C • 4 H • 4 O

Products: • 1 C • 4 H • 4 O

Therefore, the balanced equation for the combustion of methane is: CH₄ + 2O₂ --> CO₂ + 2H₂O