Reactants and Products in Chemical Reactions

In a chemical reaction, substances (elements and/or compounds) called reactants are changed into other substances (compounds and/or elements) called products. You can’t change one element into another in a chemical reaction — that happens in nuclear reactions. Instead, you create a new substance with chemical reactions.

A number of clues show that a chemical reaction has taken place, for example:

• Something new is visibly produced

• A gas is created

• Heat is given off or taken in

Chemical equations show the reactants and products, as well as other factors such as energy changes, catalysts, and so on. With these equations, an arrow is used to indicate that a chemical reaction has taken place. In general terms, a chemical reaction follows this format:

Reactants→Products

For example, take a look at the reaction that occurs when you light your natural gas range. Methane (natural gas) reacts with the oxygen in the atmosphere to produce carbon dioxide and water vapor. The chemical equation that represents this reaction is written like this:

You can read the equation like this: One molecule of methane gas reacts with two molecules of oxygen gas to form one molecule of carbon dioxide gas and two molecules of water vapor. The 2 in front of the oxygen gas and the 2 in front of the water vapor are called the reaction coefficients. They indicate the number of each chemical species that reacts or is formed.

Methane and oxygen (oxygen is a diatomic — two-atom — element) are the reactants, while carbon dioxide and water are the products. All the reactants and products are gases (indicated by the g’s in parentheses).

In this reaction, all reactants and products are invisible. The heat being evolved is the clue that tells you a reaction is taking place. This is a good example of an exothermic reaction, a reaction in which heat is given off. A lot of reactions are exothermic. Some reactions, however, absorb energy rather than release it. These reactions are called endothermic reactions.

You have to supply a little energy to get a reaction going. The energy you have to supply to get a reaction going is called the activation energy of the reaction.