# What is enthalpy?

Enthalpy (H) is the heat content of a system at constant pressure. The heat that is absorbed or released by a reaction at constant pressure is the same as the enthalpy change, and is given the symbol ($\Delta$H) . Unless otherwise specified, all reactions in this material are assumed to take place at constant pressure. Therefore, there is a connection between enthalpy and the first law of thermodynamics, where energy cannot be created nor destroyed.