# 5: Energy and Enthalpy

• 5.1: First Law of Thermodynamics (Summary)
Summary for Chapter 3 "First Law of Thermodynamics" in Fleming's Physical Chemistry Textmap.
• 5.2: Prelude to Thermodynamics
Thermodynamics is the study of how energy flows into and out of systems and how it flows through the universe. People have been studying thermodynamics for a very long time and have developed the field a great deal, including the incorporation of high-level mathematics into the process. Many of the relationships may look cumbersome or complicated, but they are always describing the same basic thing: the flow of energy through the universe.
• 5.3: Work and Heat
Joule was able to show that work and heat can have the same effect on matter – a change in temperature! It would then be reasonable to conclude that heating, as well as doing work on a system will increase its energy content, and thus it’s ability to perform work in the surroundings. This leads to an important construct of the First Law of Thermodynamics: The capacity of a system to do work is increased by heating the system or doing work on it.
• 5.4: Reversible and Irreversible Pathways
It is convenient to use the work of expansion to exemplify the difference between work that is done reversibly and that which is done irreversibly. The example of expansion against a constant external pressure is an example of an irreversible pathway. It does not mean that the gas cannot be re-compressed. It does, however, mean that there is a definite direction of spontaneous change at all points along the expansion.
• 5.5: The Joule Experiment
Joule's experiment concluded that dq=0 (and dT=0) when a gas is expanded against a vacuum. And because dV>0 for the gas that underwent the expansion into an open space, the internal pressure must also be zero!
• 5.6: Calorimetry
As chemists, we are concerned with chemical changes and reactions. The thermodynamics of chemical reactions can be very important in terms of controlling the production of desired products and preventing safety hazards such as explosions. As such, measuring and understanding the thermochemistry of chemical reactions is not only useful, but essential!
• 5.7: Reaction Enthalpies
Reaction enthalpies are important, but difficult to tabulate. However, because enthalpy is a state function, it is possible to use Hess’ Law to simplify the tabulation of reaction enthalpies. Hess’ Law is based on the addition of reactions. By knowing the reaction enthalpy for constituent reactions, the enthalpy of a reaction that can be expressed as the sum of the constituent reactions can be calculated.
• 5.8: Lattice Energy and the Born-Haber Cycle
An important enthalpy change is the lattice energy. This is the energy necessary to take one mole of a crystalline solid to ions in the gas phase. A very handy construct in thermodynamics is that of the thermodynamic cycle. This can be represented graphically to help to visualize how all of the pieces of the cycle add together. A very good example of this is the Born-Haber cycle, describing the formation of an ionic solid.
• 5.E: First Law of Thermodynamics (Exercises)
Exercises for Chapter 3 "First Law of Thermodynamics" in Fleming's Physical Chemistry Textmap.

Thumbnail: A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake. Image used with permission (CC SA-BY 3.0; Nikthestunned)