How to Balance Chemical Equations

Materials

• Pencil or pen
• Sheet of paper

Time Required

• 5-10 minutes

Experience

• Basic understanding of atoms and inorganic molecules
• Basic multiplication skills

Introduction

Chemical equations provide a formula for a chemical reaction between molecules that may include a single element or multiple elements. Generally, they follow the format of reactants to products, where "reactants" are the starting materials of your reaction and "products" are the end result. Abbreviations of element names are used to facilitate the equations. Abbreviations can be found in a periodic table of elements.

It is important to balance chemical equations in order to follow the Law of the Conservation of Mass. In simplified terms, the law states that there must be an equal number of atoms of each element in the reactants as in the products.

The instructions will examine balancing simple equations that contain 2 molecules for reactants and for products. The examples will only use whole numbers and will not discuss equations that involve complex ions, which is a molecule that has a charge.

When approaching a chemical equation, it is important that you understand the difference between coefficients and subscripts. The coefficient is placed in front of a molecule, while the subscript follows certain atoms as shown in the first picture.

In a molecule, the coefficient denotes the amount of that molecule present. The subscript of an atom indicates the amount of that atom in the molecule. For example, in the first picture the coefficient for the second term indicates that 3 molecules of H2 are present, and the subscript of the first term signifies that 2 atoms of nitrogen (N) are present per molecule of N2. If there is not a subscript present on an element, you can infer there is only one atom of that element.

Adding a coefficient in front of a molecule multiplies all atoms within that molecule by the number of the coefficient. If an atom has a subscript, the coefficient and the subscript multiply to yield the total amount of that atom in the molecule. For example, in the second picture, the coefficient for ammonia (NH3) on the products side is 2. The 2 is multiplied by the subscript of hydrogen which is 3, yielding a total number of hydrogen atoms equal to 6.

The coefficient is the part that can be changed and added when balancing an equation. Changing the coefficient changes the total number of that molecule. The subscript, however, cannot be changed. Altering a subscript would change the molecule itself.

1. Create a chart for the reactant and product sides of the equation
2. Count the number of atoms for each element on both sides of the equation
3. Fill in the charts with the appropriate atom counts

1. Choose an element with which to begin
2. Add a coefficient to the start of the molecule containing this element on one side
3. Choose a number for the coefficient that would cause the atom count to equal the atom count of that element on the other side of the equation
4. Alter the chart based on the change caused by the coefficient

In the above example, Fe was chosen as the element to start. A coefficient of 2 was added to the first term so that the new reactant atom count of Fe would be 2 in order to equal the atom count of 2 for the product Fe.

1. Add coefficients for the other elements as done in step 3
2. Alter the chart as the coefficients change the atom counts
3. It might be necessary to go through the process more than once as changing a coefficient can alter the atom count for an element that has already been balanced
4. Continue until the atom counts for each element on one side is equal to the atom counts on the other side

In the example, Cl was the next atom chosen. In order to equal the new atom count of 6 for the reactant Cl, the molecule containing Cl in the products must have a coefficient of 3.

The coefficient brings the product atom count of Mg up to 3. In order to compensate for this change, the reactant molecule containing Mg was given a coefficient of 3.

1. Begin with an atom that appears in one molecule on either side of the equation. As shown in the picture, oxygen appears in 3 different molecules on the product side. As oxygen will be involved in several molecules that must be given a coefficient to accommodate the other atoms, it is simpler to save it until the end when all other atoms have been balanced.

2. It is best to balance oxygen and hydrogen last. These atoms are typically involved in more than one molecule per side, as explained in the previous tip.

For more information about chemical equations:
http://www.visionlearning.com/en/library/Chemistry/1/Chemical-Equations/56

For a website tool that automatically balances chemical equations:
http://www.webqc.org/balance.php