Chemical energy

Reactions that absorb energy from the surroundings are said to be endothermic. When the heat flow is into a system, the process is endothermic.

When a reaction results in the evolution of heat, it is said to be exothermic that is, energy flows out of the system. For example, in the combustion of methane, energy flows out of the system as heat.

CH₄(g) + 2O₂(g) ➜ CO₂(g) + 2H₂O(g) + Energy(heat)

Internal energy

The internal energy (E) of a system is the total energy contained within a system, partly as kinetic energy and partly as potential energy.

Potential energy is a factor of:

• Attractive and repulsive interaction between molecules;
• Attractive interactions between electrons and nuclei within molecules;
• Repulsive interactions between electrons within molecules; and
• Repulsive interactions between nuclei within molecule.

Law of conservation of energy

The law of conservation of energy states that energy may be converted from one form to another, but the total quantity of energy remains constant. The energy of the universe is constant. The first law of thermodynamics is a specific statement of the conservation of energy, especially useful in discussing chemical reactions.

The first law of thermodynamics relates the change in internal energy of a physical or chemical change taking place in a container to the flows (transfers) of energy into or out of the container. These energy transfers are of two kinds: work and heat.