Thursday, December 18th, 2014

Comments on, and answers to, the 2012 AP Chemistry FRQ’s

For the record (and just in case you didn’t know the background to this years exam), I had a conversation with Trevor Packer today (@AP_Trevor on Twitter) who told me that 2% of the schools administering the AP chemistry exam on Monday, had 50% of their kids see DIFFERENT questions to the ones released (and answered here). This is to create a pool of questions that can be statistically analyzed and used again, and is the reason that we no longer get the green question booklets and the reason that the kids are not supposed to talk about the questions.

Question 1:

For the first time in over a decade, the equilibrium question repeated a K theme in consecutive years. Very straightforward, solid chemistry here, nothing to report. BTW – Ka and molar mass suggest that HA is benzoic acid.

Question 2:

Quite a novel way to test some combustion analysis, and I think it will have made lots of quite able kids think a lot – in that respect I like it. I also like the originality of the question for such a simple reaction and situation, so in that respect, good. There are many different ways to solve (b), (c) and (d) which means this could be a pain to grade!

Question 3:

They called ethene, ‘ethene’ and did NOT reference ethylene – HOORAY FOR IUPAC!

(a), (b) and (c) are uninspiring to say the least, but like question 2, (d) and (e) show some originality.

Question 4:

As we all know, the net ionic equation writing has been a shadow of its former self of late but this reaches perhaps an all time low. This question is close to being an embarrassment for an AP exam. I think it’s a terrible shame that the NIE’s have been decimated over the years, but this takes the cake. Someone at the CB/ETS ought to be ashamed.

Here’s a few ways to make this question better;

(a) – Make the acid weak, and then follow up with a question about testing for CO2 (limewater test).

(b) – Almost ANYTHING would help here! Formation of magnesium nitride perhaps?

(c) – Ask about excess hydroxide ions being added and the formation of a complex and then follow up with a question about THAT. At LEAST don’t FLAG the precipitate!

Question 5:

A pretty easy question but one that will probably be scored very harshly and will result in much lower scores than one might anticipate. The grading of these bonding questions has been really meticulous in the past, to the point of being obsessive, and I bet there will be lots of people that thought they scored full credit that end up with half or less. I don’t see the point of essentially having all five parts ask about bonding, and I think it goes against the idea that questions should be more diverse. It’s really the same thing asked over and over again.

Question 6:

Run of the mill with the exception of (e)(iii) which is NUTS.

(a) – Disappointing, since there are much more imaginative ways to ask about reactivity series, and it would be nice to perhaps talk about OBSERVATIONS (or lack of them) rather than just giving bland equations. Isn’t this the kind of thing they WANT to do more of?

(e)(ii) – I would MUCH prefer it if the question did NOT tell us that a precipitate forms and the kids have to work it out. It would be better that way.

(e)(iii) – I was puzzled at first when I did not see the option of ‘zero’ mentioned in the question (like it had been in (e)(i)), since I thought if the switch is open then the circuit would be broken and the voltage would be zero. I actually got this wrong as a result, until Paul Cohen pointed out that the BIZARRE manner in which the circuit is connected means that even with the switch open, the circuit is still complete. This is VERY, very odd and frankly very, very sneaky. I have no problem with the odd curve ball here and there*, but lets not start asking the kids physics and electricity circuit questions on the AP chemistry exam!

*There is of course a valid argument to say that odd, tricky questions will help to distinguish the very best kids, but when (traditionally) the only distinction for a 5 has been getting in the 60-70% range, if a really great kid misses a single, tricky point, in the end that ISN’T actually going to distinguish him or her from a kid that gets the really hard point.

OK, before you take a look at my FIRST DRAFT answers PLEASE understand the following;

1. They are first draft and will likely containing errors and have omissions.

2. Please let me know what you think, and correct me where necessary.

3. Thanks to the venerable Paul Cohen (veteran chemistry educator, experienced AP chemistry reader and table leader) for picking me up on a couple of minor points already!

Going out a limb is always dangerous, so please be respectful of my willingness to do so! Thanks.

Here are my FIRST DRAFT answers

Comments

  1. Thanks for the comments, tweaking as I go!

  2. elpeddi says:

    On question 3, delta H is sum of the heat of formation of the products minus sum of the heat of formation of the reactants, so based on Hess’s Law and bond enthalpy that makes the value negative not positve which means it would be exothermic in part C not endothermic (or am I totally off on my calculations here)?

    • Breaking is endo (+ve), making is exo (-ve) and Delta H is the SUM of those numbers IF you use the signs in the manner that I did. I think my answer is correct.

  3. Thanks for all the suggestions, typo corrections etc.

    I think I have fixed most things now, and you should be looking a version of the answers that has a time stamp of ‘5/10/12 10:48 AM’ in the footer.

    I will pull these after another 24 hours.

  4. bmeighan says:

    Adrian, thanks for posting the draft answers. I think you might have made a mistake on your draft on #3d. You used the first order integrated rate law to calculate k but you used ln[a] at time 0 – ln[a] at time t = -kt . The first order integrated rate law shows ln [a] at time t – ln[a] at time 0 = -kt . When I plugged into the equation as written on the formula sheet, I got -.209 min-1for k.

    • bmeighan says:

      Adrian, I see that you had the right answer, just had the formula shown reversed and I made a math error in coming up with .209 min-1

    • I wrote it backwards, sorry – it’s been amended now.

  5. Hi Adrian, on question 2, in order to get the oxygen pressure it seems much more straightforward to simply subtract the pressure of the hydrocarbon from the total pressure since no change in temperature or volume is reported