Chris
Chemical reactions: They’re fundamental to chemistry; they make new things by rearranging other things. They can blow stuff up ... or freeze things quickly. In short, they are awesome.

Brittny
But why do some chemical reactions release massive amounts of energy, while others absorb energy? In a chemical reaction, the main change that occurs relates to the way atoms are connected (or bonded) to each other. In order to change those connections, bonds must be broken and new bonds must be formed. Let’s break down how energy is transferred in these reactions.

Chris
To understand the energy implications of chemical reactions, it’s important to keep in mind two key ideas:

1. It takes energy to break bonds.
2. Energy is released when bonds are formed.

To understand this, consider the chemical reaction between vinegar and baking soda. That’s right —the classic baking soda volcano experiment. The chemical reaction behind this science fair favorite involves baking soda—also known as sodium bicarbonate to chemists—and vinegar, otherwise known as acetic acid.

These compounds react to form the molecules sodium acetate, water, and carbon dioxide. The baking soda and vinegar are called the reactants. The sodium acetate, water, and carbon dioxide that are formed are called the products.

Before the atoms in acetic acid and sodium bicarbonate can be rearranged to form the products, some of the bonds between the atoms in those molecules must be broken, and because the atoms are attracted to one another, it takes energy to pull them apart.

Then, when the products are formed (sodium acetate, water, and carbon dioxide) energy is released because atoms that have an attraction for one another are brought back together.

By comparing the energy absorbed when bonds in the reactants are broken with the energy released when bonds in the products are formed, you can determine whether a chemical reaction releases energy or absorbs energy overall.

Brittny
Chemical reactions that release energy are called exothermic. In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants. Exothermic reactions are accompanied by an increase in temperature of the reaction mixture.

Chemical reactions that absorb (or use) energy overall are called endothermic. In endothermic reactions, more energy is absorbed when the bonds in the reactants are broken than is released when new bonds are formed in the products. Endothermic reactions are accompanied by a decrease in temperature of the reaction mixture.

Chris
You can use energy level diagrams to visualize the energy change during a chemical reaction. To understand these diagrams, compare the energy level of the reactants on one side with that of the products on the other side.

Consider, for example, a diagram that charts the energy change when a candle burns. Wax (C₃₄H₇₀) combusts in the presence of oxygen (O₂) to yield carbon dioxide (CO₂) and water (H₂O). Because more energy is released when the products are formed than is used to break up the reactants, this reaction is exothermic.

Brittny
All of this stuff relates to thermodynamics—the study of heat and its relationship to energy and work. Using thermodynamics, you’ll learn how to calculate the precise amount of energy used or released by chemical reactions. Classifying a chemical reaction as exothermic or endothermic is simple. It comes down to weighing the energy needed to break bonds in the reactants with the energy released when the products are formed.

It’s a simple idea, but one with a lot of power.