We’ve lived through the nonsense of the inclusion of PES in the new AP Chemistry curriculum. Many think the exclusion of quantum numbers, stinks. Some people HATE the removal of colligative properties from the course. For me, these (and other things) provoke varying degrees of outrage that range from complete indifference to utter incredulity. Then we come to EU6D and LO 6.25 and another ∆G° mystery. Just what precisely is going though the mind of the TDC/CB with those?
Let’s clear one thing up immediately. The chemistry is simple. Very simple. Essentially it’s nothing more than the application of the equation that links ∆G° with K, i.e.,
∆G° = – R T ln K
In the good old days it used to be largely sufficient to plug numbers into that equation to calculate a value for the change in Gibb’s Free Energy or the equilibrium constant. Becasue of the math of the equation, kids would quickly see that when K > 1, a positive value for ln K would result, and in turn ∆G° would be negative. When K < 1 the opposite would be true of course, and if K happened to be equal to 1 then ln K =0 and in turn ∆G° = 0.
All of that tells us that the sign of ∆G° (positive, negative or even 0), can be used as an indicator of the ratio of products to reactants at equilibrium. Fine, no problem, simple and straightforward, so why complicate the issue with a specific reference to ‘thermal energy’ (RT), and its value of 2.4 kJmol-1? I’ve already asked about the mystery of the actual value of RT, but putting that aside for a moment, here’s the next question.
Is there a reason why the CED has isolated a WHOLE Enduring Understanding statement (EU6D) for that simple chemistry? Why couldn’t kids simply be asked about the application ∆G° = – RT ln K and an understanding that K > 1 mean products being favored at equilibrium and a negative ∆G° (and vice-versa)? What’s the point of the whole ‘thermal energy’/RT specific reference? I’ve only managed to come up with one thing, but I remain unconvinced that even it makes much sense.
It seems to me that the CED is saying that even when (for example), K = 2/1 = 2, or K =1/2 = 0.5, that this is still considered relatively close to products and reactants being ‘equally favored’ (or at least in the words of the CED, having ‘significant concentrations’) IN THE GRAND SCHEME OF THINGS. Even though K’s = 2 and 0.5 hardly seem ‘equally favored’ in terms of products and reactants, it could be literally millions of times smaller (or bigger), for example, with the massive Ka’s of a strong acids or the tiny Ka’s of weak ones. So is the whole point of the existence of EU6D really only a question of scale comparison? If so, it goes way beyond my examples of K = 2 and K = 0.5. Some AP teachers choose to use values of 0.001 and 1000 as examples of K’s where the products/reactants are not (in their words) strongly favored at all. Even though that makes perfect sense to me (since the range can be many factors of 10 bigger and smaller than even those), K’s such as 0.001 and 1000 produce a range of approx +17 to -17 for ∆G°, and NOT +2.4 to -2.4 that the CED references. The question becomes, what is considered a significant concentration? Very ambiguous.
As you can tell, I don’t get the point of a whole hoopla surrounding ∆G° = – RT ln K getting its own EU, but I think it is bound up in the obsession that the CB/TDC has regarding algorithmic calculations (and perhaps calculations in general). They just don’t want kids using equations to calculate numbers, and in the process of following that edubabble mantra, they’ve turned a simple relationship into a convoluted mess. We have seen this already on the new exam with removal of Nernst, coupled with persistent questioning surrounding non-standard cells. Sometimes it’s just BETTER to do a calculation and be done with it. This is what schools of education have got wrong. This is another example of that.
A concept such as the one tied up in EU6D can really only be sensibly tested with a formula and a bit of math. Becasue of the edubabble mantra of ‘no algorithmic calculations’, now it has to be artificially set up with the insertion of the largely meaningless +/-2.4 kJmol-1 value, in order to allow it still be be tested (somewhat) quantitatively. Why not test it quantitatively, PROPERLY? It’s madness, but typical of education ‘research’.
As a result I feel that this probably won’t be tested on the free response section of the exam. Why not? Well, any kid could pick up their calculator, apply the formula, and probably – horror of horrors – answer the question by performing a calculation! The CB would HATE that! If on the other hand this is asked in a MCQ context, then we can go down the edubabble route of RT and 2.4.
One final observation. Of the six exams that have been produced by the College Board that relate to the new curriculum, there hasn’t been a single point devoted to LO 6.25. Perhaps, in the words of another AP chemistry teacher, “Maybe the question writers are scratching their heads on this one, too.“