Stoichiometry: Chemical Formulas and Equations

Chemical equations


What happens to matter when it undergoes chemical changes?

The law of conservation of mass:

Atoms are neither created, nor distroyed, during any chemical reaction

Thus, the same collection of atoms is present after a reaction as before the reaction. The changes that occur during a reaction just involve the rearrangement of atoms.

In this section we will discuss stoichiometry (the "measurement of elements").

Chemical equations

Chemical reactions are represented on paper by chemical equations. For example, hydrogen gas (H2) can react (burn) with oxygen gas (O2) to form water (H20). The chemical equation for this reaction is written as:

The '+' is read as 'reacts with' and the arrow '' means 'produces'. The chemical formulas on the left represent the starting substances, called reactants. The substances produced by the reaction are shown on the right, and are called products. The numbers in front of the formulas are called coefficients (the number '1' is usually omitted).

Because atoms are neither created nor destroyed in a reaction, a chemical equation must have an equal number of atoms of each element on each side of the arrow (i.e. the equation is said to be 'balanced').

Steps involved in writing a 'balanced' equation for a chemical reaction:

  1. Experimentally determine reactants and products
  2. Write 'un-balanced' equation using formulas of reactants and products
  3. Write 'balanced' equation by determining coefficients that provide equal numbers of each type of atom on each side of the equation (generally, whole number values)

Note! Subscripts should never be changed when trying to balance a chemical equation. Changing a subscript changes the actual identity of a product or reactant. Balancing a chemical equation only involves changing the relative amounts of each product or reactant.


Consider the reaction of burning the gas methane (CH4) in air. We know experimentally that this reaction consumes oxygen (O2) and produces water (H2O) and carbon dioxide (CO2). Thus, we have accomplished step #1 above. We now write the unbalanced chemical equation (step #2):

Now lets count up the atoms in the reactants and products:

We seem to be o.k. with our number of carbon atoms in both the reactants and products, but we have only half the hydrogens in our products as in our reactants. We can fix this by doubling the relative number of water molecules in the list of products:

Note that while this has balanced our carbon and hydrogen atoms, we now have 4 oxygen atoms in our products, and only have 2 in our reactants. We can balance our oxygen atoms by doubling the number of oxygen atoms in our reactants:

We now have fulfilled step #3, we have a balance chemical equation for the reaction of methane with oxygen. Thus, one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.


The physical state of each chemical can be indicated by using the symbols (g), (l), and (s) (for gas, liquid and solid, respectively):



1996 Michael Blaber