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The thermodynamic quantity E + PV is called the heat content or Units for Enthalpy of the system and is represented by H.i.e. the enthalpy may be defined mathematically by the equationH = E + PVThus ,if H2 is the enthalpy of the system in the final state and H1 is the value of the initial state,thenH2 = E2 + PV2-------------------------------------------(1)and, H1 = E1 + PV1 -------------------------------------------(2)

Subtracting (2) from (1) we get, ?H = ?E + P?VHence ,the enthalpy change accompanying a process may also be defined as the sum of the increase in internal energy of the system and the pressure-volume work done i.e. the work of expansion.Physical Concepts of Enthalpy:In the above discussion,the enthalpy has been defined by the mathematical expression H = E + PV.Let us try to understand what this quantity really is.Every substance or system has some definite energy stored in it,called the internal energy.This energy may be of many kinds. 

The energy stored within the substance or the system that is available for conversion into heat is called the heat content or enthalpy of the substance or the system.Like internal energy,the absolute value of the heat content or enthalpy of a substance or a system cannot be measured and fortunately this is not required also.In the thermodynamic processes, we are concerned with only the changes in enthalpy.(? H) which can be measured experimentally.Further, it maybe mentioned that as E and V are extensive properties,therefore enthalpy is also an extensive property. 

My forthcoming post is on Units for Entropy and specific heat equation will give you more understanding about Physics

Some Important Points of Enthalpy:(i) The quantity of heat supplied to a system at constant pressure (isobaric process) 1.e QP is equal to the increase in its enthalpy,?H.QP = ?H(ii) For exothermic reactions, the sign of ?H is negative.(iii) For endothermic reactions, the sign of ?H is positive.