Enthalpy

Enthalpy is a concept used in science and engineering when heat and work need to be calculated. The name comes from the Greek word "enthalpos" (ενθαλπος), meaning "to put heat into". The idea and the word were made up by the Dutch scientist Heike Kamerlingh Onnes in 1909.

When a substance changes at constant pressure, enthalpy tells how much heat and work were added or removed from the substance.

Enthalpy is similar to energy, but not the same. When a substance grows or shrinks, energy is used up or released. Enthalpy accounts for this energy. Because of this, scientists often calculate the change in enthalpy, rather than the change in energy.

Consider a system containing a fixed amount of gas. Any heat energy entering the system will:

1. Increase the Internal Energy of the gas

or

2. Lead to some work being done to expand the system.

The work done is the product of pressure and volume. Enthalpy takes both these factors into account, therefore: ${\displaystyle {\begin{aligned}Enthalpy(H)&=Internal\ Energy(E)+\left(Pressure(P)\times Volume(V)\right)\\H&=E+(P\times V)\\\end{aligned}}}$