A foundational skill in the field of chemistry, balancing chemical equations is essential to understanding chemical reactions. The atoms and molecules of chemicals often recombine into other molecules. Chemical equations show this recombination using the symbols of various periodic elements and their numerical amounts.
For example, the burning of propane to create water and carbon dioxide can be depicted as: C3H8 + O2 H2O + CO2. Representing a transformative chemical change, the arrow symbol in this chemical equation acts very much like the equal sign (=) in the traditional mathematical equation.
Because matter cannot be created or destroyed, chemical equations must contain the same number of atoms on each side. You can use different techniques to balance a simple chemical equation, but a traditional balancing process takes just a few steps.
1. Record the number of atoms for each element
Elements without a subscript exist as a single atom. Otherwise, the subscript indicates the number of atoms for that element. Using the provided chemical equation as your guide, record the number of atoms for each element. First, do this for the left (pre-change) side of the equation. Then, do it for the right (post-change) side of the equation. To balance the equation C3H8 + O2 H2O + CO2, you should list C=3, H=8, and O=2 for the left side and C=3, H=2, and O=7 for the right side. It may be helpful to list this information in two vertical columns so that each element sits beside its counterpart on the other side of the equation.
2. Save the most common elements for last
With your two coordinated columns in place, you are ready to add coefficients to balance your equation. However, you should approach this process strategically. For easier balancing, you will want to save extremely common elements, such as hydrogen and oxygen, for last. Because hydrogen and oxygen are present in so many molecules, you are quite likely to encounter them on both sides of your equation.
3. Start with single elements
Check your chemical equation for single elements that have no subscripts. If you find any, this is the best place to start the balancing process. Begin with any single elements that appear on both sides of the equation and then move on to those that appear on one side or the other. In the case of C3H8 + O2 H2O + CO2, you should balance the single carbon atom first.
4. Use coefficients to balance your equation
Because there are three carbon atoms on the left side of the equation C3H8 + O2 H2O + CO2 and there is only a single carbon atom on the right, you must add a coefficient of 3 to the right side in order to balance this element. To record this coefficient, place it in front of the carbon dioxide (CO2) molecule to triple it. Now that you have three carbon dioxide molecules, you not only have a total of 3 carbon atoms on the right side of your equation, but you have increased the total number of oxygen atoms to 7. With your carbon atoms balanced at 3 on both sides of your equation, you are now ready to move on to your hydrogen atoms. Because you can add coefficients but never change molecular subscripts, you must add a coefficient of 4 to your water (H2O) molecule on the right side of your equation. Now you are balanced with 8 hydrogen atoms on both sides. However, you must also add three oxygen atoms to the right side of your equation to make a total of 10. Finally, you must balance the number of oxygen atoms in your equation. This means adding a coefficient of five to the oxygen (O2) on the left side of your equation. Now, every element on both sides of your equation has an equal number of atoms!