Here are my initial thoughts, and draft answers for the 2022 AP Chemistry released FRQs.
DOWNLOAD THE DRAFT ANSWERS HERE
As an entity, the FRQ section seems staggeringly easy. Of course, the MCQ could balance this with some really hard stuff, but we’ll never know.
Whilst I understand that for several years now there has been a significant emphasis on moving away from algorithmic calculations, the simplicity of the calculations on this FRQ as whole seems mildly insulting to the word “Advanced”. Is this really what kids are doing in college?
It seems like using the periodic table as a means to calculate a molar mass is now deprecated. I think there is an argument to say that calculating a molar mass by reading the periodic table is actually a “chemistry skill”, so I’d rather see questions not given with molar masses.
1(b) feels a little ambiguous to me. Maybe an argument could be constructed either way?
1(f) shows that the obsession with that acid-base ratio in buffers lives on!
If 2(c) is the sig fig question, then that’s a bit sneaky.
I’d leave out 2(e), and double the points for 2(f) with no reference to the expression.
Why directly point to Q in 2(g)? Kind of gives the game away.
I wouldn’t want to grade 3(c), and there are arguments for all kinds of other stuff here like a wash bottle and a funnel to help transfer the solid, disposable pipets to approach the mark etc., but I guess a lot of leeway is going to given.
I’d rather 3(d) was a limiting reactant calculation.
There are schools of thought that WILL allow a hydrogen bond to be defined as the IMF where the delta+ H that is covalently bonded to N, O, or F (in this case in water) interacting with ANY electonegative part of another molecule (that is not necessarily itself connected to H in the other molecule (in the case the N connected to Cl), i.e., that H-bonding occurs between NCl3 and water, However, I hope that is not necessary here (even if it is allowed), and I see a number of problems with applying H-bonding to NCl3 and water in this context.
1. The CED in 3.1 defines hydrogen bonding as, “a strong type of intermolecular interaction that exists when hydrogen atoms covalently bonded to the highly electronegative atoms (N, O, and F) are attracted to the negative end of a dipole formed by the electronegative atom (N, O, and F) in a different molecule”.
Now, I believe that Cl and N have very similar electonegativities (3.0 for both), so is N really, “the negative end of a dipole formed by the electronegative atom (N, O, and F)”?
2. To get around the problem in 1., are you going to allow the lone pair on the N to be the “negative end”? Maybe, but IF you DO allow H-bonding between water and NCl3, how are you going to distinguish between the two substance’s solubility with such similar iMFs?
If the answer to 2. is that NCl3 has much larger LDFs (via polarization), so it’s difficult to separate the NCl3 molecules from one another, and therefore harder to achieve the first part of the solvation process of separating the solute molecules, then how do we compare that to the definite H-bonding that exists in NH2Cl that would also inhibit solute separation?
Another workaround to 2. is to discuss the number of sites available for H-bonding which are greater in NH2Cl, so that would work. This is the most likely way to distinguish between the two compounds if you introduce H-bonding for NCl3.
For these reasons, that I think are quite complex, I prefer to think of no H-bonding between water and NCl3 here. I think that is the pragmatic way to go, and makes most sense, and whatever happens at the reading I hope that H-bonding is not required for NCl3‘s interaction with water.
I wish they’d call MnO4– manganate(VII), but that’s just me.
For 6(b), I would like an explicit note in the CED that talks about reading glassware scales. I don’t like the idea that the science practices might infer something, or that the lab experience might teach something. What I’m talking about is a specific sentence that says something like, “the AP exam expects students to estimate the last figure to be a 0 or a 5”. (OWTTE).
I have a real issue with (b)(iii) in terms of assumed or required recall knowledge. In order to answer this question there’s a need to know that oxalic acid (or the hydrogen oxalate ion) is weak, and with no Ka value given, and that not explicitly stated, I can only assume that the TDC are asking students to make the assumption that;
- because oxalic acid/hydrogen oxalate is not listed as an example of a strong acid in the essential knowledge of 8.2, that they are to assume that it is weak, OR
- because there is definitely no required knowledge of the solubility of silver chlorate(VII) (it’s not among the required solubility rules) that a student should assume that the change must be due to H+, OR
- because 7.13 in the CED is about pH and solubility, that it must be the acid that’s in play, OR
- because the questions uses the word neutral that we should draw the conclusion that the acid is the vital component,
but whichever one we settle on, it doesn’t sit well with me.
For 1a, is the answer supposed to be grams?
Thanks for the heads up. Here’s something interesting that is worth mentioning.
In 2016, in Q1(a)(ii), the question asked for the answer in units of kJ/molrxn, and J/molrxn was accepted. This came up at the de-brief at BCCE, and I commented on that being odd at the time, and the Chief Examiner’s attitude was, “it doesn’t really matter” (I’m paraphrasing, but that was the general, prevailing response). As such, there has been precedent for allowing such things to slide. I wouldn’t encourage deliberately contradicting the question, but units are WAY less important than most people think.
For Q4, the NCl3 is practically non-polar (0.6 Debye) because the similar EN of N and Cl. (Also due to the relative position the lone pair and the Cl atoms, N may even be a bit positive in charge?). As a result NCl3 does not form H bond with water and does not mix well with water.
Chloroamine is more polar and at least its H’s can still form H-bond with the oxygen of water.
Well, as I say above, I think that some may argue for “H-bonding” (of some description) between NCl3 and water, so …
2c should be 7.5, not 7.50.
Perhaps this test was ‘easy’ because they were accounting for students who took first-year chemistry during a difficult COVID year. I think they didn’t want too many low scores this year….
Ah, significant figures, the non-chemistry part of the AP chemistry exam where the rules aren’t formally stated by the College Board and we are guessing how the unwritten rules may be applied!
Thanks so much for putting this together. If by any chance you know the process for AP reading, how are sig figs handled? I’ve heard everything from that they are a single point deduction on EACH frq for however many violations are present, to that only one frq specifically checks for proper sig figs (and will specify this will be checked, and otherwise if your answer is in the ballpark sig figs are irrelevant to scoring, and finally other people say that they are scored in the one point deduction one way on each numerical calculation question, but the only requirement is to be +/- 2 sig figs. Let’s just say I was to hypothetically ignore sig figs and just write out the super long decimal my calculator gives me. Would this not matter because sig figs are only checked for once or would there likely be a deduction on each frq because I would be outside the +/- 2 digit gray/grace area? For the record, I know sig figs I just had misconceptions about how they were handled on scoring and figured I would make at least one accidental error somewhere on the test which would make all the extra time and work ensuring proper sig figs would be wasted, so I might as well just not worry about it.I thought it was clever how you mentioned 2c – I totally glossed over the decimal alignment for the entropy values while taking the exam! Thanks!
Traditionally, only a single point is considered for sig figs on the whole of the FRQ. It is NOT flagged during the exam. At the reading the question is determined, and either you get the point or you don’t. In the past, on some occasions +/- 1 sig fig have been allowed, and on other occasions no leeway has been given. For more, read this blog post.