Download Thoughts on Thermodynamics - Principles of Chemistry II - Handout | CH 302 and more Study notes Chemistry in PDF only on Docsity! Thoughts on Thermodynamics In organizing your study plans for thermo, it is essential to be able to distinguish these seemingly endless stream of very similar concepts from each other. This is best done by creating a hierarchy of important facts about thermodynamics—note that the relative importance of this material doesn’t run from front to back, it is more like from back to front. Thus we start with ∆G 1. ∆G : The most important thing about themodynmics: Thermodynamics tells you whether a reaction is spontaneous or not, whether it occurs or not. To do this we look at the Gibb’s Free Energy, ∆G and ask the following: If ∆G positive (+) the reaction is not spontaneous—it doesn’t happen If ∆G negative (-) the reaction is spontaneous—it happens So what is ∆G? It is the free energy of a system. It is a combination of three kinds of energy: • heat energy, q used to calculate the enthalpy, ∆ H • work, usually a change in P or V (P∆V) for a reaction involving gases • T∆S, the entropy, which is a measure of the disorder in a system We can combine these three forms of energy in a couple of ways: • ∆G = ∆H – T∆S which is a really famous equation used to calculate free energy • ∆E = q + w = ∆H – P∆V which describes all the kinds of internal energy in a system Enthalpy, H: The heat of reaction: Very simply, this term provides a quantitative measure of how much heat is given off in a reaction. • Endothermic reaction: ∆Η is +. The reaction is endothermic. The surroundings get colder. • Exothermic Reaction: ∆H is -. The reaction is exothermic. The surroundings get hotter. Calculating H: There are two ways to do this. • Experimentally: calculate ∆H = mC∆T using a bomb calorimeter. This is how they measure the calories of food products. • Hess’ Law. Hess’ Law Hrxno = Σ ∆Hfo products – Σ ∆Hfo reactants Because of the first Law of Thermodynamics, we know that energy is conserved. So if we want to calculate the ∆H for a reaction, we can put together any combination of reactions that cancel out to leave the ∆Hrxn we want. There are two ways we use Hess’ Law, heats of formation and bond energy: • Heat of formation, ∆Hfo values in Appendix K. ∆Hfo is the heat of reaction when elements in their standard state react to make an product. • Bond Energy: Rather than use Appendix K, we can apply Hess’ Law using average values for each and every broken and formed bond from Tables on p15-8 and 15-10 of the notes.
