Thermodynamics is the study of the change in energy during a chemical reaction. While this may sound simple, it involves several types of energy that impact every part of a reaction. When you finish this chapter, you will be able to predict reaction spontaneity, energy changes, and understand the relationships between thermodynamics and reaction equilibrium. You'll also come to appreciate The Net Equation's extensive "Reference" section, which includes hundreds of thermodynamic values--finding the information you need has never been so easy!
Before we begin, a few new concepts must be discussed. Since thermodynamics involves energy transfer, we need to establish areas between which the transfer can occur. In thermodynamics, the system is part of the universe under examination, often the atoms or molecules involved in a reaction. The surroundings constitute the rest of the universe, excluding the system under study. Strangely, standard conditions for thermodynamics are not STP: thermodynamic values usually assume a temperature of 25 °C, instead of 0 °C; standard pressure remains 1 atmosphere (760 mm Hg). Standard thermodynamic conditions also imply that the number of moles reacting is the smallest amount required for a balanced, no-fractions reaction. Finally, when we say a reaction is "favorable" or not, we are stating that it will eventually occur given enough time; this is a distinct concept from the rates of reaction discussed in the "Chemical Reactions" chapter.
Armed with these definitions, we're ready to proceed. In this chapter, we'll discuss:
Endothermic and Exothermic Reactions
Enthalpy and ΔH
Heats of State Changes
Gibbs Free Energy
ΔH, ΔS, ΔG, and K: Predicting Reaction Favorability
Click the "next" button to continue in this section.