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Integrated Sequence	Physics	Chemistry	Organic	Biology

Thermochemistry

Internal energy change

Enthalpy change

Thermodynamic work

Conservation of energy in chemical systems

Hess' Law of Heat Summation and the Standard State - Applied Thermochemistry

Bond-Dissociation Enthalpies

Web Resources

Chem1 Virtual Textbookï¿½-ï¿½The First Law of Thermodynamics
Begins with excellent primer on the 1st Law of Thermodynamics and finishes with a clear explanation of the concept of enthalpy.

Chem1 Virtual Textbookï¿½-ï¿½Molecules as energy carriers and converters
Excellent presentation of a crucial set of concepts. Highly recommended. Make sure that you understand this material!

Purdue Universityï¿½-ï¿½Energy, Enthalpy, and the First Law of Thermodynamics
Beginning with fundamental thermodynamic principles, carefully builds to convey a good understanding of the difference between internal energy and enthalpy. Recommended.

HyperPhysicsï¿½-ï¿½Enthalpy

Special points of emphasis

Work, Energy, and Power

The Ideal Gas and Kinetic Theory

Thermochemistry

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.

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