Explain the terms enthalpy and enthalpy change.

Enthalpy: Enthalpy is the total energy associated with the system and is defined as the sum of internal energy and product of its pressure and volume. It is denoted by H.
Mathematically,
H = U + PV
where U is the internal energy change, P and V are respectively the pressure and volume of the system. H is also called heat content of the system. Enthalpy is a state function and every substance has a definite value of enthalpy in a particular state. The absolute value of enthalpy of a substance cannot be determined, but the change in enthalpy (∆ H) accompanying a process can be determined.

Change in enthalpy (∆H): It may be defined as the difference in the enthalpies of the product and reactant species taking part in a chemical reaction at a constant pressure. For example, if H_R represents the enthalpies of the reactants and H_P represents the enthalpies of the products, then ∆H = H_p – H_r where ∆H gives the change in enthalpy.

The enthalpy change (∆H) is equal to the heat evolved or absorbed in a chemical reaction at constant pressure and constant temperature,