Chemical Reaction

Many chemical reactions take place in labs. However, most chemical reactions do not. Where do they occur? They happen in the world all around you. They even happen inside your own body. In fact, you are alive only because of the many chemical reactions that constantly take place inside your cells.

What Is a Chemical Reaction?

A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right).

Reactants → Products

Chemical reactions may occur quickly or slowly. Look at the two pictures in the Figure below. Both represent chemical reactions. In the picture on the left, a reaction inside a fire extinguisher causes foam to shoot out of the extinguisher. This reaction occurs almost instantly. In the picture on the right, a reaction causes the iron tool to turn to rust. This reaction occurs very slowly. In fact, it might take many years for all of the iron in the tool to turn to rust.

[Figure 2]

Q: What happens during a chemical reaction? Where do the reactants go, and where do the products come from?

A: During a chemical reaction, chemical changes take place. Some chemical bonds break and new chemical bonds form.

Same Atoms, New Bonds

The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. This happens because chemical bonds break in the reactants and new chemical bonds form in the products.

Consider the chemical reaction in which water forms from oxygen and hydrogen gases. The Figure below represents this reaction. Bonds break in molecules of hydrogen and oxygen, and then new bonds form in molecules of water. In both reactants and products there are four hydrogen atoms and two oxygen atoms, but the atoms are combined differently in water. 

[Figure 3]

Types of Chemical Reactions

The chemical reaction in the Figure above, in which water forms from hydrogen and oxygen, is an example of a synthesis reaction. In this type of reaction, two or more reactants combine to synthesize a single product. There are several other types of chemical reactions, including decomposition, replacement, and combustion reactions. The Table below compares these four types of chemical reactions.

Q: The burning of wood is a chemical reaction. Which type of reaction is it?

A: The burning of wood—or of anything else—is a combustion reaction. In the combustion example in the table, the fuel is methane gas (CH₄).

Energy In and Energy Out

All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In terms of energy, there are two types of chemical reactions: endothermic reactions and exothermic reactions.

• In exothermic reactions, more energy is released when bonds form in products than is used to break bonds in reactants. These reactions release energy to the environment, often in the form of heat or light.

• In endothermic reactions, more energy is used to break bonds in reactants than is released when bonds form in products. These reactions absorb energy from the environment.

Q: When it comes to energy, which type of reaction is the burning of wood? Is it an endothermic reaction or an exothermic reaction? How can you tell?

A: The burning of wood is an exothermic reaction. You can tell by the heat and light energy given off by a wood fire.

From Reactants to Products

All chemical reactions—including a candle burning—involve reactants and products.

• Reactants are substances that start a chemical reaction.

• Products are substances that are produced in the reaction.

When a candle burns, the reactants are fuel (the candlewick and wax) and oxygen (in the air). The products are carbon dioxide gas and water vapor.

Relating Reactants and Products

The relationship between reactants and products in a chemical reaction can be represented by a chemical equation that has this general form:

Reactants → Products

The arrow (→) shows the direction in which the reaction occurs. In many reactions, the reaction also occurs in the opposite direction. This is represented with another arrow pointing in the opposite direction (←).

Q: Write a general chemical equation for the reaction that occurs when a fuel such as candle wax burns.

A: The burning of fuel is a combustion reaction. The general equation for this type of reaction is:

Fuel + O₂ → CO₂ + H₂O

Q: How do the reactants in a chemical reaction turn into the products?

A: Bonds break in the reactants, and new bonds form in the products.

Breaking and Making Chemical Bonds

The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms end up in different combinations in the products. This makes the products new substances that are chemically different from the reactants.

Consider the example of water forming from hydrogen and oxygen. Both hydrogen and oxygen gases exist as diatomic (“two-atom”) molecules. These molecules are the reactants in the reaction. The Figure below shows that bonds must break to separate the atoms in the hydrogen and oxygen molecules. Then new bonds must form between hydrogen and oxygen atoms to form water molecules. The water molecules are the products of the reaction.

[Figure 4]

Signs of Chemical Reactions

A change in color is just one of several potential signs that a chemical reaction has occurred. Other potential signs include:

• Change in temperature. Heat is released or absorbed during the reaction.

• Formation of a gas. Gas bubbles are released during the reaction.

• Formation of a solid. A solid settles out of a liquid solution. The solid is called a precipitate.

• Color Change - A substance may change color during a reaction, indicating that new substances are formed.

• Odor Change: A new smell may develop as reactants are transformed into products, signaling a chemical change.

• Light Emission: Some reactions release energy in the form of light, such as combustion reactions.

Examples of Chemical Reactions

Look carefully at the Figures below, below, and below. All of the photos demonstrate chemical reactions. For each photo, identify a sign that one or more chemical reactions have taken place. 

[Figure 5]

A burning campfire can warm you up on a cold day.

[Figure 6]

Dissolving an antacid tablet in water produces a fizzy drink.