Work, Energy, and Power
The Ideal Gas and Kinetic Theory
The Properties of Light
|Why doesn't chemistry refer to heat flow simply as 'heat flow'? Why 'enthalpy change'? Heat is not an inherent property or condition of a system, so it is not proper to refer to 'heat energy'. Heat refers to a transfer of energy. However, by specifying constant pressure, we can conceptualize a new state function for the system, the enthalpy, H, where H = E + PV. However, when you say enthalpy change, you do mean heat flow. Think about it. To say that enthalpy change equals heat flow is simply the 1st law of thermodynamics, i.e. the heat flow results from the change in internal energy and work.|
However, when you say enthalpy, as something in itself, you are not refering to heat. That would make no sense. Heat is not a state function. What you are refering to when you refer to enthalpy in itself is a property of the system that includes the internal energy plus the work involved to push back the environment to create the space the system occupies. Enthalpy can be thought of as the 'thermal potential' of the system.
Enthalpy is a pretty good stand-in for internal energy in chemical discussion. When it changes, heat flows. In fact, enthalpy change equals internal energy change if the volume is constant. However, there is a bit of a danger of too much reliance on enthalpy as a stand-in for internal energy if you lose sight of the nature of internal energy itself and forget to think about what is going on at the particle level.