Enthalpy

What is enthalpy definition?

Enthalpy is the property of a system that is equal to E + PV.

• E is the internal energy of the system,
• P is the pressure
• V is the volume of the system.

The change in enthalpy equals the energy flow as heat at a constant pressure. Enthalpy is used to predict the heat flow in chemical reactions. Enthalpy or enthalpy of formation is the change in the enthalpy that occurs during a chemical reaction.

Many enthalpies of chemical reaction have been measured and tabulated. It is possible to perform enthalpy calculation using these tabulated enthalpy change values, so it is not necessary to make calorimetric measurements for all chemical reactions.

What is Hess's Law?

Hess's Law states that if a reaction is carried out in a series of steps, the enthalpy change for the reaction will be equal to the sum of the enthalpy changes of the individual steps. The overall enthalpy change for the process is independent of the number of steps or the particular nature of  the path by which the reaction is carried out.