It’s never been OK, on the AP exam, to offer a simple, ‘because of Le Châtelier’s principle‘ argument as the explanation for the shift of an equilibrium position caused by an external stress. This is in much the same way that it has never been acceptable to say, ‘because as you go across the period first ionization increases‘ as an explanation for the trend in first ionization energy from Na to Ar. In short, one can’t state the ‘trend/rule’ as the ‘reason’.
Fair enough, but what has always been acceptable is to use the words, ‘because of Le Châtelier’s principle‘, followed by a more expansive explanation that includes the idea that the stress must be relieved, and that whatever the shift in equilibrium subsequently is, that stress relief was the reason for the shift. This is what I call ‘expanded LCP‘, and is an explanation that shows an understanding that simply stating the rule does not.
In the new curriculum we find a couple of LO’s that deal with such explanations. Firstly, LO 6.8;
The student is able to use Le Châtelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium.
As far as this LO goes, it seems like business as usual and we should assume that explanations in the ‘expanded LCP’ spirit should still be entirely acceptable. However, we also have LO 6.10;
The student is able to connect Le Châtelier’s principle to the comparison of Q to K by explaining the effects of the stress on Q and K.
The inclusion of the specific language of 6.10 seems to make it inevitable that we are going to see questions in the future, that specifically ask the kids to answer questions regarding equilibrium shifts in terms of changes in Q and K. This has not been directly targeted in the past, but 6.10 looks like a direct attempt to force kids into using a Q versus K explanation as opposed to an ‘expanded LCP’.