Using Information from the 2024 AP Chemistry Reading to Improve Exam Performance

Question #1

Part

Mistakes or Misconceptions

(a)

• Circled a hydrogen atom that is not attached to the carboxylic acid group (–CO₂H)
• Circled a chemical bond or an atom of carbon or oxygen.

(b)

• Made errors when converting the volume of 500. milliliters into units of liters.
• Made algebraic errors (e.g., multiplying instead of dividing).

(c)

• Reported a pK_a value of approximately 8, which suggests the following mistakes.
  
  • confusion about the location of the half-equivalence point (pH = 3.9) and the location of the equivalence point (pH ≈ 8) on the titration curve
  • reporting the volume of the titrant (8 mL) at the half-equivalence point instead of the pH of the solution (3.9) at the half-equivalence point
• Even though the value of pK_a was requested in the question, students used the estimated value of  pK_a to calculate the K_a value of lactic acid.

(d)(i)

• Made an incorrect conclusion about the particle diagram, in which two C₃H₅O₃^– particles and four C₃H₆O₃ particles are shown. Since [C₃H₅O₃^–] = ½ [C₃H₆O₃], students assumed incorrectly that the solution in the flask represents the half-equivalence point of the titration experiment.
• Wrote answers that showed confusion between the half-equivalence point and the equivalence point of a titration.

(d)(ii)

• Used the general term “base” instead of the more specific term “conjugate base”.

(d)(iii)

• Drew a titration curve for experiment 2 that contained one of the following features.
  • The curve did not begin at an initial pH of 2.
  • The curve was identical to the titration curve for experiment 1.
  • The pH at the halfway point of the curve (i.e., where pH should be equal to pK_a) was much higher than pH = 4.

(e)(i)

• Calculated the combined mass of the solutes C₃H₆O₃ and NaOH (i.e., 4.5 g + 2.0 g = 6.5 g) instead of using the total volume of the solution (200.0 mL) and the density (1.00 g/mL) to calculate the mass of the combined solutions (200. g).
• Used the final temperature of the solution (23.2°C) as the value of ΔT instead of calculating the change in temperature (23.2°C – 20.0°C = 3.2°C).
• Made algebraic errors.

(e)(ii)

• Did not include the negative sign with the calculated value of ΔH_rxn. This reaction is classified as exothermic.
• Did not divide the amount of heat (2.7 kJ) by the quantity “mole of reaction” (0.0500 mol).
• Added the number of moles of each reactant (0.0500 mol) together, to give an incorrect value for the quantity “mole of reaction” (0.100 mol).

• Stated that the value of the molar enthalpy of reaction would be larger, not smaller.
• Did not include a specific reference to the temperature, which is a measurement recorded during this calorimetry experiment.
• Re-stated the prompt, stating only that the calculated value of heat or the value of the molar enthalpy of reaction would be smaller.

Question #2

Part

Mistakes or Misconceptions

(a)(i)

• Used an incorrect value for the molar mass of CO₂.
• Used a conversion factor of (44 g CO₂ / 2 mol CO₂)
• Made algebraic errors (e.g., dividing instead of multiplying).

(a)(ii)

• Used a value for the ideal gas constant (R) with inconsistent units (e.g., 8.314 J mol^–1 K^–1 instead of 0.08206 L atm mol^–1 K^–1).

• Didn’t convert the temperature from units of degrees Celsius into units of Kelvins.
• Assumed incorrectly that the gas sample was under STP conditions (i.e., 22.4 L mol^–1).
• Used the number “2,” the coefficient of CO₂ from the balanced equation, as the number of moles of CO₂ instead of the value that was given in the question (0.0114 mol CO₂).

(b)(i)

• Wrote an answer that showed confusion between the surface area of a single particle and the surface area of the entire solid sample. Example: “The surface area of the powder is less than the surface area of the chunks because the particles of the powder are smaller and take up less space.”

• Did not discuss the collisions between the particles.
• Showed confusion between time and rate (e.g., “the time would be greater because of the increased frequency of collisions between particles.”)

• Stated that the volume of CO₂ produced in the second experiment would be greater, based on an incorrect justification such as more collisions or greater surface area.
• Provided an incorrect justification for the claim that the volume of CO₂ would be the same in both experiments, such as the following.
  • The temperature and/or the pressure remains the same.
  • The experiment obeys the law of conservation of matter (or mass).

• Chose H₂C₄H₂O₄ as the limiting reactant, based on the fact that 0.0133 mol H₂C₄H₂O₄ is less than 0.0149 mol NaHCO₃, which ignores the stoichiometric ratio of the reactants.
• Chose NaHCO₃ as the limiting reactant, but gave an incorrect justification such as the following.
  
  • 1.251 g NaHCO₃ is less than 1.543 g H₂C₄H₂O₄.
  • The molar mass of NaHCO₃ is less than the molar mass of H₂C₄H₂O₄.
  • There are 2 moles of NaHCO₃ for every 1 mole of H₂C₄H₂O₄.
  • The mass of NaHCO₃ changes as you go from Trial 3 to Trial 4.

(d)

• Gave an explanation for the increase in entropy that did not include particle-level reasoning. Examples include the following.
  
  • The total number of moles of the products is greater than that of the reactants.
  • There are more moles of gas in the products than in the reactants.
  • The phase of matter changes from aqueous (aq) to gaseous (g).
  • Gases are more random (or chaotic or disordered).
  • The reaction involves the formation of gaseous, liquid, and aqueous substances (i.e., there is no specific attribution to the formation of gaseous particles as the reason for the increase in entropy)
• Did not use particulate-level phrases to describe the properties of the gas particles, such as “more microstates,” “more dispersed,” “greater number of arrangements,” “greater distribution of energy,” etc.

(e)

• Agreed with the claim by stating that an endothermic reaction has a negative ΔH value.
• Stated that endothermic reactions are not thermodynamically favorable.
• Disagreed with the claim, but didn’t provide any justification.
• Disagreed with the claim, but only referred to the positive ΔH value or only referred to the positive ΔS value in the justification.
• Stated that an endothermic reaction (i.e., a reaction with a positive ΔH value) that also has a positive ΔS value will only be thermodynamically favorable at certain temperatures. The phrase “at high temperatures” is more specific than “at certain temperatures.”

• Stated that pK_a2 = 6.07, but did not show the setup for this calculation.
• Calculated the value of pK_a1 (1.82) instead of calculating the value of pK_a2 (6.07).
• Multiplied the values of K_a1 and K_a2 together to give a value of 1.3 × 10^–8. Then took the negative log of that K value (7.89).

• Stated that the ratio of [C₄H₂O₄^2–] to [HC₄H₂O₄^–] is 1-to-1 because the pH of the solution is 7.00, or because the molar masses of these two substances are similar.
• Made a calculation error with the Henderson-Hasselbalch equation, obtaining a value of –0.93 instead of +0.93 (i.e., 10^–0.93 = 0.12, whereas 10^0.93 = 8.5).
• Used the base e from the natural log (ln) function to perform the calculation instead of using base 10 (i.e., e^0.93 = 2.5, whereas 10^0.93 = 8.5).

Question #3

Part

Mistakes or Misconceptions

(a)

• Assigned an oxidation number of +1 (instead of zero) to Ag in the element Ag(s).
• Assigned an oxidation number of +2 (instead of +1) to Ag in the compound Ag₂S(s).

(b)(i)

• Provided an incorrect explanation of the fact that sterling silver is better classified as a substitutional alloy, such as the following.
  • Ag has a larger atomic radius than Cu.
  • Ag has a larger atomic mass than Cu.
  • Sterling silver is made up of more than one type of metal.

(b)(ii)

• Provided an incorrect or insufficient explanation of the fact that Ag has a larger atomic radius than Cu, such as the following.
  • Ag is located below Cu on the periodic table.
  • Ag has more electrons/orbitals/subshells/etc. than Cu
  • Compared to Cu, the valence electrons of Ag experience a weaker attraction to the nucleus.
• Did not show understanding of the fact that the difference in the atomic radius of two elements located in the same group is primarily a function of the principal quantum number (n) of the occupied electron shells or the valence shells.

(c)

• Calculated the number of moles of Ag₂S (0.04144 mol) instead of calculating the number of moles of Ag (0.08289 mol).
• Used the value 92.5% silver, given at the beginning of the question, to calculate the number of moles of Ag present in 10.27 g Ag₂S(s). The value 92.5% is the percentage of Ag by mass in the alloy. It is not the percentage of Ag by mass in the compound Ag₂S (87.06%).

(d)(i)

• Tried to combine two reduction half-reactions instead of combining a reduction half-reaction and an oxidation half-reaction.
• Wrote a chemical equation that was not balanced.
• Ignored the information given in the description of the electroplating experiment, which stated that Rh(s) and O₂(g) are formed as products.

• Multiplied the half-cell potentials by the balancing coefficients. (+0.80 V × 4) + (–1.23 × 3) = –0.49 V
• Added the two reduction half-cell potentials together (0.80 V + 1.23 V = 2.03 V).

• Showed confusion about the relationship between the sign of the cell potential E° and the thermodynamic favorability of a reaction.
• Stated that a galvanic cell requires an external power source.
• Stated that E° is negative and/or stated that ΔG° is positive, without mentioning that the reaction is thermodynamically unfavorable.
• Justified the need for an external power source in terms of things such as breaking chemical bonds or overcoming the activation energy for the reaction.

• Used an incorrect conversion factor involving the relationship between moles of electrons and moles of Rh(s), such as the following. ( 12 mol e^– / 1 mol Rh )  or  ( 1 mol Rh / 3 mol e^–  ).
• Reported a final answer with an incorrect number of significant figures. The correct answer should be expressed with two sig figs (3900 s or 4.0 × 10³ s).

Question #4

Part

Mistakes or Misconceptions

(a)

• Reported the temperature with no decimal places (38°C).
• Reported the temperature with two decimal places (38.50°C).
• Misread the lines in the diagram (30.85°C, 35.35°C, or 40.15°C).
• Used incorrect units, such as grams or milliliters.

(b)

• Drew arrows with the same lengths as or with shorter lengths than the arrows shown in the “Before” diagram.
• Drew arrows in the form of curved lines instead of straight lines.

(c)

• Used the mass of the water (52.0 g) instead of using the mass of the metal (98.1 g).
• Combined the masses of the metal and the water to get a value of 150.1 g.
• Used an incorrect value of ΔT such as the following.
  
  • the initial temperature of the metal (100°C)
  • the final temperature of the mixture (38.5°C)
  • the change in temperature for the water (38.5°C – 25.0°C = 13.5°C)
  • the difference between the two initial temperatures (100.0°C – 25.0°C = 75.0°C)
• Reported the value for the specific heat as a negative number.
• Made algebraic errors.

• Stated that the magnitude of ΔT of aluminum would be larger than the magnitude of ΔT of the metal used in the first experiment.
• Showed confusion about the inverse relationship between the specific heat and the change in temperature (when the mass and the amount of heat are constant).
• Used the inequality symbol ( <  or  > ) incorrectly.
• Showed confusion between the magnitude of ΔT and the rate of change of temperature (e.g., stated that one metal heats up more quickly than the other).
• Showed confusion between the terms heat and temperature (e.g., stated that Al absorbed less heat than the metal used in the first experiment).
• Referred to the temperature, but not the change in temperature.
• Used the vague word “ it ” when comparing the different metals, resulting in a response that was unclear regarding which metal the student was referring to.

Question #5

Part

Mistakes or Misconceptions

(a)

• Used parentheses ( ) in the K_c expression instead of hard brackets [ ].
• Wrote the K_p expression instead of the K_c expression.
• Wrote the reciprocal of the K_c expression.
• Used the number “2” as a multiplier for the term [HI] instead of an exponent.
• Used a plus “ + “ sign in between the terms [H₂] and [I₂]

(b)(i)

• Drew a number of molecules of HI that was inconsistent with the expression written in part (a).
• Drew four molecules of HI, possibly based on the assumption that the forward reaction proceeds all the way to completion.

(b)(ii)

• Stated that increasing the temperature would make the reaction more thermodynamically favorable.
• Stated that increasing the temperature would increase K_c and produce more HI.
• Hypothesized that a catalyst was added to the system.
• Hypothesized that the pressure was increased. This statement is too vague. It is possible to increase the total pressure of the system by adding an inert gas, which would not cause the number of moles of HI(g) in the system to increase.

• Stated that the number of moles of HI(g) would increase because the decrease in pressure would result in a shift toward the side of the equation with more moles of gas, which is the right side.
• Stated that the number of moles of HI(g) would increase based on the incorrect assumption that the concentration of HI must remain constant. In other words, if the total volume increases, then the number of moles of HI must also increase in order to keep [HI] constant.
• Stated that the number of moles of HI(g) would remain the same, but provided an incorrect justification, such as the following.
  
  • The temperature remains constant.
  • Nothing was added to the system or removed from the system.
  • A larger container would not affect the number of moles of HI(g).
  • The value of K_c remains the same.
  • The reaction had already reached a state of equilibrium.
• Wrote a justification that did not show understanding of the stoichiometric ratio between the reactants and products (e.g., There are two reactants for two products.)

Question #6

Part

Mistakes or Misconceptions

(a)

• Provided an incorrect or insufficient justification for the claim that the reaction is second order with respect to NO₂, such as the following.
  
  • The graph on the right is linear.
  • The second order graph is linear.
  • 1/[NO₂] is constant.
  • 1/[NO₂] has a constant half-life.
  • The relationship between 1/[NO₂] and time is proportional. (Note: “proportional” is not a synonym for “linear.”)

(b)

• Provided the correct numerical answer with the correct units, but did not show any setup for the calculation.
• Ignored the stoichiometry, assuming that the rate of appearance of O₂ is equal to the rate of disappearance of NO₂.
• Multiplied by 2 instead of dividing by 2.
• Attempted to use information found in a second order rate law (i.e., an exponent of 2) by performing a calculation involving taking the square root of the given quantity or raising the given quantity to the 2^nd power.
• Used incorrect units in the answer.

(c)(i)

• Drew a Lewis diagram for NO₂⁺
  
  • in which at least one atom does not have a complete octet
  • in which at least one atom has more than an octet
  • that has a total of 18 electrons instead of 16 electrons
  • that represents a valid resonance structure of the NO₂ molecule

(c)(ii)

• Assumed incorrectly that the single electron on the central atom in NO₂ has no effect on the bond angle of the molecule.
• Provided an incorrect or insufficient justification for the claim that the bond angles are different in NO₂ and NO₂⁺, such as the following.
  
  • NO₂ is bent because it’s polar; NO₂⁺ is linear because it’s nonpolar.
  • NO₂ is a neutral molecule, whereas NO₂⁺ is an ion.
  • The NO₂ molecule contains 1 single bond and 1 double bond, whereas the NO₂⁺ ion contains 2 double bonds.

Question #7

Part

Mistakes or Misconceptions

(a)

• Did not show sufficient work in the setup for the calculation to earn the point (e.g., omitted the molar mass of NaCl from the work shown).
• Did not use the volume of the solution (100.0 mL) when performing the calculation, Instead, multiplied the molarity of the solution (0.340 M) by the molar mass of NaCl.
• Didn’t include units in the answer.
• Made algebraic errors.

(b)

• Made errors in writing Step 2, such as the following.
  
  • Added more than 100 mL of water.
  • Didn’t mention the type of glassware used.
  • Added NaCl to a volumetric flask (or a beaker) and then added 100 mL of water.
  • Added NaCl to a graduated cylinder with 50 mL of water (without further steps added).
  • Omitted important details such as NaCl or water.
• Made errors in writing Step 4, such as the following.
  
  • Added water and filled to 100.0 mL.
  • Added water to a container that is not a volumetric flask.
  • Transferred the solution (from a beaker or a graduated cylinder) to a volumetric flask and filled to the mark with water without rinsing the glassware that had previously contained the solution.

(c)

• Didn’t compare the distance between X and Y.
• Stated that the distance between X and Y would remain the same or become larger.
• Only stated that the solvent front will not travel as far up the paper.
• Implied that a chemical reaction occurs between X and Y.