Wednesday, August 18, 2010

acid/base equilibrium

I've taught first semester of general chemistry (or introduction to general chemistry) as a grad student and as a professor. I'm familiar with the concepts and feel very comfortable explaining these concepts. (I'm always looking for new and fresh ideas on presentation, however, so I welcome all input in this area.)

I have never actually taught second semester of general chemistry. The concepts, as I've discovered in my review, are not concepts I've dealt with in a lot of scientific detail. In graduate school, I focused on organic chemistry which is not really covered in this section other than the basics of equilibrium and the functional groups of acid/bases (and various definitions).

In my review of kinetics, equilibrium, electrochemistry and other topics, I was most struck by acid/base chemistry. So simple and yet I think this may be one of the most difficult topics covered in the entire course (first or second semester). I'm the first one to admit that one strategy in first-year chemistry is to complete many, many problems from the back of each chapter. You can practice your way into perfection and actually ace an exam without really understanding some of the larger concepts behind the problems in the book. This is especially true for acid/base behavior.
The basics of acids and bases are just that- very, very basic (haha, pun intended here). We all can relate with the sour taste of lemons (acid) or the slippery feeling of bases (milk, sodium hydroxide). This is probably the easiest lab in the book for an introductory class- you test the pH of various acids and bases and describe the characteristics of each.
On a deeper level, however, the behavior of these ubiquitous species is very, very difficult to describe chemically. You must have a certain grasp of math (mostly basic algebra), logarithms, and understand the concept of chemical equilibrium. A reaction only proceeds to the extent that the species is soluble, reactive or otherwise able to break into new chemical compounds. This is the crux of acid/base chemistry.
It is almost impossible to do the problems in the acid/base equilibrium chapter without truly understanding the concept of the dissociation behavior of an acid or base. A weak acid/base reaction vs a strong/acid strong base reaction vs a strong acid/weak base reaction all have different assumptions under which you carry out the calculations for the equilibrium constant.
Let's take a hypothetical example here: HA + OH- = H+ (superscript +) + H20
The equilibrium constant (extent to which the acid and base actually react together to form H+ and water) is written as [H20][H+]/[HA][OH-] With the assumption that H20 concentration is a constant it is removed from the equation.

So- the way the starting material is plugged into this equilibrium equation is dependent on whether or not it is a strong acid or weak acid (just to use acids as an example). For a strong acid, dissociation is nearly complete (if not totally complete) and the equilibrium of the reaction will shift toward the water/H+ product. So 1.0 moles of starting material will give 1 mole of each H+ and H20. (based on molar ratios of 1: and assuming 1 is the limiting reactant)

If the acid is weak, the equilibrium of the reaction will stay shifted toward the reactant. In this case, we use 1.0 moles of HA - X as the value of how much HA will be left at equilibrium. Because X is extremely small, HA will remain essentially at 1.0 for the purposes of the equilibrium equation. The other side of the equation will be represented by x (a very small number) in equal amounts for both products. Therefore, it will be x2 (superscript) and you have to solve a math equation (square root of both sides).

Include this in the concept of buffers and other more complicated acid/base problems and it gets really, really confusing on when you are making what assumption if you don't really understand the concept of acid/base equilibrium.

Has anybody taught this and have any advice for me on teaching it?