The concept of energy conservation is also widely used in chemistry. Chemical reactions are determined by the laws of thermodynamics. In thermodynamics, the internal energy of a system is the energy contained within the system, excluding the kinetic energy of motion of the system as a whole and the potential energy of the system as a whole due to external force fields.
In thermodynamics, the internal energy includes the translational kinetic energy of the molecules (in the case of gases), the kinetic energy due to rotation of the molecules relative to their centers of mass, and the kinetic energy associated with vibrational motions within the molecules.
In chemical reactions, energy is stored in the chemical bonds between the atoms that make up the molecules. Energy storage on the atomic level includes energy associated with electron orbital states. Whether a chemical reaction absorbs or releases energy, there is no overall change in the amount of energy during the reaction. That’s because of the law of conservation of energy, which states that:
Energy cannot be created or destroyed. Energy may change form during a chemical reaction.
For example, energy may change from chemical energy to heat energy when gas burns in a furnace. The exact amount of energy remains after the reaction as before. This is true of all chemical reactions.
In an endothermic reaction, the products have more stored chemical energy than the reactants. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. The excess energy is generally released to the surroundings when the reaction occurs.
Example: Combustion of Hydrogen
Consider the combustion of hydrogen in air. In a flame of pure hydrogen gas burning in the air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases energy.
Energetically, the process can require the energy to dissociate the H2 and O2, but then the bonding of the H2O returns the system to a bound state with negative potential. It is more negative than the bound states of the reactants, and the formation of the two water molecules is, therefore, an exothermic reaction, which releases 5.7 eV of energy.
2H2(g) + O2(g) → 2H2O(g)
The balance of energy before and after the reaction can be illustrated schematically with the state in which all atoms are free taken as the reference for energy.