There is a state function, the internal energy E (in some texts U), which has the following properties:

• in an isolated system E remains constant
• addition of work, symbol w, to a closed system will increase the internal energy by the amount of work expended.

         ΔE   =  q   +  w        

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Definition of enthalpy, H and  ΔH

Use of internal energy or change in internal energy,  Δ E , is not very convenient in chemistry.  The reason for this is that when chemical reactions occur or samples are heated, the volume does not stay constant.  If one is therefore interested in only q, the  ΔE is complicated by an additional w.  To avoid this a new quantity called enthalpy is defined, given the symbol H.

    H = E + PV     or

    ΔH = ΔE + PΔV

Since at constant pressure PΔV = -- w if no other external form of work is present, then:

   ΔH = w + q + PΔ V
and
   ΔH = q

Therefore at constant pressure ΔH will yield the heat transferred.  All thermodynamic tables use this as the tabulated "heat of reaction," etc.