- Principles of Chemistry II: Honors
Spring 2014, Unique 51880
Lecture Summary, 25 February 2014
Over the past couple of days we have been discussing how to
influence the equilibrium position of any chemical reaction.
This is described by Le Chatelier's principle: If a system in
equilibrium is stressed, the system will react in such a way to
relieve the stress. We considered three stresses: 1) changing
the amount of a reactant or product in the reaction vessel; 2)
changing the pressure of the system; and 3) changing the temperature
of the system. In all cases we were able to derive an
expression that not only confirmed our prediction from Le Chatelier,
but in each case was able to quantify how far the reaction would
Acid - base equilibrium: Today we also started discussing the application of chemical equilibrium to the study of acids and bases. We proposed two definitions of an acid and base, one based completely on interactions in water (Arrhenius) and one based on moving a proton from a reactant to product (Bronsted-Lowry). Bronsted-Lowry is the more general definition, and so most of our discussion of acid-base chemistry will follow this definition. No matter how we define the acid and the base, we can only understand the proton transfer by considering the molecules that are created after the proton has moved - i.e the conjugate base of the acid and the conjugate acid of the base. The extent of proton transfer in our acid-base reaction will depend on the stability of the conjugate molecules thus created.