2021 Live Review 8 | AP Chemistry | Free-Response Medley & Exam Strategies

The session begins with a welcome and an overview of the AP Chemistry exam preparation. The focus is on a free response medley, which covers various challenging topics such as equilibrium, kinetics, thermodynamics, and molecular structure through exam-style questions. The presenter shares a handout with a link and QR code for further resources and emphasizes the importance of practicing different skills like experimental analysis, interpreting graphs, and drawing particle diagrams. Tips are provided to avoid common mistakes and the presenter reminds the audience about the depth of resources available on AP Daily for each topic.

The first question discussed is about nitrogen oxides, starting with the reaction of NO gas with O2 gas to form NO2 gas. The presenter guides the audience through drawing a particle level representation of the product mixture after the reaction, emphasizing the importance of using the balanced chemical equation to identify the limiting reactant and ensuring atom conservation in the diagram. The process includes calculating the mole ratio, identifying NO as the limiting reactant, and constructing the diagram with the correct representation of oxygen and nitrogen atoms.

The script moves on to discuss the calculation of the equilibrium constant (K) for a reaction involving NO and NO2 gases at 298 Kelvin. It covers the importance of unit consistency and the use of the correct value for the ideal gas constant (R). The presenter demonstrates how to use the Gibbs free energy (ΔG) to calculate K, emphasizing the need to either memorize or derive the expression relating ΔG to K. The calculation results in a value of 0.70 for K. The presenter also explains how to determine the equilibrium pressure of the product using an ICE (Initial, Change, Equilibrium) table and predicts the outcome based on the value of K.

The presenter agrees with a student's hypothesis that decreasing the temperature will increase the amount of N2O3 in the equilibrium mixture for an exothermic reaction. The discussion then shifts to Lewis structures, where the presenter explains how to calculate formal charges and use them to determine the better representation of the bonding in N2O3. The presenter also discusses the Lewis structure of HNO2 and the hybridization of oxygen atoms within it, correcting a student's misconception about the hybridization being the same for both oxygen atoms.

The focus is on a titration experiment where HNO2 is titrated with KOH, and the presenter explains how to determine the initial concentration of HNO2 using the titration curve and the equivalence point volume. The presenter also addresses a student's hypothesis about the concentrations of HNO2 and NO2- in the solution after the equivalence point, explaining why the hypothesis is incorrect due to the nature of the neutralization reaction and the complete reaction of HNO2 at the equivalence point.

The presenter discusses an experiment to determine the enthalpy of combustion of ethanol, where the mass of ethanol combusted is determined by weighing the alcohol burner before and after combustion. The data from the experiment is used to calculate the heat energy absorbed by water, and the presenter explains how to use the heat of combustion and the moles of ethanol combusted to find the heat released per mole of ethanol. The presenter also covers error analysis, explaining how the presence of water in the alcohol burner or using a different substance with a lower specific heat, like olive oil, would affect the final temperature of the water in the beaker.

The presenter delves into a kinetics problem involving the reaction between NO and Br2. The data from an initial rates experiment is used to determine the order of the reaction with respect to each reactant. The presenter explains how to use the principles of collision theory to explain why the rate of disappearance of Br2 decreases as the reaction proceeds. The rate law is then written based on the orders found, and the presenter calculates the specific rate constant (k) using the data from the experiment. The discussion also includes an analysis of a proposed mechanism for the reaction and its consistency with the experimental observations.

The presenter addresses a scenario where a second scientist repeats an experiment with identical concentrations of Br2 and NO in a smaller container, explaining that the initial rate of appearance of NOBr will be equal to the rate observed in the original experiment due to the same concentration and temperature conditions. The presenter concludes with general tips for the AP exam, emphasizing the importance of reading questions carefully, showing work for calculations, and using the formula sheet effectively. The presenter also mentions that solutions for the questions will be posted for review and thanks the audience for their participation.