Untitled Document

Enthalpy

Enthalpy is a measure of the total energy of a thermodynamic system. The SI unit of enthalpy is the Joule, but other historical, conventional units are still in use, such as the small and the large calorie.

Heat of reaction The total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead. Enthalpy change is defined by the following equation:

ΔH = H_final − H_initial

where

ΔH is the enthalpy change

H_final is the final enthalpy of the system, expressed in joules. In a chemical reaction, Hfinal is the enthalpy of the products.
H_initial is the initial enthalpy of the system, expressed in joules. In a chemical reaction, Hinitial is the enthalpy of the reactants.

For an exothermic reaction at constant pressure, the system's change in enthalpy equals the energy released in the reaction, including the energy retained in the system and lost through expansion against its surroundings.

In a similar manner, for an endothermic reaction, the system's change in enthalpy is equal to the energy absorbed in the reaction, including the energy lost by the system and gained from compression from its surroundings.

For endothermic reactions Hfinal > Hinitial and ∆H is positive (+∆H)
For exothermic reactions Hfinal < Hinitial and ∆H is negative (-∆H)

Using enthalpy simplifies certain descriptions of energy transfer, as it is not affected by energy transferred to the environment through the expansion of the system

Relationship to heat

The increase in enthalpy of a system is exactly equal to the energy added through heat, provided that the system is under constant pressure and that the only work done on the system is expansion work:

∆H = ± q

where
ΔH is the change in enthalpy of the system (under the restrictions mentioned above), and
q is the energy added to the system through heat.