The balancing of chemical equations is based on the Law of Conservation of Mass, which states that matter cannot be created nor destroyed. This means that the number of reactant atoms must be the same as the number of product atoms.
We call these elements, which don't change after a chemical reaction, immutable groups.
Intro: Why Must We Balance Chemical Equations?
A chemical equation serves as a symbolic representation of a chemical reaction and so knowing the numbers of each compounds involved is essential.
For the following equation the numbers in blue are called your coefficients and represent the number of moles for each compound shown.
This written chemical reaction tells us a story that states, “2 moles of hydrogen, H2 , react with 1 mole of oxygen , O2 , to produce 2 moles of water, H2O. ”
The numbers in red are the subscripts and combined with the coefficients give us the number of each particular element.
Determining the number of each element will also be needed for later calculations dealing with reaction stoichiometry, limiting reagent and identifying reaction types.
Instructions for Balancing
Balancing a chemical reaction doesn’t really require algebra, an Algebraic Balance or even a calculator. All you need to do is follow these instructions and you’ll be balancing like a pro.
STEP 1: Set up a list for the elements that are reactants and another list for the elements that are products. If your numerical values in both lists don’t match then you’ll have to balance the chemical equation.
The following chemical reaction deals with the combustion of gaseous butane:
STEP 2: Start at the top and going down both lists make sure to balance each element one by one.
STEP 3: At this point notice there are 4 carbon atoms as reactants versus 1 carbon atom as a product.
Balance the number of carbon atoms by placing a “4” in front of CO2. The “4” is distributed so the number of carbon and oxygen atoms are both affected.
STEP 4: Continuing down the lists we next balance the number of hydrogen atoms.
The reactant side has 10 hydrogen atoms, while the product side has only 2 hydrogen atoms. So placing a 5 in front of H2O will give 10 hydrogen atoms (2 x 5).
In addition the 5 in front of H2O is distributed to oxygen. We now have 5 oxygen atoms from H2O and 8 oxygen atoms from CO2, giving us a current total of 13 oxygen atoms.
STEP 5: At this point we need to balance our oxygen atoms and in order to have 13 oxygen atoms on both sides we place “6.5” in front of O2.
STEP 6: Although the chemical equation is balanced because both lists have identical numerical values, you want your coefficients to be whole numbers and not decimals or fractions.
STEP 7: This additional step is only necessary when you have a decimal or fraction as a coefficient. When balancing a chemical equation you need all the coefficients to be whole numbers.
In this example, in order to convert 6.5 into a whole number we multiply the entire chemical equation by 2.
FOR EXPERTS ONLY: Eventually if you keep up with your chemistry studies you’ll be asked to do more advanced chemical reactions (redox, acids and bases, precipitation) while using simple stoichiometry.
You may have to balance redox reactions in acidic solutions or basic solutions. Again you won’t have to balance these reactions algebraically or with advanced math.
You may even be given just the reactants without the products. In this case you’ll have to deal with charges that are written as superscripts. These charged ions will be in the form of a cation and an anion and will include finding the net ionic equation.