Chemical energy is stored and released by the making and breaking of bonds between atoms. It is typically released when bonds are formed, and it is required to break them. Sometimes, however, compounds can be formed that store energy and release it later by undergoing chemical reactions, or rearranging themselves into molecules that, together, have less energy. These compounds can be created by natural processes, and by man. It is also possible to produce electricity chemically. There are many examples of chemical energy in action, both natural and man-made, including photosynthesis, respiration, combustion, explosives and batteries.
A chemical reaction will take place when the atoms involved can reach a lower energy state by rearranging themselves in a different way. For example, two hydrogen molecules can combine with one of oxygen to produce two water molecules. Some energy — such as a naked flame or spark — is required to break the bonds within the existing molecules, but much more is released by the formation of the new bonds. The hydrogen and oxygen molecules can be regarded as storing energy that can be released under the right circumstances. The opposite reaction, splitting water into hydrogen and oxygen, takes a great deal of energy, which is why water is very stable.
Under the right circumstances, it is possible to create molecules that can store a lot of energy, but this must first be supplied from somewhere. One of the best examples of chemical energy storage is photosynthesis by green plants. In this case, sunlight provides the power to combine carbon dioxide from the atmosphere with water to produce sugar molecules, which the plant uses as food. Since sugar has more energy than carbon dioxide and water, they cannot combine directly. Photosynthesis, however, is a complex process that creates the sugar indirectly in a series of steps, using the power of the Sun.
Cellular respiration can be viewed as the opposite of photosynthesis. When a person or other animal eats plant material, the sugar molecules are broken down, producing carbon dioxide and water. Since these, together, have less energy than the sugar, some is released. This is stored in a molecule called adenosine triphosphate (ATP) by adding a phosphate group to another molecule called adenosine diphosphate (ADP). It can be released again, when required, by the removal of this phosphate group; although some energy is required to do this, much more is released by the new bonds that the unattached phosphate group forms.
Combustion and Fuels
One of the most familiar examples of chemical energy is combustion. This is usually a reaction in which carbon and hydrogen in organic substances, such as wood or oil, combine with oxygen in the air to produce carbon dioxide, water, light and heat. It can also involve other elements, however. Combustion drives the motor car, powers the majority of electricity generating stations, and provides heat and cooking facilities for many homes.
The fuels used for combustion processes can be regarded as chemical stores of energy, much of which has ultimately come from the Sun. Coal, oil and natural gas come from the remains of ancient plants and animals that got their energy from photosynthesis or by eating plants that did so. These organic materials became buried in mud and silt, eventually forming the deposits that are exploited today.
These substances are also energy stores. Their molecules consist of atoms that can rearrange themselves into other molecules that have far less energy, and when this happens, the difference is released as light and heat. Modern explosives are typically nitrated organic compounds, which means that they are carbon-hydrogen compounds that have had nitrogen-oxygen groups added to them. This is usually a relatively unstable formation: with quite a small stimulus, the existing bonds will break, and the atoms will regroup into molecules with much lower energy, such as carbon dioxide, water and nitrogen. The light and heat released, combined with the extremely fast transformation of a solid or liquid into gases, produces a violent explosion.
Batteries: Electricity from Chemicals
Chemical reactions can also be used to produce electricity. Atoms of some elements can easily give out electrons, while others like to gain electrons. Batteries exploit this fact by arranging two different elements or compounds in such a way that electrons can flow from one to the other when they are connected in a circuit, forming an electric current. A wide variety of different substances can be used to convert chemical energy to electricity in this way, and so there are many different types of battery that can be used to power phones, small computers, and electric circuitry in cars, among other things.