Lesson 3: Chemical Reactions and Chemical Equations

In this lesson you will study some of the fundamental principles which govern all chemical reactions - in particular, those which deal with mass. However, we will start with observing chemical reactions and what is involved with distinguishing between chemical reactions and physical changes.

You will not be concerned with predicting whether two chemicals react or, if they do, what products are formed. Rather, given that information, you will learn how to predict the relative amounts of the various reactants and products that would be consumed or produced. This is the area of chemistry known as stoichiometry.

Another purpose of this lesson on chemical equations is to show you how to use chemical equations to represent chemical reactions. This includes learning how to balance chemical equations, and how to use balanced equations to come up with the weight relationships that you need to solve various kinds of chemical problems. In addition, you will learn how to categorize chemical reactions by looking at the equations that are used to represent them.

Much of this lesson involves using ratios and proportions. The central idea is that if two quantities (in our case, amounts of chemicals in a reaction) maintain a constant ratio to one another, then given one you can always determine the other - provided you know what that ratio is. For example, if the number of shoes in a room is always twice the number of people, then given the number of people, you can always figure out how many shoes are in the room (and vice versa).

As you work through the lesson and its many examples, try always to keep clear in your mind what the two quantities are and what their ratio is. If that much is clear, you should find the examples and problems become easy to follow and to work out.