How does enthalpy change differ from heat flow?

1 Answer
Truong-Son N.
May 23, 2015

Well, Enthalpy is defined as follows:

#H = U+PV#

#DeltaH = DeltaU + Delta(PV)#

#= DeltaU + (P_1 + DeltaP)(V_1 + DeltaV) - P_1V_1#

#= DeltaU + P_1DeltaV + V_1DeltaP + DeltaPDeltaV#

#= q + w + P_1DeltaV + V_1DeltaP + DeltaPDeltaV#

#= q cancel(- P_1DeltaV + P_1DeltaV) + V_1DeltaP + DeltaPDeltaV#
(with #w = -PDeltaV# for volume expansions/contractions)

As a result,

#DeltaH = q + V_1DeltaP + DeltaPDeltaV#

where:

  • #H =# enthalpy
  • #U =# internal energy
  • #q =# heat flow
  • #w =# work
  • #P =# pressure
  • #V =# volume

So you can see that enthalpy is heat flow along with a constant-volume pressure change, and enthalpy and heat flow are only the same if the pressure is constant (#q_p = DeltaH#) (like in a coffee-cup calorimeter).

We can then say that enthalpy is equivalent to heat flow in a constant-pressure system open to the air, whereas internal energy is equivalent to heat flow in a constant-volume system closed to the air.

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