Reflection on the 2018 AP Chemistry Free Response Questions

AP reflection

Now that the 2018 administration of the AP Chemistry Exam is in the books, all of us AP Chemistry teachers now have an opportunity to reflect on the year as we turn our attention toward preparing for the fall.

One part of this process is the review of the released Free Response Questions from this year’s exam. Every year the College Board releases the FRQs from the operational exam (Form O, the version of the exam most students in the United States take) forty-eight hours following the conclusion of the administration. The released FRQs since the redesign of the exam in 2014 can be found HERE and questions from 1999-2013 can be found HERE.

The official scoring guidelines will be posted in late summer after the reading of the exam, followed by the extremely valuable Chief Reader Report, scoring statistics, and sample student responses. Until then, teachers around the world post their draft answers on the National AP Chemistry Teachers Facebook Page or the AP Chemistry Teacher Community and reflect on the exam and what we can learn from it.

I strongly suggest that all AP teachers take the time to fully answer every released FRQ in order to gain a deeper understanding and “feel” of the test. Additionally, read the Chief Reader Report when it is released, read what others post online, and engage in the conversation around how the exam is scored. At BCCE this summer, the AP Chief Reader Dr. Paul Bonvallet will give a presentation in which he reviews the exam. It is always a very informative talk.

My draft answers to the 2018 FRQs, as well as my prediction of how points will be awarded are attached to this post as a PDF. I will now reflect on each of the questions, the exam overall, and highlight things that I think teachers, especially new teachers, can take away from this test.

Question 1

(a)-(c) - The question is very straightforward at the beginning, and I enjoyed the unique way the limiting reactant question was presented. I appreciated the use of more than two reactants and that the limiting reactant could be determined without doing much (or really showing any!) mathematics, if a student had a strong understanding of proportional reasoning and immediately recognized the equimolar solutions.

(d) - Reading graphs and glassware to proper levels of precision is an important skill and has shown up repeatedly since the redesign. Only one point on every exam will be awarded for proper significant figures and it will always be a lab scenario. Note that it may or may not be specified for the student to report the value with the proper number of sig figs. My bet is that this was the sig fig question, though others have suggested part (b) given that the volume of the solution provided was so precise (100.00 mL). See  2010 #3 for a very similar question.

(e) - Part (i) is very straightforward, and I liked that they gave a value for the heat capacity of the solution that differed from that of water. I may modify some of my practices to do this, instead of always assuming it to be 4.184 J/g·°C.

Part (ii) is likely going to stir the pot AP teacher community due to the use of the “per mole of reaction” concept. For those who are not familiar, “per mole of reaction” basically means “every time the reaction runs as written”. We will have to wait to see until after the reading which calculated value(s) earned credit. To learn more about this concept, read this article written by James Spencer , the co-chair of the AP Exam Redesign Commission.

(f) - Would a student get credit for simply citing that thermodynamic quantities are intensive properties? Likely not based on conversations I’ve had with previous AP readers. The student would have to explain in their answer that intensive properties do not vary with amount or state their reasoning in terms of “per mole.”

(g) - Remember, just because Question 4 from the Legacy Exam is gone, that does not mean that students do not have to write equations. In fact, this year they had to write three! This was very straightforward, as sodium was the only spectator ion.

Question 2

(a) - I really liked this question. It was a unique way of testing the qualitative aspects of stoichiometry and a reminder of the importance of particle diagrams both for exam success, but also for strong stoichiometric understanding. Have your students draw a ton of particle diagrams, especially in first-year chemistry, when balancing equations and doing stoichiometry.[1] I wonder how many students did not realize this was a limiting reactant problem…

(b)-(c) - These questions deal with equilibrium and are very straightforward. For b (ii) I wonder how many students got stuck when they hit the quadratic and didn’t know what to do. I hope not many, as the question does not ask for a specific value to be calculated. This is a good reminder that quantitative proficiency does not equate to conceptual understanding and that the AP will never require students to solve a quadratic equation to get an answer to any problem.

(d) - This was a very straightforward bonding question. Have your students practice filling in skeletal structures like this, they pop up frequently for more complex molecules. Two different structures are possible, and would both earn points since students were not asked to consider formal charge.

(e)-(f) - A very simple, straightforward weak acid /strong base titration with predictable questions.

Question 3

(a) - Students will likely forget that the 4s electrons will be removed from an iron atom before the 3d electrons. Some teachers have expressed concern over Aufbau exceptions, but I see no problem here as the Course and Exam Description (CED) does not require knowledge of any exceptions on the exam.

(b)-(c) - Basic atomic structure and Coulomb’s Law questions. I do wonder how specific or vague the wording will need to be for (b) to earn the point.

(d)-(e) - A simple redox titration. I am surprised at the numerous uses of molarity in the FRQs this year. Maybe I am wrong that it is abnormal, but at this point on the exam I was thinking, “wow, more solution stoichiometry!”

(f) - An absurd question, if you know what you’re doing, that got a lot of hilarious posts on social media. Make sure student know proper equipment vocabulary!

(g)-(i) - The wording on part (i) will cause students points here I am afraid and perhaps it could have been phrased a bit more clearly. But with an FRQ that has already been criticized by students and teachers for its length (9 parts, tying the longest FRQs of the redesigned exam with 2014 #1 and 2015 #1). This is likely only to be a single point.

Question 4

(a) - Again students were asked to explain how a non-polar substance can have a higher boiling point than a polar substance. This is a favorite question, it seems, as the test writers try to exploit the common misconception that comes from simply memorizing that LDFs are “weak”.

(b) - A pleasant surprise that must be worth two points. As I say to my students, “Give me the points!”

Question 5

(a) - A good use of particle diagrams to test understanding of weak acids. I would rather them have had to choose which was the most accurate representation. The fact that they were told Figure 1 was better was a huge advantage!

(b) - Students will assume “-x” is negligible even though they calculated it by determining [H+]. This will cost many a point. The writers seem to be testing students understanding of when to ignore “-x” in various ways in recent years.

(c) - This will be a bloodbath, just as it was in 2016 #6. Use Q v. K, it is your friend

Both (b) and (c) harken back to 2016 #6 where the modal score was a 0/4. See what was said about this question on what is now called the Chief Reader Report. This is a great question to foster good class discussion.

Question 6

(a) - As far as I know, this is the first time students have been asked to explain the purpose of the salt bridge, as opposed to drawing the direction of ion flow, so it will be interesting to see what phrasing earns credit. Many probably still think electrons flow through it…

(b) - Very simple, get those points! Though the algebraic sign of Eo will likely get many students.

Question 7

(a) - Simple PES identification. It would have been nice to see a follow up question that had more depth, but that means that there was an easy point available for rate constant units in part (b).

(b)-(c) - Straightforward first-order kinetics and half-life question. Teachers and students have been grumbling that it is unfair because nuclear chemistry is not part of the course. However, I do not share their frustration. The CED clearly mentions radioactive decay as an example of first-order kinetics in Essential Knowledge Statement 4A.3(e) under Learning Objective 4.3 and even if it did not, students should clearly recognize that if the half-life is citable, then it must be constant and therefore the process is first-order, no matter what that process is.

I would hope this question scores very high.

Overall Impressions

This was a very fair free response section that was well balanced between conceptual and quantitative understanding. The last few years we have seen a consistency develop in the style of the exam and evidence that it is very faithful to the CED. One of the goals of the redesign was to create an exam that was faithful to a set of standards, not just to previous iterations of exams.[2] The more exams we see, the more I think that the Test Development Committee is succeeding in this aspect.

What are your thoughts of this years FRQs? I am excited to hear how the reading goes in a few weeks at to debrief at BCCE in South Bend, Indiana with everyone!


[1] See the following articles and resources about visualizing chemistry using particle diagrams and using the table method (BCA) to solve stoichiometry problems.
Bridle, Chad A. and Ellen J. Yezierski. Evidence for the Effectiveness of Inquiry-Based, Particulate-Level Instruction on Conceptions of the Particulate Nature of Matter. Journal of Chemical Education, 2012;89;(2), 192-198. DOI: 10.1021/ed100735u.
Dukerich, Larry. “Conceptual Chemistry.” https://www.chemedx.org/blog/conceptual-chemistry.
Hemling, Melissa. “Using Visual BCA Tables to Teach Limiting Reactants”, https://www.chemedx.org/blog/using-visual-bca-tables-teach-limiting-reactants.
Posthuma-Adams, Erica. “Simple Activities to Implement Particle-Level Diagrams", https://www.chemedx.org/blog/simple-activities-integrate-particle-level-diagrams.
Prilliman, Stephan J. “Integrating Particulate Representations in to AP Chemstry and Introductory Chemsitry Courses.” Journal of Chemical Education 2014 91 (9), 1291-1298. DOI: 10.1021/ed5000197.

Underwood, Kaleb. “A Visual and Intuitive Approach to Stoichiometry.” https://teachchemistry.org/professional-development/webinars/a-visual-and-intuitive-approach-to-stoichiometry.
[2] David Yaron. Reflections on the Curriculum Framework Underpinning the Redesigned Advanced Placement Chemistry Course. Journal of Chemical Education201491;(9), 1276-1279. DOI: 10.1021/ed500103e

 

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Comments 2

Michael Farabaugh | Tue, 05/15/2018 - 01:18
These are my scoring guidelines. Here are some of my thoughts about the answers and the point distributions.   I think 3(g) will likely represent the sig fig question. I think most students will report the temperature change in 1(d) to one decimal place without thinking about sig figs at all. The CB can make the sig fig question a little more subtle than that. They have already asked students in other parts of the FRQs to convert from grams to moles. Therefore 3(g) is not really about grams to moles, but rather how should one report the answer to the proper number of sig figs. The moles of Fe2O3 must contain exactly 4 sig figs in order to earn credit because the mass of Fe2O3 was reported to 4 sig figs.  Student must get their sig figs straight from the data table, but many will ignore this.   For 1(e)(ii), I think that an answer of 300 kJ/mol should earn zero points, because the student made two separate mistakes: wrong magnitude and wrong sign. An answer of –300 kJ/mol should earn 1 point (for the correct sign but the wrong magnitude). I like giving them 1 point for having the negative sign, since this was mentioned in the stem of the question. We anticipate that we're going to see lots of answers of "300" and not very many answers of "1200" In my opinion, the second point, magnitude of ΔHrxn, should be harder to earn.   For question 2, I think part (b)(i) should be worth 1 point. You either use the equation to calculate K correctly or you don't. In part (e)(i), you have to get the moles of KOH from determining the volume of KOH on the titration curve and doing the M x V calculation. Then you have to recognize the 1-to-1 mole ratio between KOH and HNO2 and do the molarity calculation for the acid. That's a lot of manipulations for only 1 point. Therefore I think 2 points for 2(e)(i) seems reasonable.   I'll be curious to see what represents the "minimum" acceptable answer in 5(c). Can a student simply state that diuluting a weak acid increases the % ionization? Do they have to mention Q vs. K? Can they say "shift toward the side with more particles"? There were 2 points allocated in 2016 #6(b) for the dilution question, and it was interesting to see what sort of responses did (or didn't) earn the justification point for that one.   I think the most popularly missed portion of #7 is going to be the units in 7(b). And yet, are we going to award the point for an answer of "0.70" in 4(b), even if they don't include the units of atm? Hmm. I wish they had asked students to include units in 4(b), considering that they could get an answer in either atm or torr.   Well, this post is long enough, so I guess I'll stop there. Thanks for sharing your reflections.

Kaleb Underwood's picture
Kaleb Underwood | Tue, 05/15/2018 - 23:12

Thanks for your always insightful comments, Michael! 

The more I think about it, the more agree with you on the sig fig point. The math works out too nicely for the graph to be it. I also agree with your point redistribution in question 2, and in fact you identified where I was torn in where to award points.

I was surprised to see the unit point appear in #7, but I can't imagine any other way the points could be distributed. The reason I was surprised is that for 2017 #2(e)(ii) it was all or nothing. But I guess every exam will be scored different depending on the content of the exam, even on similar (or almost identical) questions.