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Using Information from the 2022 AP Chemistry Reading to Improve Exam Performance

Michael Farabaugh | Wed, 08/10/2022 - 18:19
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In June AP Chemistry readers either traveled to Salt Lake City or worked online from home to read and score student responses from the 2022 exam. I attended the in-person Reading and served as a table leader for a group of at-home readers. I enjoyed the opportunity to do the following.

 

  • Re-connect with my colleagues, many of whom I had not seen in person since 2019
  • Discuss strategies for teaching chemistry with a diverse, talented group of readers
  • Reflect on ways to help students improve the quality of their responses to free-response questions

 

On the last day of the Reading, the question leaders presented a brief overview of how students performed on each question, including examples of common student errors. This information, in addition to advice for helping students improve exam performance, will be included in the Chief Reader Report, a document that serves as a valuable planning tool for AP teachers. The Chief Reader Report will be available on AP Central in the fall. Links to reports from previous years are listed below.

I attended the Biennial Conference on Chemical Education (bcce2022.org) from July 31 to August 4, which was held on the campus of Purdue University in West Lafayette, IN. During the AP Chemistry symposium, Chief Reader Kyle Beran led a session entitled “Review of the 2022 AP Chemistry Exam,” which was a more detailed version of the information that had been shared at the Reading in June. Kyle will also present this information in the form of a webinar for AACT in September, entitled “Lessons Learned from the AP Chemistry Exam.” Teachers can register and watch this webinar for free by visiting teachchemistry.org. Afterwards, the recording of the webinar will be available to AACT members.

In this article, I will list the mistakes or misconceptions that were encountered during the 2022 AP Reading. The 2022 released free-response questions (as well as questions from previous years) 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 2022 free-response questions so that you can work through each question on your own. Then you will have a better understanding of the context regarding various mistakes and misconceptions. If you would like to see a preliminary version of the scoring guidelines for these free-response questions, you can find it here: PDF icon 2022 FRQ SG DRAFT Farabaugh.

 

Question #1

Part

Mistakes or Misconceptions

(a)
  • Since this question part was designated as “the sig fig question,” any student who expressed their final answer by rounding to something other than three significant figures (e.g., 0.27 g or 0.2723 g) did not earn this point.

(b)
  • Disagreed with the student’s claim
  • Agreed with the student’s claim, but didn’t give a justification or gave an incorrect justification
  • Didn’t make the connection between the decrease in percent yield and the fact that some of the solid may have dissolved in water during the rinsing step

(c)
  • Calculated only one of the following quantities for heat, but not both
    • the heat absorbed as the solid is heated from 25°C to 159°C (16.5 J)
    • the heat absorbed as the solid melts at 159°C (20.6 J)
  • Made mistakes with unit agreement (J vs. kJ)

(d)
  • Stated that salicylic acid has more hydrogen bonding sites than methyl salicylate, but did not mention anything about the relative strength of the intermolecular forces
  • Referred to an intermolecular force other than hydrogen bonding, such as London dispersion forces.

(e)
  • Didn’t make the connection between the pH of the solution at the half-equivalence point and the pKa of the weak acid

(f)
  • Stated that the acid has a higher concentration than the conjugate base
  • Stated that the conjugate base has a higher concentration than the weak acid, but didn’t give a justification
  • Didn’t do either of the following in their justification.
    • compare the pH of the solution to the pKa of the acid
    • recognize that when pH = 4, this is a point in the titration that is beyond the half-equivalence point

(g)
  • Gave the correct answer (4.20), but didn’t show any work (e.g., pKa = –log(6.3×10–5))

(h)
  • Drew a titration curve that didn’t include all three of the following features
    • at 0 mL, pH ≈ 3.11
    • at 5 mL, pH = pKa value that was calculated in part (g)
    • at 10 mL, a vertical inflection or rapid increase in pH

 

Question #2

Part

Mistakes or Misconceptions

(a)
  • Failed to include the oxidation numbers or gave incorrect oxidation numbers
  • Stated that the oxidation number of H in CH3OH is “positive” without specifically stating that it is +1
  • Stated that the oxidation number of H in CH3OH is +4 (because they multiplied +1 by the number of H atoms in the molecule)

(b)
  • Drew a Lewis diagram with the wrong number of total valence electrons
  • Drew a Lewis diagram in which the octet rule is violated

(c)(i)
  • Didn’t use the coefficient of 2 for H2 in the ΔS calculation.
  • Set up the formula for the ΔS calculation “backwards,” in the form of  “reactants minus products” instead of “products minus reactants.”

(c)(ii)
  • Made calculation errors
  • Didn’t pay attention to the fact that there are different units for ΔH (kJ/molrxn) and ΔS (J/molrxn · K)

(d)
  • Didn’t count the number of particles in the diagram correctly
  • Made errors such as 3 × 12 atm = 36 atm
  • Gave the correct answer (3.6 atm), but didn’t show any work (e.g., (3/10)(12 atm))

(e)
  • Wrote the K expression as Kc, with brackets [ ] around each substance instead of writing the Kpexpression with the symbols for the partial pressure of each substance
  • Wrote the symbols for partial pressure, but also surrounded each one of them with brackets [ ]
  • Didn’t use the coefficient of 2 for H2 as the exponent in the K expression
  • Wrote the expression upside-down, as reactants over products

(f)
  • Didn’t use the coefficient of 2 for H2 as the exponent in the calculation

(g)
  • Stated that the number of moles of CH3OH would increase
  • Stated that the equilibrium position would shift toward the reactants
  • Contradicted themselves, by stating that number of moles of CH3OH would decrease because the value of Q is greater than the value of K
  • Didn’t compare Q and K in their answer
  • Revealed misconceptions regarding Q and K, such as the following
    • The equilibrium shifts toward the products, causing Q to become larger than K
    • The equilibrium shifts toward the products, causing K to become larger than Q

 

Question #3

Part

Mistakes or Misconceptions

(a)
  • Drew a Lewis electron dot diagram for the Al atom
  • Drew a Bohr model or shell diagram for the Al atom
  • Switched the numbers for the energy level of the subshell with the number of electrons that occupy that subshell (e.g., 2s1 2s2 6p2 1s3 1p3)
  • Wrote “3p” at the end of the electron configuration instead of “3p1”

(b)
  • Stated that the Al3+ ion has more protons than the Al atom
  • Stated that the attractive force of the nucleus is distributed over all of the electrons in an atom or ion, causing the electrons in Al to experience a weaker attraction to the nucleus than the electrons in Al3+ because the attractive force per electron is lower in Al
  • Wrote general statements without discussing principles of atomic structure, such as the following
    • The species with more electrons has a larger size
    • A positive ion is smaller than the parent atom
    • Al3+ has a greater effective nuclear charge than Al
    • Al has more electron-electron repulsions than Al3+

(c)
  • Chose a piece of glassware with an inappropriate level of precision (e.g., Used a 250 mL beaker instead of a 200.00 mL volumetric flask)
  • Used the volumetric flask to measure 200.00 mL of water and then added the water to another container
  • Didn’t mention anything about making sure that all of the solid dissolves
  • Added water up to the 200.00 calibration mark on the volumetric flask and then added the solid to the flask afterwards
  • Added 200.0 mL of water that had been measured in another container to the solid solute in the volumetric flask

(d)
  • Drew a particle diagram that didn’t include all three of the following features
    • conservation of matter, with 8 Ag particles and 4 Al particles
    • conservation of charge, with 2 Ag+ ions and 2 Al3+ ions
    • correct phases of matter, with 6 Ag atoms in solid phase and 2 Al3+ ions in aqueous phase

(e)
  • Gave the correct answer (2.46 V), but didn’t show any work (e.g., 0.80 + 1.66)
  • Added the two E° values for the reduction half-reactions together  (e.g., E° = 0.80 V + (–1.66 V) = –0.86 V is incorrect)
  • Added the two E° values for the oxidation half-reactions together (e.g., E° = –0.80 V + 1.66 V = 0.86 V is incorrect)
  • Multiplied the E° value for the reduction of Ag+ by three; didn’t recognize that E° values are intensive properties that do not depend on the stoichiometric coefficients in the equation (e.g., E° = (3)(0.80 V) + 1.66 V = 4.06 V is incorrect)

(f)
  • Stated that ΔG° would be positive because the reaction is thermodynamically favorable
  • Stated that ΔG° would be negative without giving an explanation
  • Wrote a general statement without discussing thermodynamic favorability or referring to the equation ΔG° = –nFE°  (e.g., Simply stating that ΔG° < 0 because E° > 0 is insufficient)

(g)
  • Stated that ΔG would be zero without giving an explanation
  • Stated that ΔG would be zero because E° = 0 (i.e., confusion about E° and E)
  • Stated that ΔG would be zero because n = 0 (i.e., ΔG = –( 0 )FE)

 

Question #4

Part

Mistakes or Misconceptions

(a)
  • Gave the correct answer (3.1 × 10–5 mol), but didn’t show any work (e.g., 0.0016 g × (1 mol / 51.48 g))
  • Didn’t pay close attention to units (e.g., Gave an answer of 0.0016 mol)

(b)
  • Used the word “bond” without clarification
  • Didn’t differentiate between intramolecular forces (i.e. chemical bonds) and intermolecular forces such as hydrogen bonding
  • Used vague language such as “NH2Cl has hydrogen bonds” instead of clearly identifying the attractions between the molecules of solute and solvent
  • Focused on attractions between the molecules of pure solute instead of the attractions between the molecules of solute and solvent
  • Stated that “like dissolves like” because both NH2Cl and H2O are polar molecules
  • Failed to recognize that the nonbonding pair of electrons on the N atom in NCl3 is capable of forming a hydrogen bonding interaction with an H atom on a nearby water molecule
  • Included incorrect information such as “bonds are broken” or “ions are formed”
  • Identified the intermolecular forces correctly but didn’t connect the relative strength of the IMFs to the observed difference in solubility

(c)
  • Gave the correct answer (4.10 kJ), but didn’t show any work (e.g., 15.0 g × (1 mol / 120.36 g) × (32.9 kJ / 1 mol))
  • Included the wrong unit in the final answer (e.g., kJ/mol instead of kJ)

 

Question #5

Part

Mistakes or Misconceptions

(a)
  • Failed to include units in the answer
  • Included the wrong units in the answer (e.g., M / hr, hr, min–1, or sec–1)
  • Used the wrong rate law equation from the equation sheet (e.g., zero order or second order)

(b)
  • Identified step 1 as the rate-determining step, but used vague wording in the justification, such as the following
    • because N2O5 is a reactant
    • because step 1 is the first step

(c)
  • Stated that the value of the rate constant k would increase because the rate increases.
  • Stated that the value of the rate constant k would remain the same, but gave a justification that was incorrect or incomplete, such as the following
    • the reaction rate remains constant
    • it is a first-order reaction
    • the time required for the reaction to reach completion is also doubled
  • Gave an answer that indicated confusion between the rate constant k and the equilibrium constant K.

 

Question #6

Part

Mistakes or Misconceptions

(a)
  • Gave a large range of wavelength values instead of a single optimum wavelength of 525 nm

(b)(i)
  • Failed to recognize the precision of the graduated cylinder (e.g., 92 mL or 92.00 mL instead of 92.0 mL)
  • Misread the calibration marks on the graduated cylinder (e.g., 90.2 mL instead of 92.0 mL)

(b)(ii)
  • Gave the correct answer (70.0 mL), but didn’t show any work
  • Made a calculation error, giving a final answer of 143 mL; failed to recognize that it is not possible to dilute 143 mL of a stock solution to a final volume of 100.0 mL

(c)
  • Failed to recognize that rinsing the cuvette with the standard solution in Step 3 should remove any drops of water remaining in the cuvette from rinsing in Step 2

 

Question #7

Part

Mistakes or Misconceptions

(a)
  • Gave the molecular geometry instead of the hybridization (e.g., trigonal planar instead of sp2)
  • Gave the electron configuration of a carbon atom (i.e., 1s22s22p2)

(b)(i)
  • Didn’t include the charges on the ions (e.g., Ksp = [Ag]2[C2O4])
  • Had the wrong charge on the Ag ion (e.g., Ksp = [Ag2+][C2O4])
  • Had the wrong charge on the C2O4 ion (e.g., Ksp = [Ag+][C2O4–])
  • Didn’t use the coefficient of 2 as the exponent for [Ag+] in the Ksp expression

(b)(ii)
  • Made errors in setting up the equation, such as the following
    • 5.40 × 10–12 = s3
    • 5.40 × 10–12 = 2s3
  • Made algebraic errors when solving for the molar solubility s.

(b)(iii)
  • Didn’t write a net-ionic equation (e.g., 2 HClO4 + Ag2C2O4 → H2C2O4 + 2 AgClO4)
  • Included spectator ions on both sides of the equation (e.g., 2 H+ + 2 ClO4– + 2 Ag+ + C2O42– → H2C2O4 + 2 ClO4– + 2 Ag+)
  • Didn’t write a balanced equation (e.g., 2 H+ + 2 ClO4– + C2O42– → H2C2O4)
  • Didn’t include charges on ions (e.g., 2 H + C2O4 → H2C2O4)

 

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

  • Always show your work when solving problems, even when simple calculations are performed.
  • Read carefully, so that you answer the question that is being asked.
  • Double-check calculations and evaluate the reasonableness of your final answer, to reduce the risk of careless errors.
  • Use the correct the units in calculations involving equations or dimensional analysis. This may help you to avoid mistakes like the one that is frequently encountered with the Gibbs free energy equation.
  • Review common mistakes related to unit conversions.
  • Practice making a measurement on an electronic balance, thermometer, graduated cylinder, or buret that takes into account the precision of that piece of equipment.
  • Consider experimental errors in a laboratory experiment. Use claim, evidence, and reasoning to make connections between various laboratory situations and the experimental results or calculations.
  • Simply making a general statement, stating a memorized trend, or stating a relationship between two variables is not the same as giving an explanation or justification based on chemical principles.
  • Practice writing justifications in which there is a connection between intermolecular attractive forces and observed properties (e.g., phase of matter, boiling point, melting point). A response that merely identifies the types of intermolecular forces is insufficient. Students must compare the relative strength of the forces.
  • Understand the difference between a pure substance and a solution, with respect to the interactions between the particles in a sample of matter.
    • In a pure substance, focus on the attractive forces between the particles of that pure substance
    • In a solution, focus on the attractive forces between the particles of solute and solvent
  • Be as specific as possible with your word choice. Avoid circular arguments that merely restate the information in the question, without giving an explanation or justification.
  • Avoid using ambiguous language such as the following examples.
    • Using the word “it” in your response can be confusing to the reader
    • Using the word “bonds” can imply confusion about intramolecular and intermolecular forces
    • Stating that a molecule “has” hydrogen bonds or another intermolecular force is inappropriate
    • Using the phrase “like dissolves like” to explain solubility is not a sufficient explanation
    • Making general statements about the properties of atoms or ions without specifically referring to principles of atomic structure is incomplete
  • Make sure you can distinguish between the following concepts commonly confused on the AP exam.
    • Lewis electron dot diagrams
    • electron configuration
    • orbital hybridization
    • molecular geometry
    • bond angles
  • Practice drawing correct Lewis structures for molecules and polyatomic ions. Important note: During the process of drawing a Lewis diagram, a student may decide to erase some of the dots or lines. It may be difficult for a reader to tell if a pair of electrons or a bond has been completely erased or not. It is better if the student crosses out the original diagram and draws a new one, to ensure that the details of their intended final answer are clearly visible to the reader.
  • Understand how to balance a chemical equation in terms of mass and charge. Know the difference between a net-ionic equation and a molecular equation.
  • Perform a laboratory experiment in which you prepare a solution of known molarity. When dissolving a solid in water, key features of this procedure include the following.
    • Choosing an appropriately sized volumetric flask based on the final volume of solution
    • Distinguishing between the volume of water added and the final volume of the solution
    • Ensuring that all of the solid solute dissolves before the final volume of solution has been adjusted to the calibration mark on the flask
  • Understand the rules for assigning the correct oxidation numbers to atoms in chemical formulas.
  • Practice doing heat calculations associated with a heating/cooling curve for a pure substance. (e.g., temperature on the y-axis and heat on the x-axis)
    • When calculating the heat associated with a temperature change, q = mcΔT
    • When calculating the heat associated with a phase change, q = (number of moles)(ΔH)
  • Practice answering questions that involve particulate diagrams, including the following.
    • Drawing or completing a particulate diagram related to a chemical process
    • Focusing on details such as conservation of mass, charge balance, and phase of matter
    • Making a claim and justification based on the information in the diagram
    • Answering both conceptual questions that don’t involve calculations as well as quantitative questions that do involve calculations
  • Understand the meaning and significance of the sign for quantities such as ΔS°, ΔH°, ΔG°, and E°.
  • Be familiar with spectrophotometry experiments, including the following.
    • Choosing the optimum wavelength for a spectrophotometry experiment
    • Preparing solutions with different concentrations in order to create a calibration plot
    • Making the connection between absorbance and concentration through the Beer-Lambert law
    • Understanding how changes in experimental conditions affect the observed absorbance of a solution
  • Practice analyzing the pH curves from acid-base titrations, including the following examples.
    • Determining the pKa of an acid from the titration curve
    • Identifying which species, the acid or the conjugate base, has the higher concentration at various points throughout the titration
    • Using information from the titration curve to calculate the molarity of an unknown solution
    • Sketching a titration curve based on certain information, such as the initial pH, the initial concentration of the reactants, and the Ka or pKa of the acid
  • Practice converting from Ka to pKa and vice versa.
  • Know how to write an equilibrium constant expression, especially how to differentiate between the Kc expression and the Kp expression.
  • Practice solving equilibrium problems in which you have to calculate the value of the reaction quotient Q.
  • Know how to use the comparison of Q and K to predict the direction in which a chemical system will shift in order re-establish equilibrium.
  • Practice solving problems related to electrochemistry, in order to do the following.
    • Identifying which species is oxidized and which species is reduced
    • Combining two half-reactions together to produce an overall equation for a reaction that is thermodynamically favorable
    • Calculating the value of the overall standard cell potential E° for the reaction
    • Relating the sign of E° to the thermodynamic favorability of the reaction
    • Understanding how the cell potential E can vary under nonstandard conditions, especially as the system approaches and reaches equilibrium
  • Know how to choose and apply the appropriate integrated rate law equation (zero order, first order, or second order) to solve for an unknown quantity.
  • Know how to write the correct units for the rate constant k, based on the reaction order and the experimental data given in the question.
  • Practice writing a justification to connect a proposed reaction mechanism and the observed rate law for the overall reaction.
  • Be familiar with the effect, if any, that various changes in experimental conditions (e.g., concentration, temperature, the presence of a catalyst, etc.) will have on the following.
    • reaction rate
    • half-life of a reactant
    • time required for the reaction to reach completion
    • rate constant
  • Be able to do the following with respect to solubility equilibrium.
    • Writing the net ionic equation for the dissolution of the solid
    • Calculating the molar solubility of the solute from the Ksp value and vice versa

I hope this article has been helpful to AP Chemistry teachers as they begin to kick off a new school year. I encourage you to watch Chief Reader Kyle Beran’s AACT webinar in September. Any teachers who are interested in applying to become an AP reader can get more information at the following web site.

You are welcome to log in and add your comments to the conversation below. I look forward to connecting with you.

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Comments 3

Yu-Sung Wu's picture
Yu-Sung Wu | Fri, 08/12/2022 - 14:02

This is a very detailed summary report for each FRQ.  As an AP chem teacher, I appreciate the insight you provided to teachers and students.  Some of the suggestions could also be applied to teach honors chemistry, such as including units during dimensional analysis.

Yu-Sung