How does enthalpy change differ from heat flow?

Question

Serenity Allen · Accepted Answer

Enthalpy has the following definition: #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) Consequently, #DeltaH = q + V_1DeltaP + DeltaPDeltaV# where 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).  Thus, we can state that heat flow in a constant-pressure system open to the air is equivalent to enthalpy, whereas heat flow in a constant-volume system closed to the air is equivalent to internal energy.

Mateo Aguilar · Answer

undefined Enthalpy change refers to the change in the total heat content of a system at constant pressure, while heat flow refers to the transfer of thermal energy between two bodies due to a temperature difference. Enthalpy change takes into account both the heat transferred and any work done on or by the system, whereas heat flow only considers the thermal energy transferred between the bodies.