(BCR) Balancing Chemical Equations Lesson

Balancing Chemical Equations

You should understand that chemical equations must be balanced because it's the law (law of conservation of matter), but how do you go about balancing chemical equations? For a verbal demonstration of how to balance equations, please watch the videos below. This will help get you started in practicing what to do. 

Steps to Balancing a Chemical Equation

When balancing a chemical equation, there are a number of steps that need to be followed.

  1. Identify the reactants and the products in the reaction. The reactants are located on the left of the arrow and the products are on the right of the arrow.
  2. Count the number of atoms of each element in the reactants and the number of atoms of each element in the products.
  3. If the equation is not balanced (the number of atoms of a particular element is not the same), change the coefficients of the molecules until the number of atoms of each element on either side of the equation balance (are the same).
  4. Check that the atoms are in fact balanced.

Let's go through the steps by looking at the reaction in which hydrogen (H2) and oxygen (O2) combine to form water (H2O) as seen in the chemical equation below.

H2 + O2 → H2O

Step 1: Identify the reactants and the products. You can determine from the chemical equation that hydrogen and oxygen are the reactants, and water is the product.

H2 + O2 → H2O

Reactant   |   Product

Step 2: Count the number of atoms of each element. Like equations in math, equations in chemistry must balance. There must be the same number of each type of atom in the products as there is in the reactants. In equation 1, count the number of hydrogen and oxygen atoms on each side of the arrow. There are two hydrogen atoms in both reactants and products. There are two oxygen atoms in the reactants but only one in the product. Therefore, equation 1 is not balanced.

Step 3: Use coefficients to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction. For example, two molecules of hydrogen would be written as 2H2. A coefficient of 1 usually isn't written. Coefficients can be used to balance the equation as follows:

2H2 + O2 → 2H2O

Step 4: Check to ensure that the equation is balanced. The equation above shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. The two molecules of hydrogen each contain two hydrogen atoms. There are now four hydrogen atoms in both reactants and products. There are 2 atoms of oxygen in both reactants and products. The equation is correctly balanced since there are the same number of each type of atom on each side of the equation.

Balancing a chemical equation involves a certain amount of trial and error.  In general, however, you should follow the steps above while continuing to repeat steps 2 and 3 until the equation is balanced.  One of the most important things to remember is when you balance chemical equations, never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients.

Practice Problems

Use the steps given above to balance the following chemical equations:

N2 + H2 → NH3

  • Step 1:   Identify the reactants and the products in the reaction.  

N2 + H2  → NH3

Reactant   |   Product

  • Step 2:   Count the number of atoms of each element.  

N2 + H2  → NH3

Reactant   |   Product

N - 2         N - 1

H - 2         H - 3

  • Step 3:   Use coefficients to balance chemical equations.

First Try:

N2 + H2 → 2NH3

Reactant   |   Product

N - 2         N - 1     2

H - 2         H - 3     6

The nitrogen atoms are now balanced, but the hydrogen atoms are not so must continue adding coefficients.  

Second Try:

N2 + 3 H2 →  2NH3

Reactant   |   Product

N - 2                N - 1   2

H - 2    6          H - 3   6

  • Step 4:  Check to ensure that the equation is balanced. Yes! There are 2 nitrogen atoms on each side of the equation and 6 hydrogen atoms on each side of the equation.

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