Often, energy is released or absorbed during a chemical reaction.
The energy for the welding torch is released when hydrogen and oxygen combine to form water (see the equation below). This welding torch burns hydrogen and oxygen to produce temperatures above 3,000°C. It can even be used underwater.
2H2 + O2 → 2H2O + energy
Again, the reverse of the welding torch reaction absorbs energy to take place.
This is the equation of this reaction: 2H2O + energy → 2H2 + O2
Sometimes, energy is released during a chemical reaction. Where does this energy come from? Chemical bonds break and form when atoms gain, lose, or share electrons. When such a reaction takes place, bonds break in the reactants and new bonds form in the products. In reactions that release energy, the products are more stable, and their bonds have less energy than those of the reactants. The extra energy is released in various forms—light, sound, and heat.
Sometimes, energy is absorbed during a chemical reaction. In reactions that absorb energy, the reactants are more stable, and their bonds have less energy than those of the products. Hence, to supply energy to the bonds of the less stable products, energy is absorbed. For an example, see the following equation.
2H2O + energy → 2H2 + O2
In this reaction, the extra energy needed to form the products can be supplied in the form of electricity.
Electrical energy is needed to break water into its components. This is the reverse of the reaction that takes place in the welding torch.
Endothermic and Exothermic
As you have seen, reactions can release or absorb the energy of several kinds, including electricity, light, sound, and heat.
When heat energy is gained or lost in reactions, special terms are used.
Endothermic reactions absorb heat energy. Exothermic reactions release heat energy.
You may notice that the root word therm refers to heat, as it does in thermos bottles and thermometers.
You might already be familiar with several types of reactions that release heat. Burning is an exothermic chemical reaction in which a substance combines with oxygen to produce heat along with light, carbon dioxide, and water.
Sometimes the energy is released rapidly. For example, charcoal lighter fluid combines with oxygen in the air and produces enough heat to ignite a charcoal fire within a few minutes.
Other materials also combine with oxygen but release heat so slowly that you cannot see or feel it happen. This is the case when iron combines with oxygen in the air to form rust.
The slow heat release from a reaction also is used in heat packs that can keep your hands warm for several hours.
Some chemical reactions and physical processes need to have heat energy added before they can proceed. An example of an endothermic physical process that absorbs heat energy is the cold pack.
The heavy plastic cold pack holds ammonium nitrate and water. The two substances are separated by a plastic divider. When you squeeze the bag, you break the divider so that the ammonium nitrate dissolves in the water. The dissolving process absorbs heat energy, which must come from the surrounding environment—the surrounding air or your skin after you place the pack on the injury.
Energy in the Equation
The word energy often is written in equations as either a reactant or a product.
Energy written as a reactant helps you think of energy as a necessary ingredient for the reaction to take place. For example, electrical energy is needed to break up water into hydrogen and oxygen. It is important to know that energy must be added to make this reaction occur.
Similarly, in the equation for an exothermic reaction, the word energy often is written along with the products. This tells you that energy is released.
You include energy when writing the reaction that takes place between oxygen and methane in natural gas when you cook on a gas range. This heat energy cooks your food. Energy from a chemical reaction is used to cook. (CH4 + 2O2 → CO2 + 2H2O + energy)