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Using Information from the 2023 AP Chemistry Reading to Improve Exam Performance

Michael Farabaugh | Sun, 08/13/2023 - 15:56
text under magnifying glass: Insights from the AP Chemistry Exam Reading

I have been an AP Reader since 2011, and I am grateful for the opportunity to participate in this process. It has helped me to improve the quality of my instruction and assessment. Here are a few of the benefits, in my opinion.

  • I develop a better understanding of common student mistakes and misconceptions.
  • Reading through free-response questions gives me ideas for writing my own AP-style questions.
  • I enjoy connecting with and learning from a talented cohort of chemistry educators during the Reading.

Last year I wrote this article for ChemEd X1 in which I summarized student errors from the 2022 AP Reading. What follows below is an article that follows the same format, based on the information from the 2023 AP Reading.

I attended the AP Reading in Salt Lake City in June. On the final day of the Reading, each question leader (for one of the seven free response questions from the operational form of the exam) shared a few examples of student errors. In the fall, the Chief Reader Report will be available on the College Board website. This document provides the following information.

  • an overview of each free-response question
  • a summary of how students performed on each question
  • examples of student errors observed during the AP Reading
  • suggestions for improving student preparation for the exam

Links to Chief Reader Reports from previous years are listed below.

At the end of July I attended the ChemEd 2023 Conference (chemed.uoguelph.ca), at the University of Guelph in Ontario, Canada. During the AP Chemistry symposium, Chief Reader Kyle Beran led a session entitled “Review of the 2023 AP Chemistry Exam.” Kyle presented the results of the 2023 AP Chemistry Exam, along with scoring guidelines and instructional strategies to improve student success on the exam.

On September 28 (at 7 PM ET), AP Chemistry Curriculum Director Jamie Benigna will present this information as a webinar for AACT entitled “Lessons Learned from the AP Chemistry Exam.” Teachers can register for this webinar by visiting this website. Afterward, the recording of the webinar will be available to AACT members.

The seven released free-response questions from the 2023 exam can be found on the AP Chemistry Exam page at AP Central. If you have not already done so, I suggest that you download the 2023 Released Free-Response Questions so that you can work through each question on your own. This will give you a better understanding of the context regarding various student errors. A preliminary version of the scoring guidelines can be found here: PDF icon 2023 FRQ SG DRAFT Farabaugh.

Question #1

Part

Mistakes or Misconceptions

(a)(i)
  • Skipped one or more subshells (e.g., 1s22s22p63s23p63d5).
  • Mislabeled the energy level of the d subshell (e.g., 1s22s22p63s23p64s24d5).
  • Wrote the electron configuration for Mg (1s22s22p63s2) instead of Mn.

(a)(ii)
  • Chose the 3d subshell instead of the 4s subshell.

(b)
  • Subtracted the mass of the empty beaker from the final mass of the beaker and its contents. This value represents the mass of the MnxCly(s) sample instead of the mass of Cl in the sample.

(c)
  • Wrote the correct answer for the number of moles of Cl, but did not show any supporting work or calculations.
  • Used the molar mass of C (12.01) instead of the molar mass of Cl (35.45).

(d)
  • Wrote the correct empirical formula, but did not show any supporting work or calculations.
  • Used decimal values instead of whole numbers for the subscripts in the empirical formula.
  • When determining the empirical formula, divided the smaller number of moles by the larger number of moles instead of doing the calculation the other way around.

(e)
  • Did not refer to the details of the experimental data.
  • Used vague, unclear language (e.g., “it would have a lower mass.”)
  • Did not relate the smaller amount of Cl to the smaller quantity of MnxCly(s) present in the beaker at the end of the experiment.
  • Stated that the number of moles of Cl would remain the same because the empirical formula of the compound would not change.

(f)(i)
  • Attempted to combine all three of the half-reactions listed in the table into one net ionic equation.
  • Wrote an equation that contained identical species on both sides.

(f)(ii)
  • Calculated a value of E° that was inconsistent with the equation written in part (f)(i).
(f)(iii)
  • Used the wrong value of n in calculations with the equation ΔG° = –nFE°.
  • Used the wrong units in the final answer or made mistakes when converting from units of joules (J) to kilojoules (kJ).
(f)(iv)
  • Agreed with the student’s claim, stating that the anode always loses mass.
  • Gave a definition of the law of conservation of mass without discussing the processes that occur in this particular galvanic cell.
  • Focused only on the electron transfer. Since the mass of the electrons is very small, the change in mass must be negligible.
  • Wrote that one electrode loses mass and the other electrode gains mass, without referring to the transfer of mass between the electrodes or stating that this battery is a closed system.

 

Question #2

Part

Mistakes or Misconceptions

(a)
  • Calculated the number of moles of Cl instead of the number of grams of Cl.
  • Gave an answer such as 130 g Cl or 132.9 g Cl that was rounded to something other than three significant figures. (The quantity 1.25 mol AlCl3 has three significant figures. Therefore the calculated value for the number of grams of Cl should be expressed with three significant figures.)

(b)
  • Calculated the sum of the three values of ΔH that are listed in the table (e.g., –583 + 326 + 243 = –14), without doing either of the required algebraic manipulations for ΔH2 or ΔH4.
  • Performed only one of the two required algebraic manipulations (e.g., changed the sign of ΔH2 or multiplied ΔH4 by 1.5, but did not do both of these manipulations).
  • Calculated a value of ΔH that is twice as large as it should be (i.e., +2547), based on the following equation:  2 AlCl3(g) → 2 Al(g) + 6 Cl(g).

(c)(i)
  • Misread the diagram and reported an incorrect value for bond length.
(c)(ii)
  • The x-value of the energy minimum of the student’s curve had a value other than 220 ± 10 pm.
  • The y-value of the energy minimum of the student’s curve had a value other than –425 ± 20 kJ/mol.
(d)(i)
  • Eliminated Diagram 1 because the Al atom has less than an octet.
  • Eliminated Diagram 3 because it contains a double bond.
  • Eliminated Diagram 2, but did not give a rationale related to VSEPR or molecular geometry.
  • Eliminated Diagram 2, but did not explain how the lone pair on the central atom affects the bond angles and/or the molecular geometry of the molecule.
(d)(ii)
  • Stated that the formal charge is zero for the entire compound instead of referring to the formal charges of the individual atoms.
  • Stated that the formal charges were equal to the charges on the ions (i.e., Al = +3 and Cl = –1).
  • Confused the number of valence electrons of the atom with the formal charge on that atom (i.e., Al = 3 and Cl = 7)
  • Chose Diagram 3 because the Al atom has a complete octet with only three electron domains.

(e)
  • Wrote a Kp expression without using the symbol P (which denotes partial pressure).
  • Forgot to square the value of PAlCl3 in the denominator.

(f)
  • Made a calculation error that resulted in an incorrect value of Kp.

 

Question #3

Part

Mistakes or Misconceptions

(a)
  • Wrote the formula of solid calcium carbonate as dissociated into separate ions.
  • Did not eliminate the chloride ion from both sides of the equation as a spectator ion.
  • Wrote incorrect chemical formulas or formulas with incorrect or missing charges.

(b)
  • Did not use the data in the table for trials 1-3 to explain why trial 5 should have a shorter reaction time than trial 6.

(c)
  • Stated that the solid in trial 2 has a greater surface area than the solid in trial 3, but did not refer to the greater number of collisions between reactant particles.
  • Stated that trial 2 has a greater number of particle collisions than in trial 3, but did not refer to the difference in the surface area for the two solids.

(d)
  • Gave a justification that did not refer to the experimental information from the data table.

(e)
  • Calculated the molarity of the HCl that was consumed in the reaction instead of the molarity of HCl remaining in the beaker after the reaction is complete.
  • Did calculations based on a 1-to-1 mole ratio between CaCO3 and HCl, even though the balanced equation provided in this question shows a 1-to-2 mole ratio between CaCO3 and HCl.
  • Made errors related to units, such as calculating molarity by dividing the number of moles by the volume of the solution in milliliters.

(f)
  • Gave a justification that did not refer to the temperature data from the table.

(g)(i)
  • Incorrectly used the value of the final temperature of the solution (21.90) as the value of the change in temperature (ΔT).

(g)(ii)
  • Did not convert the answer from part (g)(i) from units of joules (J) into kilojoules (kJ).
  • Although this reaction had been classified as exothermic in part (f), the student did not include the negative sign with the calculated value for the change in enthalpy (ΔH) for this reaction.

 

Question #4

Part

Mistakes or Misconceptions

(a)
  • Used an incorrect value for the molar mass of CH3NH3Cl.
  • Wrote the correct answer for the mass of CH3NH3Cl, but did not show any supporting work or calculations.

(b)
  • Did not mention that the electronic balance should be used in order to weigh the solid.
  • Did not mention that the buret should be rinsed with a small amount of 0.100 M CH3NH2(aq) before it is filled with this same solution.
  • Did not mention that buret should be filled with 0.100 M CH3NH2(aq) after it is rinsed with this same solution.

(c)
  • Based on mistakes in calculations related to Ka, Kb pH, or pOH, the student claimed that the pH of the second buffer was either less than or greater than the pH of the first buffer.

 

Question #5

Part

Mistakes or Misconceptions

(a)(i)
  • Used the wrong value of the ideal gas constant R (e.g., 8.314) in the calculations.
  • Made algebraic mistakes (e.g., n = RT/PV) when solving for the variable n.

(a)(ii)
  • Used an incorrect equation involving the variables pressure and temperature (e.g., P1T1 = P2T2)
  • Calculated the new value for gas volume at 271 K (i.e., 5.49 L) and reported this answer as the new pressure value.
  • Made algebraic mistakes when solving for the new pressure at 271 K.
  • Solved for the number of moles (n) instead of the pressure (P).

(b)
  • Reversed the correct orientation of the H2O molecules so that oxygen atom (white circle) was pointing toward the negatively charged chloride ion.
(c)
  • Did not refer to information from the particle diagram, which indicates that the acid (HA) molecules are only partially dissociated in aqueous solution.
  • Used words such as “dissolve” or “solubility,” which revealed confusion between the percent ionization of an acid in aqueous solution and the solubility of the acid in water.
  • Stated that HNO2 has the smallest Ka value (of the three acids listed in the table), but did not indicate that the value of Ka is less than one.

 

Question #6

Part

Mistakes or Misconceptions

(a)
  • Did not list dipole-dipole attractions for HBr(l).
  • Did not list hydrogen bonding for HF(l).

(b)(i)
  • Stated that the intermolecular forces present in HF(l) are stronger than those present in HBr(l), without mentioning the specific type of attractive force responsible for this difference.
  • Did not compare the relative strength of the intermolecular forces present in HF(l) with those present in HBr(l).
  • Compared the relative strength of the covalent bonds present in HF and HBr.

(b)(ii)
  • Did not convert the quantity 6.85 g HF into moles.
  • Used an incorrect value for the molar mass of HF.
  • Used the value for enthalpy of vaporization for HBr (17.3 kJ/mol) instead of the value for HF (25.2 kJ/mol)

(c)
  • Stated that Br has a larger size than F without discussing the difference in the number of occupied electron shells or energy levels.

 

Question #7

Part

Mistakes or Misconceptions

(a)
  • Stated that the number of ions in the diagram was incorrect, but did not refer to the correct ratio for Sr2+ and OH–.
  • Stated that there was an error, but did not describe how this error should be corrected.

(b)(i)
  • Assumed that [Sr2+] = [OH–].
  • Calculated the value of [OH–] by dividing 0.043 M by 2 instead of multiplying 0.043 M by 2.

(b)(ii)
  • Based on a transcription error, used the values 0.43 M and 0.86 M in the calculation instead of using the values 0.043 M and 0.086 M.
  • Forgot to square the value for [OH–] in the calculation.
  • Obtained an incorrect value for Ksp based on an incorrect formula, such as the following: (0.043)(2)(0.043)2 = 1.6 × 10–4

(c)
  • Mentioned the common ion effect or Le Châtelier’s principle, but did not explain why the value of [OH–] in the second solution would be less than the value in the first solution. 
  • Stated that the value of Ksp would be affected by the presence of the common ion.
  • Stated that the value of [OH–] in the second solution would be equal to the value in the first solution.

 

Based on these mistakes and misconceptions, the following represents a list of suggestions for improving performance on the AP Chemistry exam.

  • Read the information provided in the question carefully, so that you answer the question that is being asked.
  • When you are asked to give an explanation or justification, you will often see phrases such as “based on…” or “in terms of…” Such phrases are intended to serve as a guide for the specific information that the question is looking for in a student’s response.
  • Practice answering free response questions that require both explanations and justifications, so that you can use appropriate vocabulary and communicate your understanding of chemical concepts clearly.
  • If a question asks you to provide an explanation or justification based on information from a data table, make sure to provide specific information from that data table in your response.
  • If a question asks if you agree or disagree with a certain claim, make sure to clearly indicate whether you agree or disagree with the claim.
  • The instructions on the AP Chemistry Exam state that you “must show your work to receive credit for your answer.” This advice is especially important when simple calculations are performed to determine a numerical quantity.
  • Utilize dimensional analysis to perform simple unit conversions such as converting from moles to grams or vice versa.
  • Play close attention to the correct units when setting up and performing calculations.
  • Be familiar with the guidelines for how to round off the final answer in a calculation to the proper number of significant figures.
  • When writing an electron configuration, be careful to write the correct numbers and letters for each energy level and orbital subshell.
  • Understand the distinction between an electron configuration and an orbital diagram.
  • For a question similar to #1(a)(ii), see FRQ #1(e)(ii) from the 2015 AP Chemistry Exam.
  • Practice solving problems in which the mass data collected during an experiment is used to determine the empirical formula or the identity of an unknown compound.
  • Pay attention to the details when doing empirical formula calculations. For example, if you determine that there is a 2-to-1 mole ratio between Cl and Mn, it is incorrect to write the empirical formula as Mn2Cl.
  • Always use whole numbers, not decimals, for the subscripts in an empirical formula.
  • If you are asked to describe the details related to experimental results, avoid the use of vague, ambiguous language. For example, “Because some of the solid splatters out of the beaker, the final mass of the beaker and MnxCly(s) will have a lower mass” is more specific than writing “It will have a lower mass.”
  • Practice combining half-reactions in the proper way in order to produce the overall equation for the reaction that occurs in a galvanic or electrolytic cell.
    • You cannot combine two reduction half-reactions or two oxidation half-reactions. The electrons will not be cancelled out on opposite sides of the equation.
    • You cannot combine three half-reactions to produce an overall balanced equation.
    • Multiplying the coefficients of a half-reaction by N does not change the value of the cell potential E°.
  • When using the equation ΔG° = –nFE°, pay attention to the signs of ΔG° and E°.
    • If E° is positive, then ΔG° is negative.
    • If E° is negative, then ΔG° is positive.
  • When doing calculations with the equation ΔG° = –nFE°, pay attention to the units of your final answer.
  • 1 joule (J) is equal to the product of 1 volt (V) times 1 coulomb (C).
  • Practice solving Hess’s Law problems, and make sure that you know how the value of ΔH is affected when a thermochemical equation is reversed or when the coefficients in an equation are multiplied by a constant N.
  • Be familiar with the details of the potential energy curve that shows potential energy on the y-axis and internuclear distance on the x-axis. The bond between two atoms is represented by the potential energy minimum on this curve, and you can identify both the bond energy (y-axis, absolute value) and the bond length (x-axis).
  • Practice drawing the correct Lewis diagrams for various molecules and polyatomic ions. Review the procedure for determining the correct molecular geometry based on VSEPR theory.
  • Review the rules for assigning formal charges to each atom in a Lewis diagram. If more than one Lewis diagram is possible for a molecule, review the guidelines for determining the most preferred representation of the bonding, based on formal charges.
  • Know the difference between the Kc expression and the Kp expression.
  • Practice solving problems in which you are given the total pressure of a gas mixture and a particle diagram that represents that gas mixture. Be familiar with how to use this given information to determine the partial pressure of each gas in the mixture.
  • Practice writing net ionic equations. Pay special attention to the following details.
    • Correct chemical formulas and charges on ions
    • Factors such as states of matter or acid strength, which affect whether a chemical formula should be written exactly as it appears or if it should be dissociated into separate ions
    • Removing spectator ions that appear identically on both sides of the equation
    • Making sure that the net ionic equation is balanced in terms of both atoms and charge
  • Practice problems that involve using the data from a calorimetry experiment to determine the value of ΔH for a chemical reaction, dissolution of a solid, or a phase change. Important details include the following.
    • Classifying the process as endothermic or exothermic based on the temperature data from the experiment
    • Using the correct algebraic sign for ΔH
    • Paying attention to the units used with both q and ΔH
    • Relating the number of moles of the limiting reactant to the quantity mole of reaction
  • Review techniques involving the proper use of laboratory equipment, such as the procedure for filling a clean buret that has been rinsed with distilled water.
  • Be familiar with different laboratory methods for the preparation of buffer solutions.
  • Be familiar with the proper use of the Henderson-Hasselbalch equation or the Ka expression to calculate the pH of the following types of buffer solutions.
    • [conjugate base] = [conjugate acid]
    • [conjugate base] < [conjugate acid]
    • [conjugate base] > [conjugate acid]
  • Be familiar with the information given on the AP Chemistry Equations and Constants sheet, especially when choosing the appropriate value for the ideal gas law constant R.
  • Practice drawing particle diagrams that illustrate ion-dipole attractive forces between ions and water molecules. For a question similar to #5(b), see FRQ #4(c) from the 2015 AP Chemistry Exam.
  • Practice drawing particle diagrams that illustrate a strong acid (100% ionized) and a weak acid (partially ionized). For a question similar to #5(c), see FRQ #5(a) from the 2018 AP Chemistry Exam.
  • Review the connection between the Ka value for a monoprotic acid and the percent ionization in solution. In other words, if the value of Ka for an acid is less than one, the percent ionization of the acid should be less than 100%.
  • Know the difference between a covalent bond that exists within a molecule (i.e., intramolecular force) and the attractive forces that exist in between molecules (i.e., intermolecular forces). It is very important to use precise language and terminology. A phrase such as “it requires more energy to break the bonds” is ambiguous. It is difficult to determine if the student is referring to covalent bonds within molecules or intermolecular forces between molecules.
  • There is an important distinction between merely knowing a periodic trend (e.g., the fact that Br has a larger atomic radius than F) and being able to explain this trend in terms of atomic structure and/or the arrangement of electrons.
  • Be precise with your word choice when describing the details of electronic structure. If one atom “has more occupied energy levels” than another atom, this is not equivalent to stating that “it has more subshells” or “it has more valence shells.”
  • Practice answering questions in which a particulate diagram is used to represent the ions present in an aqueous solution of an ionic compound.
  • Practice solving problems that involve saturated solutions, especially those including the following details.
    • You are given the value of Ksp, and you must calculate the concentration of each ion in a saturated solution.
    • You are given the concentration of one ion in a saturated solution, and you must calculate the concentration of the other ion and the value of Ksp for the ionic compound.
    • You are asked a question about how the presence of a common ion affects the solubility of an ionic compound.

I hope this article has been helpful to AP Chemistry teachers as they begin a new school year. I encourage you to watch the AACT webinar on September 28, 2023 presented by Jamie Benigna. Any teachers who are interested in applying to become an AP Reader can get more information at the following web site.

Please feel free to log in and comment below. I look forward to connecting with you.

 

  1. Farabaugh, M., Using Information from the 2022 AP Chemistry Reading to Improve Exam Performance, August 2022, Chemical Education Xchange.

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