Thoughts on the 2021 AP Chemistry Released FRQs

Written by Adrian
On May 10, 2021
Categories: 2021 Exam

Here are some initial thoughts on the AP Chemistry 2021 released FRQs. I will publish my draft answers later.

Firstly, don’t forget that if you take the in-person, paper and pencil test, you are potentially at a huge disadvantage. Any student taking the exam digitally at home can have any number of helpers present, and the College Board has no way of detecting that. Anyway, moving on to the released questions.

It’s another, really easy exam, that will still be answered badly by a number of students. Some of those students are woefully underprepared, while others have absolutely no intellectual business taking the exam or even being in an AP chemistry class. Others will just write poor answers. Significant figures, missed units, math errors, and misreading questions all contribute to poor answers, as do problems with English language. The English issue is especially true of students that have English as their second language, but that problem also afflicts students that cannot express themselves clearly or precisely, or in terms of what the CB is looking for.

Question 1:

In (a) avoid the temptation of writing [H3O+]2 in the numerator of the Ka expression. That is generally frowned upon, even though the math works out that way ultimately.

In (b), if you use 0.25 in the denominator, say that the ionization is negligible. I wrote about sig figs and pH years ago, but I am hoping that the CB does not go down the road of applying the sig fig point to pH calculations – ever!

In (c) since there is no mention of formal charge and/or “better/best” structures, more than one will likely get full credit.

(d) is kind of a neat way of asking about Kw, Ka, and Kb, and brings up the prickly point of where an equivalence point of a weak acid and a weak base titration would lay. I like this question.

In (e) I don’t think it should be necessary to flag anything other than H changing oxidation state, but we will see.

I don’t like (g) much, since there is potential for the catalyst to change a lot during the reaction depending on the mechanism. I’m assuming the answer being sought is simply ‘no change’ (with an explanation), but still …

Question 2:

Look out for an obsession with dispersal rather than disorder in (d).

In (i), units will kill some students, and make a mess of the calculation.

Question 3:

In (f) ranges of acceptable values will be given, but this always brings up unnecessary room for debate IMO,

Question 4:

In (a) the question misses a trick by pointing out the 4-minute maximum temperature and could have been left open-ended to make it more interesting.

Question 5:

(b) is interesting since I don’t recall that concept being asked ever before. Should be easy for most students to work out, but I like the originality there.

Question 6:

I’m OK with (d), but I wonder if the Ka2 value for sulfuric acid is going to lead to the better-prepared kids to overthink this. I think that there are more elegant ways to test knowledge of K/Q/Le Chat├ólier, my favorite being changing the amount of water present.

Question 7:

Parts (b), (c) and (d), although simple, will probably be a bloodbath of non-explanation!

 

5 Comments

  1. Steve

    Thanks for sharing your thoughts! I had the same thought for 6(d). I do not envy the readers who will be grading question 7.

    Reply
  2. Grant Colijn

    Thanks for this, I always appreciate your insights.

    I’m curious as to your thoughts regarding 2(b). Most students will likely treat this as a “more electrons = greater polarizability = stronger LDFs” question. However, given that silicon dioxide is network covalent as opposed to molecular, I wonder if the desired answer would require a covalent bonds vs intermolecular forces comparison.

    Reply
    • Adrian

      Hi Grant. Well, yes, the question is expecting a comparison of IMF in the molecular case, and covalent bonds in the network case. As you clearly know, there are no molecules of silicon dioxide, so we cannot be talking about IMF’s in that case.

      Reply
  3. Peter Moskaluk

    5(b) While the official scoring guide is not out. I have my reservations about this question.
    This is the note I posted on the AP Chem teachers chat group:

    Paul S Cohen first brought this up.

    ” 5b Would an applied voltage of 2.0 V be sufficient for the reaction to occur? Support your claim with a calculation as part of your answer.”

    I’m pretty sure the spirit of the question is that the voltage would be too low based on the reduction potential’s given.

    But this is far from a standard cell. The standard voltages given in the problem are based on 1 M aqueous ions and a salt melt is anything but that (20M and 40 M are the approx concentrations of these ions). Also the temp is certainly much higher than standard voltage.

    I see this as a question that looks nice on paper, but is one that would not be even close to reality. From what I’ve found this electrolysis will occur even at 0.5 V.

    Reply
    • Adrian

      Interesting, but as you correctly observe, it’s difficult for me to imagine that anything other than the simplest idea that the applied voltage should exceed the negative voltage of the non-thermodynamically favored reaction will be required.

      Reply

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