All chemical reactions obey the First Law of Thermodynamics (also known as The Law of Conservation of Energy). This law states that the “Energy may be converted from one form to another, but the total quantities of energy remain constant”.
When a chemical reaction happens:
- Energy is required to break bonds, and
- Energy is released when bonds are formed.
Usually in a lab experiment, the substance is not moving, so it has no kinetic energy. Also, there is no change in potential energy as the reaction vessel is not moved. The release of energy in chemical reactions occurs when the reactants have higher chemical energy than the products. The chemical energy in a substance is a type of potential energy stored within the substance. This stored chemical potential energy is the heat content or enthalpy of the substance. Therefore all the energy released or absorbed relates to changes in the Internal Energy of the substances. In chemical reactions, energy is often transferred from the “system” to its “surroundings,” or vice versa. The substance or mixture of substances under study in which a change occurs is called the thermodynamic system (or simply system.). The surroundings are everything outside the thermodynamic system.