Chemical equations express possibly the most important chemical law ever
discovered. Antoine Lavosier, considered by many to be the father of modern
chemistry, expressed this law as follows:
“We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements.”
That is, matter is neither created nor destroyed. The amount and type of chemical elements remain the same before and after a chemical reaction; they are simply rearranged into different chemical compounds.
Chemical compounds are combinations of elements which cannot be separated by physical means (by freezing, boiling, or sorting). They exist as a result of chemical bonds between atoms of different elements. A chemical compound is defined by the ratio of each type of atom involved in the compound. Here are some examples:
AlCl3 is a compound of aluminum and chlorine. In each molecule of this compound there is always just one Al atom and exactly 3 Cl atoms.
K2SO4: two atoms of potassium, one atom of sulfur and four atoms of oxygen
Fe(OH)3: one atom of iron, three atoms of oxygen, and three atoms of hydrogen.
The little numbers below and to the right of the atomic symbols (K) and groups of atoms (OH) in these compounds are called subscripts. The subscripts tell you how many of each kind of atom are in the compound. When balancing chemical equations you may never change the subscripts. Changing subscripts changes the compound. For example, H2O is harmless but H2O2 is a strong oxidizer and is used as bleach and to kill bacteria.
In chemical reactions, which are represented by chemical equations, it is often the case that more than one molecule of a compound is required to react with the others. This is shown by a coefficient as follows:
2H2O - there are (2 × 2) atoms of hydrogen (total = 4) and (2 × 1) atoms of oxygen (total = 2).
2NH4NO3 - there are (2 × 1) + (2 × 1) atoms of nitrogen (total = 4), there are (2 × 4) atoms of hydrogen (total = 8), and (2 × 3) atoms of oxygen (total = 6).
2Mg(OH)2 - there are (2 × 1) atoms of magnesium (total = 2), there are (2 × 1 × 2) atoms of oxygen (a total of 4), and (2 × 1 × 2) atoms of hydrogen (total = 4).
Coefficients in front of chemical formulas are multipliers. A coefficient of 2 means that every atom is multiplied by two. If an atom or group of atoms has a subscript, then the subscript is multiplied by two as well.
To help you to be sure you understand these ideas, try a few problems. Each problem gives you a chemical formula either with or without a coefficient. Write the number of each type of atom in a chart.
Now that we have the background necessary, let’s move on to balancing chemical equations. The following is one method for taking on this challenging task.
For example, in the unbalanced equation O2 --> O3 the LCM is six. That means that there should be 6 oxygen atoms on each side of the equation. The equation is balanced by changing coefficients until this is so:
3O2 --> 2O3
For example, in the unbalanced equation KClO3 --> KCl + O2 the K and Cl are balanced with one of each on both sides of the equation. The oxygen has an LCM of 6 so you add coefficients: 2KClO3 --> KCl + 3O2. This makes it so that K and Cl don’t balance anymore. But by noticing that the LCM for both elements is now 2 you can add a coefficient to KCl on the products side to get the equation to balance: 2KClO3 --> 2KCl + 3O2The key here is to make a series of changes, each of which gets you closer to a balanced chemical equation.
Here is an example of how to approach the problem of
balancing a chemical equation: Fe + O2
In the unbalanced equation, there is only one Fe on the left and two on the right. Putting a two in front of the Fe on the left brings the irons into balance.
2Fe + O2 ---> Fe2O3
Now the oxygen needs to be balanced. This is a common problem, here is how to solve it: