Interpreting Chemical Equations

If the formula of a molecule is like vocabulary in the language of chemistry, then chemical equations are the sentences.  Chemical equations show what happens during a chemical reaction in a shorthand that is relatively simple to learn but that is vital to the calculations necessary for stoichiometry.   It is helpful to think about chemical reactions as interactions between large populations of molecules or moles.  The idea is to write a chemical equation that maintains the relationship between the number of moles of each substance involved in the chemical reaction.  Then it is easy to convert any number of moles of a substance into mass of that substance. If you would like to review the concept of the mole, you should take a look at the next unit.

Consider the combustion reaction between nitrogen and oxygen that produces NO in a combustion engine.  If the gases were weighed, we might find the following:

You can see from this equation that mass is conserved.  The sum of the reactant masses is equal to the sum of the product masses.  But how many moles of each reactant were necessary to produce the 60.02 grams of NO?  Was there some excess reactant, that is, reactant that was not used in this reaction?   The only way to answer these questions is to convert the mass units from the equation to number of moles of each compound.
 

Now we can clearly see that all the moles of reactant where needed to make the moles of product in this particular reaction.

A chemical equation is a statement in formulas that expresses the identities and quantities of substances involved in a chemical or physical change.  The left side of the equation represents the substances before the reaction occurs, known as the reactant.  The right side shows the substances that were changed during the reaction, or the products. The arrow indicates that a chemical change occurs.