Buffers can be challenging. Over the years, I have found that kids tend to get stuck in understanding buffers more than they should. This past summer I attended an AP Institute and heard Tom Greenbowe (University of Oregon) speak on how he teachers buffers. I took some parts of his concept and made it my own. Students this year seemed to have an easier time when identifying what the net ionic reactions within the buffer system should be.
Tom described the buffer system like a person’s backyard. In the backyard is a big dog. That dog is the boss of the backyard. In a weak acid system, we are looking for what species exists in the biggest concentration. This we know is the weak acid, HA. Since the acid dissociation constant is relatively small, the HA will stay bonded together to a large extent causing the HA to remain the highest concentration.
As the reaction reaches equilibrium, a little dog is added to the yard; A-. The conjugate base will be in significantly lower concentration than HA. While not the big dog, as far as concentration is concerned, the little dog can still interact with new things in the back yard.
When making a buffer, the weak acid and the conjugate base must be added in concentrations that are approximately equal to maximize buffering capacity. That means we artificially add a bunch more little dogs to the yard. Now there is a big dog, high concentration of HA, and a bunch of little dogs, high concentration of A-. When any new perturbation to the equilibrium comes in (ie adding a small amount of strong acid or strong base) they are going to have to interact with either the big dog or the little dog. Period.
This is where my students in the past would tend to get confused. They understand Le Chatelier’s Principle. They just aren’t sure where things are interacting. Let me use this example to show you the confusion.
Kids had a hard time when trying to predict what would happen with the addition of a small amount of OH-. I know to some of you, this is a silly thing that shouldn’t be challenging, but the way I was teaching it caused kids to stumble. But now, I can ask a very simple question. Does the OH- interact with the big dog or the little dogs? It’s easy to see that the hydroxide would not interact with A-, so the answer is HA. This removes the H+ forming water and increases the concentration of A-. The net ionic equation would be
HA (aq) + OH- (aq) ↔ H2O (l)+ A- (aq). Just by adding the concept of the big dog and the little dogs, the students can eliminate some of the weird answers that they were coming up with.
The dog metaphor still follows into the Henderson-Hasselbalch Equation.
pH = pKA + log [A-]/[HA].
While the equation can look intimidating, it is really just some questions concerning the dogs. Whose yard is it? The big dog. What Ka should I use for the equation? The big dog’s. The ending of the equation is the ratio of small dogs to big dogs. Again, I don’t use this metaphor as the entirety of the unit, but it focuses the kids down to looking at what matters most.
Give it a shot, the next time you teach buffers and see if the kids can identify how buffers work with more clarity. Plus they like dogs, so there’s that…
Comentários