chemical equation

A representation of a chemical reaction in which chemical symbols represent reactants on the left side and products on the right side.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

A Chemical Equation is a way of representing a chemical process taking place. It isn't an equation in the mathematical sense of the word, where the two sides are exactly equal (although see below) - rather, it is a progression from the reactants on the left hand side to the products on the right hand side. This may be something that takes place spontaneously, or with the input of some outside influence (such as heat or light).

A very simple example would be the combustion of methane gas (natural gas).

CH₄ + O₂ → CO₂ + H₂O

Here, we see Methane (CH₄) combining with Oxygen in the form of a diatomic molecule (O₂) on the left hand side, which goes to Carbon Dioxide (CO₂) and Water (H₂O) - although the water is in the form of steam.

More complex equations are, of course, possible, especially in the case of organic chemistry.

But, have you noticed something not quite right in the above equation? Although the two sides don't have to contain the same compounds, they do have to have the same number of individual elements. Looking at what we have above, we have the following.

        Left  |   |  Right
        ------+---+-------
           1  | C |  1
           4  | H |  2
           2  | O |  3

We have to make the numbers in the left and right columns for each element balance - hence this is known as balancing the equation. The actual process is somewhat trial-and-error - although what we are doing is figuring out how many molecules of each input compound combine to form how many molecules of each output compound.

In this case, if we write the equation as follows, it is balanced.

CH₄ + 2O₂ → CO₂ + 2H₂O

        Left  |   |  Right
        ------+---+-------
           1  | C |  1
           4  | H |  4
           4  | O |  4

We have said that each molecule of Methane combines with 2 molecules of Oxygen, to produce a single molecule of Carbon Dioxide, and 2 molecules of Water. This information is essential when setting up a Stoichiometric reaction - one where exactly the right amounts of source compounds are present to completely react together and produce the resultant products with nothing of the source left. Knowing the number of molecules of each reactant involved, one can work out the volume (for gasses) or the mass (for liquids) of the reactants involved to ensure complete combustion.

This is the complete chemical equation for the combustion of Methane. And the same principles apply to any chemical equation.