Chemistry Essentials for Dummies

Chapter 7. Chemical Reactions

In This Chapter

· Differentiating between reactants and products

· Finding out how reactions occur

· Taking a look at types of reactions

· Understanding how to balance reactions

· Figuring out chemical equilibrium

· Checking out speeds of reaction

In a chemical reaction, substances (elements and/or compounds) are changed into other substances (compounds and/or elements). You can’t change one element into another element in a chemical reaction — that happens in nuclear reactions, as I describe in Chapter 4.

A number of clues show that a chemical reaction has taken place — something new is visibly produced, a gas is created, heat is given off or taken in, and so on.

In this chapter, I discuss chemical reactions — how they occur and how to write a balanced chemical equation. I also tell you about chemical equilibrium and explain why chemists often can’t get the amount of product out of a reaction that they thought they could. And finally, I discuss the speed of reaction.

Reactants and Products: Reading Chemical Equations

You create a new substance with chemical reactions. The chemical substances that are eventually changed are called the reactants, and the new substances that are formed are called the products.

Chemical equations show the reactants and products, as well as other factors such as energy changes, catalysts, and so on. With these equations, you use an arrow to indicate that a chemical reaction has taken place. Beneath or above this arrow people sometimes indicate that a specific catalyst is used, or acidic conditions or heat is applied, and so on. 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 in order to fry your breakfast eggs. Methane (natural gas) reacts with the oxygen in the atmosphere to produce carbon dioxide and water vapor. (If your burner isn’t properly adjusted to give that nice blue flame, you may also get a significant amount of carbon monoxide along with carbon dioxide.) You write the chemical equation that represents this reaction like this:

You can read the equation like this: One molecule of methane gas, CH₄(g), reacts with two molecules of oxygen gas, O₂(g), to form one molecule of carbon dioxide gas, CO₂(g), and two molecules of water vapor, H₂O(g). 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. I show you how to figure out the value of the coefficients in the section “Balancing Chemical Equations,” later in the chapter.

Methane and oxygen are the reactants, and 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, but you do feel another product of the reaction: heat. The heat being evolved is the clue that tells you a reaction is taking place.