Chemical Thermodynamics and the Equilibrium State
|In our earlier discussion of Thermochemistry, we reviewed the concepts of internal energy change and enthalpy change. This equipped us to describe the changes in a substance as a thermochemical system. We learned how to compare the products and reagents in a chemical reaction in terms of the allotment of energy between the system and its surroundings. If the system completely changed from Reagents A to Products B, does heat flow in or out? At this stage, we are ready to seek to understand spontaneity in chemical transformations. What is the availability of energy in a chemical system for drive a reaction forward? We are describing the free energy in a chemical system, energy that when it is expended during chemical tranformation increases the entropy of the universe. Free energy is expended until the equilibrium state for the system is achieved. At equilibrium, heat flows between the system and its surroundings become microscopically reversible. The forward direction is just as likely as the reverse because the entropy associated with heat flows in either direction is the same.|
On the MCAT, although you probably will run into a Le Châtelier's Principle question or two, the importance of this topic is very great besides direct questions on the MCAT because the concepts discussed here are the scaffolding on which much else is built. Most students find chemical thermodynamics to be a very abstract subject. I think this unfortunate situation occurs because of the way that chemical thermodynamics is approached in undergraduate general chemistry, without physics to make it intuitive and biology to make it concrete. If you approach chemical thermodynamics, though, with a basis in force and energy, you can relate the internal energy changes driving heat flow and entropy change to a fundamental understanding of the particle level of substances. Furthermore, when you bring chemical thermodynamics to your understanding of biochemistry, your appreciation of life processes will become much more coherent and much more interesting. This is the kind of intuitive feel for science that the MCAT writers are ultimately looking for with their exam.