Chief Reader Report on Student Responses: 2018 AP® Chemistry Free-Response Questions Number of Students Scored Number of Readers Score Distribution Global Mean 161,852 360 Exam Score 2.80 N 21,624 28,489 40,285 38,078 33,376 %At 13.4 17.6 24.9 23.5 20.6 The following comments on the 2018 free-response questions for AP® Chemistry were written by the Chief Reader, Paul Bonvallet of the College of Wooster They give an overview of each free-response question and of how students performed on the question, including typical student errors General comments regarding the skills and content that students frequently have the most problems with are included Some suggestions for improving student preparation in these areas are also provided Teachers are encouraged to attend a College Board workshop to learn strategies for improving student performance in specific areas © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Question #1 Task: Analysis of redox experiments Topics: Oxidation number, stoichiometry, thermochemistry Max Points: 10 Mean Score: 4.47 What were the responses to this question expected to demonstrate? Parts (a) through (c) explored fundamental concepts including oxidation numbers, stoichiometry, and limiting reactants within the context of an oxidation-reduction reaction In the second grouping, parts (d) through (f) focused on the interpretation of calorimetry data for the redox reaction by determining the standard enthalpy change of the reaction, H°rxn Part (g) explored student understanding of net-ionic equations Students were asked in part (a) to determine the oxidation number of the chlorine atom in NaOCl (LO 3.8; SP 6.1) In part (b) students were asked to determine the mass of Na2S2O necessary to prepare 100.00 mL of a solution of given concentration (LO 1.4; SP 7.1) In part (c) students were asked to determine the limiting reactant in the reaction, given the balanced chemical equation and the concentrations and volumes of solutions of each reactant (LO 3.4; SP 2.2, 5.1, 6.4) Part (d) asked students to interpret a graph of temperature versus time to determine the temperature change of the reaction mixture (LO 3.1; SP 1.5, 7.1) This answer carries forward to part (e)(i), where students were asked to calculate the magnitude of the heat energy released during the reaction (LO 5.7; SP 4.2, 5.1, 6.4) In part (e)(ii) students were asked to calculate the standard enthalpy change for the reaction (LO 5.7; SP 4.2, 5.1, 6.4) In part (f) students were asked to explain why the calculated value of H°rxn remains unchanged in a second experiment where the volume of each solution of reactant was doubled (LO 3.3; SP 2.2, 5.1) In part (g) students were asked to provide a balanced net-ionic equation for the reaction (LO 3.2; SP 1.5, 7.1) How well did the response address the course content related to this question? How well did the responses integrate the skills required on this question? The mean score for Question was 4.47 out of a possible 10 points The distribution of points on this question is shown below Q1: Mean 4.47 ± 2.27 Percent of Students 25.0 20.0 15.0 10.0 5.0 0.0 - 10 Score Nearly every student attempted Question Most earned at least a few points The first half of the question, parts (a)-(e)(i), was fairly accessible Most students earned points in parts (b), (d), and (e)(i) In part (b), most students were able to determine the number of moles of Na2S2O required and convert this number into grams of Na2S2O Most students correctly interpreted the graph in part (d) to determine the value of T and then successfully used this result to determine |q| in part (e)(i) © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Parts (a) and (c) were more challenging In part (a), some students assumed that NaOCl contains a chloride ion and thus reported the oxidation number of the chlorine atom as −1 Part (c) required students to evaluate tabular data to determine the limiting reactant Many students correctly chose NaOCl, although some provided insufficient justification for their choice The second half of the question, parts (e)(ii)-(g), proved challenging for many students Parts (e)(ii) and (f) required higher-level thinking and a stronger intuitive understanding of chemical principles Some students struggled to find a valid method for converting q to H°rxn in part (e)(ii), while many others gave their answer without a negative sign to indicate that the reaction is exothermic In part (f), many students had difficulty relating the concept of proportionality to H°rxn and did not address the fact that the ratio of heat energy released per moles of reactant was unchanged in the second experiment In part (g), most students failed to provide a complete, balanced, net-ionic equation Many responses failed to dissociate the sodium salts or otherwise included sodium ion in the equation What common student misconceptions or gaps in knowledge were seen in the responses to this question? Common Misconceptions/Knowledge Gaps Responses that Demonstrate Understanding Part (a): The oxidation number of Cl in NaOCl is +1 This information can be deduced from the standard +1 and −2 oxidation numbers of sodium and oxygen, respectively Claiming −1, the charge of a chloride ion Multiplying the oxidation number by the stoichiometric coefficient of NaOCl in the balanced chemical equation Reporting the number of valence electrons in chlorine Part (b): 100.00 mL Errors in algebra and/or arithmetic = 7.90 g Na2S2O3 Incorrect molar mass of Na2S2O3 Expressing molarity in units of g/L, or equating molarity with the number of moles of a substance © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Part (c): Failing to use the information in the table, i.e., claiming NaOCl as the limiting reactant based solely upon its stoichiometric coefficient in the balanced chemical equation NaOCl is the limiting reactant Given that equal numbers of moles of each reactant were present initially, it follows from the coefficients of the reactants in the balanced equation that NaOCl will be depleted first Failing to consider the balanced chemical equation, i.e., claiming that none (or all) of the reactants were limiting because each solution contains the same number of moles of reactant Choosing NaOCl because it is the reactant with the lowest molar mass Claiming that an equal number of moles of each reactant was present based only upon the solutions’ equal concentrations or equal volumes, rather than using both concentration and volume Part (d): From the graph the final temperature is 32.5C Reading the y-axis tick marks incorrectly, resulting in an incorrect initial and/or final temperature T = Tf Ti = 32.5C − 20.0C = 12.5C Rounding to the nearest ± 1°C rather than ± 0.1°C Reporting T as −12.5°C rather than +12.5°C Determining that T = 12.5°C, but then adding 273 to that result to get an answer of 285.5 K Part (e)(i): Expressing T in units of Kelvins, which does not match the units of °C in the specific heat q = mcT = (15.21 g)(3.94 J/(g·C))(12.5C) = 749 J Assuming T to be 20.0C (the initial temperature) instead of 12.5C Errors in algebraic manipulation © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Part (e)(ii): Neglecting to make H°rxn negative to indicate an exothermic reaction n NaOCl = 5.00 mL n rxn = 0.00250 mol NaOCl = 0.00250 mol NaOCl = 0.000625 molrxn Failing to convert q to kJ before solving for H°rxn Divide q by molrxn and make negative to indicate an exothermic reaction: Multiplying molrxn and q rather H°rxn = −q / molrxn = −0.749 kJ / 0.000625 molrxn than using division = −1.20 103 kJ/molrxn Failing to adjust for NaOCl stoichiometry, or adjusting incorrectly (e.g multiplying by or dividing by the sum of the stoichiometric coefficients of all reactants) Calculating molrxn from the total mass of the reaction mixture, or from the sum of the molar masses of all reactants Calculating a second value of q using a different T and/or mass, then subtracting this result from the first value of q from part (e)(i) Attempting to calculate H°rxn from bond dissociation enthalpies or standard enthalpies of formation of reactants and products Part (f): Giving an incomplete explanation, e.g., stating only that H°rxn is an By doubling the volumes, the number of moles of the reactants are doubled, which doubles the amount of energy produced Therefore, the amount of heat per mole will remain the same intensive property or that H°rxn is independent of solution volume Failing to indicate that both the number of moles and the heat evolved will double in the second experiment © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Part (g): Failing to balance with respect to mass and/or charge S2O32(aq) + OCl(aq) + OH–(aq)2 SO42(aq) + Cl(aq) + H2O(l) Including spectator ions Using incorrect ionic charges Sulfate and thiosulfate ions were often assigned a charge of −1 Breaking down polyatomic ions into elemental ions, e.g., writing sulfate ion as S2− + O2− Based on your experience at the AP® Reading with student responses, what advice would you offer to teachers to help them improve the student performance on the exam? Require students to show all work and include units, even in intermediate steps, in all calculations Have students perform calculations where some data is read from a graph or table Emphasize the similarities and differences between q (amount of thermal energy absorbed or released in a given situation) versus H°rxn (standard enthalpy change for a reaction) Pay attention to sign conventions in these two values Reinforce the concept of “per mole reaction” and its application in thermochemistry problems Practice writing net-ionic equations for chemical reactions, helping students to recognize the ionic charge of common polyatomic ions What resources would you recommend to teachers to better prepare their students for the content and skill(s) required on this question? Teachers will find sample student responses to exam questions on the exam information page on AP Central, along with specific commentary explaining why each point was or was not earned Teachers can use these samples to work with students to help them become more comfortable in practicing and producing responses within the suggested response time, so that students devote an appropriate amount of time for each question Teachers will find scoring guidelines explaining how the exam questions were scored on the exam information page on AP Central Teachers can use and adapt these scoring guidelines throughout the AP year so that students become familiar with how their responses will be scored Teachers can review elements of Q1 from the 2017 exam, Q3 from the 2016 exam, and Q2 from the 2015 exam and Q5 and Q6 from the 2014 exam Teachers can use the guidebook Quantitative Skills in the AP Sciences (2018) to assist students in developing quantitative skills throughout the course The AP Chemistry Online Teacher Community is active and there are many discussions concerning teaching tips, techniques, and activities that many teachers have found helpful It is easy to sign up for and you can search topics of discussions from all previous years Newer teachers (and career changers) might want to consider signing up for an APSI An APSI is a great way to gain in-depth teaching knowledge on AP Chemistry curriculum and exam and is also a great way to network with colleagues from around the country The Chief Reader Report Module is a brief walkthrough of the highlights of the chief reader report, by Paul Bonvallet of the College of Wooster © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Question #2 Task: Analysis of reactions of nitrogen oxides Max Points: 10 Topics: Equilibrium, thermodynamics, Lewis structures, acid-base titration Mean Score: 4.07 What were the responses to this question expected to demonstrate? Parts (a) through (f) assessed students’ understanding of equilibrium, thermodynamics, Le Chatelier’s principle, Lewis electron-dot structures, hybridization, acid-base neutralization, and titration In part (a) students were asked to draw a particle-level representation of the mixture of NO and O2 reactants that would afford the product mixture illustrated in the diagram Students were required to interpret and use the pictorial symbols, making connections between the balanced chemical equation and the particulate diagram (LO 1.17; SP 1.5) In part (b) students were required to use a table of thermodynamic data to calculate the value of the equilibrium constant for an equimolar mixture of NO and NO2 that has reached equilibrium at 298 K Students were expected to recognize the relationship between ∆G and K (LO 6.25; SP 2.3) They were then asked to determine if the partial pressure of N 2O3 product at equilibrium will be equal to 1.0 atm if the partial pressures of the reactants (NO and NO 2) in the vessel were initially 1.0 atm The students should recognize that K