Acids and Bases on a Snowy Valentine's Day
Practice problems as well as an online homework are still up. In addition, we do have a textbook that you could open and read. What we may be missing from not spending time together for 50 minutes inside a classroom is my perspective (and perhaps, my boring jokes). This post is then my attempt to provide a direction, a starting point for you to have before reading the text and solving problems. This requires additional initiative or motivation on your part. It may or may not help, but in the past, I have heard that it somehow helps. Below is an email I received a year ago:
Hello Professor de Dios, You don't know me and I am not a student of yours, but in a way I am becoming one.... I have been struggling through my own Chem2 class this semester and found your lessons online. I wanted to thank you so much for posting exactly what I have been looking for: common sense explanations to what I am supposed to be learning. I am a 40-something mother who is going back to school for Physical Therapy after many years in the corporate world. I am taking classes at a local community college working on my re-req's and here, I am dealing with an ineffective, unprofessional, and frequently scatter brained teacher. As students we are often exchanging glances wondering what is going on or what exactly he means. The dean has apologized explaining he's all they have.... You however, have changed all that!! I thank you from the bottom of my heart for taking the time to post your lessons and letting an interloper experience how a real chemistry class should sound. Best regards,XXXXXX
It has worked at least once, so perhaps it could help us well. At least, it could put chemistry into our minds during this seven day break from a face-to-face lecture inside Reiss 112.
We are currently covering in class acid-base equilibria. As I have mentioned several times, this chapter and the next one are extensions of the chapter on chemical equilibrium. The concepts are the same, we are simply delving deeper into the specific case of reactions that involve proton transfer (acid-base reactions). The math behind the quantitative treatment of equilibrium should be familiar.
Hopefully, at this time, we already know by heart the Bronsted-Lowry definition of acids and bases: An acid is a proton-donor and a base is a proton acceptor. In aqueous solutions of either an acid or a base, the following equilibria are therefore relevant.
|The figures used in this post are either taken from the text (Chemistry,
The Central Science, 12th edition, Brown, |
With pure water, we only had to concern ourselves with one reaction and its corresponding equilibrium expression: (taken from the previous lecture)
Thus, introducing an acid HA or a base B adds a reaction that needs to be considered, and of course, this comes with its own equilibrium expression. For instance, with an aqueous solution of an acid, we need to worry about
The following is the problem:
And here is my detailed answer:
The way I answered this problem (specifically part (b)) does not fit what this post described for weak acids or bases. The problem above in fact requires that we identify first what equilibrium expression we need to use. I should also note that the answer you would find in the textbook's solutions manual (posted in Blackboard) is unfortunately incorrect. Therefore, if you are able to follow what I did, I think that is a good and clear indication that you do understand what this chapter is all about.
Happy Valentine's Day!