A chemical reaction in a beaker either releases heat or absorbs it. The heat content of the system at constant pressure is called enthalpy, denoted by H. It is a state function, meaning it depends only on the current state of the system, not on the path taken to get there. The change in enthalpy, delta H, is the heat absorbed or released during a reaction at constant pressure. A negative delta H means the reaction is exothermic. A positive delta H means it is endothermic.

Enthalpy is measured in joules or kilojoules per mole. It is usually reported as the standard enthalpy change, delta H°, which is measured under standard conditions: 1 atmosphere pressure and a specified temperature, usually 298 Kelvin. The standard enthalpy of formation is the enthalpy change when one mole of a compound forms from its elements in their standard states. It is defined as zero for elements in their standard states.

Enthalpy changes can be calculated using Hess's law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for each step, regardless of the path. This allows chemists to calculate enthalpy changes for reactions that are difficult to measure directly. Bond energies can also be used to estimate enthalpy changes, though the values are averages and less accurate.

Enthalpy is central to thermochemistry. It determines whether a reaction is exothermic or endothermic, how much heat is released or absorbed, and how much energy is available for work. It is used in industry to design reactors, optimize fuel efficiency, and manage heat transfer.