Every process in this universe, whether it is in living cells, test tubes, atmosphere or water, etc., involves a change in energy. Some processes release energy, others require energy. Many chemical reactions produce huge amounts of energy, which is used to produce new raw materials such as iron, steel, copper, aluminum, etc. Energy is also used to transform these new raw materials into useful products such as trains, trucks, cars, buildings, bridges, and many other objects. The study of energy changes in chemical reactions is called chemical energetics.
Energy in the form of heat is developed or absorbed as a result of a chemical reaction. This is because in a chemical reaction old bonds are broken and new bonds are formed. Breaking bonds always consumes energy and binding always releases energy. If the energy released in forming a bond is greater than the energy expended in breaking the bond, there is a net release of chemical energy. On the other hand, energy is absorbed when the energy expended in breaking a bond is greater than the energy released in forming the bond. Thus, during chemical reactions, energy is exchanged with the surroundings.
The part of the universe that we want to focus our attention on is called a system. The rest of the universe is called the environment. In chemistry, a system is usually a substance that changes physically or chemically. For example, when studying the reaction of limestone and hydrochloric acid solution in a test tube, limestone and hydrochloric acid solution form a system. The test tube and everything around the test tube is the environment. Similarly, when studying the thermal decomposition of a compound, the sample of the compound would be the system. While the beaker, heat source, and everything else would be the environment.