A chemical reaction is a recombination of atoms that produces products with new chemical identities. Chemical reactions power life, and provide most of the energy used by today's high energy society. Humans use an uncountable number of chemical reactions to make modern society possible; for example, to make products and prepare food. Chemical reactions are often accompanied by the release or absorption of energy, or the emission or absorption of light. Chemical reactions rearrange atoms into new molecules, but no new atoms are created or destroyed.
Most chemical reactions are in one of the following four forms:
Chemical reactions provide a way to get at primary energy in fuels (both primary fuels like coal, oil, and natural gas and also secondary fuels like molecular hydrogen and gasoline). When chemicals react, chemical bonds are broken and then restructured. When chemical bonds break, energy is absorbed (it takes energy to break a chemical bond). When the atoms recombine into new molecules, energy is given off (from the chemical bonds forming). When getting energy from a fuel, more energy is given off from the new molecules forming than went into breaking the bonds in the first place.
This is why hydrocarbons have to be brought to a certain temperature before they'll combine with oxygen in combustion. The energy released from one hydrocarbon undergoing combustion will lead to another molecule undergoing the same chemical reaction. This is a chemical chain reaction. Technology like heat engines has been developed to get energy out of chemical reactions in this way. There will always be molecules in a lower energy state coming out of these reactions. The two biggest are carbon dioxide and water.
Fuel cells also get energy from chemical reactions, but of a different sort.
Chemical reactions involve the electric force, which is the attraction between atoms. Less energy is involved in chemical reactions than in nuclear reactions, because the latter involve the strong force, or attraction between nucleons, protons and neutrons, in the nucleus.
Chemical energy is the energy released by breaking the bonds between atoms in a molecule. Because these bonds are relatively weak compared to the strong force, it is easy to gain the energy (provide a spark to gasoline, and it will ignite). In a similar vein, breaking the bonds of the much stronger strong force, produces much more energy, but is more difficult. By comparison, uranium is a million times more energy dense than gasoline, because its energy is released by breaking the nucleus.
The University of Colorado has graciously allowed us to use the following PhET simulation. Explore this simulation below to see how all of the different chemical elements recombine to form new molecules.