# Chemical Thermodynamics

Chemical thermodynamics is the study of the flow of energy during chemical reactions or phase changes between the system and the surroundings.

The study of the flow of energy during chemical reactions or phase changes is chemical thermodynamics. The energy involved in a chemical system is most often thermal energy. Heat is thermal energy that is transferred from one location to another. Any system that involves a transfer of heat is studied under the domain of thermodynamics. A refrigerator or air conditioner uses the principles of thermodynamics to move heat. An internal combustion engine in a car uses the principles of thermodynamics to generate motion, called useful work, by using heat.

Thermodynamics uses systems and surroundings to analyze the movement of energy. A system is the part of the universe that is under consideration. For example, a piston or the gas within a piston can be defined as a system. Anything that is not the system is the surroundings, the part of the universe that is in immediate contact with a system. Systems are defined as needed by scientists. Systems are most often defined so that the energy and mass moving into and out of a system can be easily tracked.

A system that allows for the flow of both energy (heat) and mass in and out of it is called an open system. A system that allows for the flow of energy (heat) but blocks the movement of mass is called a closed system. A system that allows neither energy (heat) nor mass to flow is called an isolated system.

The law of conservation of energy is the backbone of thermodynamics. Energy is conserved when a system and its surroundings are studied as a whole. Because the system plus its surroundings describe the entire universe, this means energy is conserved in the universe:
$\Delta E_{\rm{universe}}=\Delta E_{\rm{system}}+\Delta E_{\rm{surroundings}}=0$
Most thermodynamic calculations involve tracing how energy changes within a system, with the implicit assumption that it cannot be created or destroyed. The law of conservation of energy also leads to the first law of thermodynamics, one of the three rules that govern thermodynamic systems.