Introduction to Chemical Equations

As we begin this section, I would like to point out a few very basic bits of information. These are things we won't dwell on, but I do want to make sure you know what they are.

First, there is the distinction between chemical reactions and chemical equations. A chemical reaction is what really happens. For example, when magnesium burns, a shiny, lustrous piece of flexible metallic ribbon is heated in the presence of a colorless, transparent gas and changes into a white-to-grey, opaque, crumbly, powdery solid. That is a chemical reaction. The chemical equation is a way of representing that reaction by using formulas to represent each of the chemicals involved.

When water is electrolyzed, we can write that H₂O becomes H₂ and O₂. "H₂O → H₂ + O₂" is an equation that represents the reaction. The reaction itself is the formation of two colorless gases, hydrogen and oxygen, when an electric current is passed through the colorless transparent liquid, water. It is important that you realize the distinction between reactions and equations.

Second, there are a few important terms that you may already know, but let's take this time to formalize it. The term "reactant" is the same as the term "reagent." It refers to each of the chemicals that are reacting with one another; in other words, what you have when you start the reaction. The chemicals that you get as the reaction proceeds are called the "products." So, as a chemical reaction proceeds, you start with reactants and you end up with products. When magnesium burns, you have magnesium as one reactant or reagent and oxygen as the other reactant or reagent. The magnesium oxide that you end up with is the product.

There are three types of equations with which you need to become familiar. They are word equations, skeleton equations and balanced equations. They are discussed below.

Word Equations

The first of the three types of equations are word equations. These are the simplest type of equation, as they don't even require formulas for the substances. An example is shown below: it illustrates a word equation for water being electrolyzed into the elements that make it up.

water → hydrogen + oxygen

We use an arrow to show which way the reaction is going. The arrow is usually read as "becomes" or "yields." The "+" sign is read as "and." Reactants go on the left side of the arrow, and products go on the right. The equation below has two reactants and two products.

methane + oxygen → water + carbon dioxide

Skeleton Equations

Word equations can be very useful. Generally, however, skeleton equations, in which we have substituted chemical formulas for the chemical names, are even more useful.

Skeleton equations are also sometimes called unbalanced equations. Using the first example above, the skeleton equation for the electrolysis of water would look like this:

H₂O → H₂ + O₂

This equation is more informative than a word equation because it shows the actual formulas of the reactants and products.

CH₄ + O₂ → H₂O + CO₂

We can also see the skeleton equation for the reaction of methane and oxygen. Note that you are not expected to know the formulas for these compounds - you don't need to know that "methane" means CH₄. In time, you will learn to turn some compound names into formulas and vice versa; for now, you'll just be working with formulas as they are given to you.

Balanced Equations

Word equations and skeleton equations should really just be seen as stepping stones on the way to balanced equations, as these are by far the most common and useful type of chemical equation. Balanced equations are important because they illustrate how mass is conserved in chemistry.

As you should remember from Lesson 2, one of Dalton's postulates was that atoms are never created or destroyed in reactions, only rearranged. This idea leads to the concept, earlier in this lesson, of conservation of mass. It also means that, when we write a chemical equation, it should not show atoms being created or destroyed. Let's take another look at our skeleton equation for water breaking down.

hydrogen: 2 oxygen: 1 H₂O → H₂ + O₂ hydrogen: 2 oxygen: 2

As you can see from the annotations, with the equation written this way, it appears that an oxygen atom is being created out of nowhere! We know from Dalton that this is impossible, and it is bad form for an equation to imply that it happens. It also makes this equation less useful for doing stoichiometry, as we will see later on.

For the equation to be balanced, it needs to have equal numbers of atoms of each element on both sides. In the next section, we will see how to take a skeleton equation and tackle the problem of balancing it. For now, just look at how changing this equation makes it balanced.

hydrogen: 4 oxygen: 2 2 H₂O → 2 H₂ + O₂ hydrogen: 4 oxygen: 2

We have added coefficients to the equation. They should be read as "X molecules (or other units) of ...". So the equation now reads as "two molecules of water forms two molecules of hydrogen and one molecule of oxygen." You can see the balanced equation for the reaction of methane and oxygen, complete with coefficients, below.

CH₄ + 2 O₂ → 2 H₂O + CO₂

These examples illustrate the three kinds of equations you will be dealing with: (1) word equations, (2) skeleton equations, and (3) balanced equations. Note that there is a progression - each one tells you a little bit more.

The word equations tell you what chemicals are reacting by name. The skeleton equations tell you the same thing but use the formulas, and the formulas tell you about the composition of the chemicals that you are working with. The balanced equations tell you how much of each chemical is involved - that is, the proportions in which they react.