"[COVERED]: We discussed Chemical Equilibria. We covered the topics of how to understand chemical reaction rates, how those rates are equal in the forward and reverse directions at Equilibrium, and how the Keq can tell us where the balance of products to reactants lies after the chemical reaction is completely done, i.e., at rest. After discussing the meaning of the Keq, we compared the reaction quotient Q to the Keq, and discussed how that comparison lets us predict whether the reaction will proceed towards creating products or reactants from that starting point. We then applied that comparison to the Thermodynamics of chemical reactions, using a formula to mathematically determine Delta G, the free energy available to drive a chemical reaction from any starting state Q to the final state, represented by Keq. We showed how this math confirmed our conceptual understanding of the relationship between starting state Q and final state Keq. We then moved on to the "standard free energy" of the reaction, which is when Q = 1, T = 25c, and P = 1 atm. We mentioned that STP, where T = 0c, is "cooler." This then brought on a discussion of what terms comprise those ratios of products to reactants, namely, aqueous species and gases only. This led to a brief overview of the states of matter, defining liquids and gases as fluids because they can flow, and solids and liquids as noncompressible states because additional pressure cannot change their volume, and thus their density and concentration remain constant. An MCAT-style critical thinking exercise allowed me to model how to solve equilibrium calculations, always defining "x" as "the amount of reactant that reacts," and I cautioned the student to continue being formal in her approach to solving calculation problems: identifying and marking down the desired quantity, identifying the variables whose values we know, and being sure to note the final quantity in terms of "x" to facilitate the logic-checking step. I took this opportunity to reinforce the idea that accuracy is what gets a great score on the MCAT. Having crested this pivotal point in understanding equilibria and solving problems based on them, we agreed it would be salient to extend the session to cover Solubility, aka "Dissociation Equilibria." Having fully erased all her old incorrect understandings, the student was readily able to see how this was just another special case of equilibrium problem. We tried her hand at a few problems. She correctly identified how to set up the Ksp (Keq for dissociation), and was able to keep the exponents straight as well as correctly plug in the values for some of the ionized species as multiples of x.
[PROGRESS]: The student continued to impress with her dedication to homework, having read the assigned chapters and done all the problems.
[CHALLENGES]: The student required a gentle reminder to be sure to identify and write down the desired and given variables, and problem-solving went more smoothly from there.
[EXTRA PRACTICE]: After the content review came to a close, we discussed the future direction of our sessions, and decided to continue with our investigation of equilibria, planning to cover acid/base and redox reactions, then to do another guided Qbank passage, following the through-line of chemical equilibria. We decided to discuss Gasses in a future session."