All hydrohalic acids, but hydrofluoric acid, are strong acids. Why is that (0:31)? Strong acids fully dissociate and have a large Ka, weak acids only dissociate to a small percentage (1:20). When looking at strength, we are comparing the stability of the conjugate acid/base pair partners (2:00). For binary acids across a period (2:37) as well as down a group (3:09) electronegativity determines acid strength. For oxyacids, we can compare acids with different numbers of oxygen atoms, which affects the inductive effect, as well as resonance structures (4:58). We can also discuss acid strength across a period (6:44) and down a group (7:34). Bases are proton acceptors. Common bases are conjugate bases of weak acids, like carboxylic acids, and amines (7:53).

Question of the Day: Acid A has a Ka = 3.5 x 10-8, acid B has a Ka = 1.2 x 10-2. Which acid is HClO, which one is HClO2?

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The FRQ questions on the AP exam often combine content from two or three different units. In today’s episode, we are emphasizing questions that require knowledge and skills from Unit 7: Equilibrium. You will rarely find an FRQ that solely focuses on Unit 7, since it lays the foundation for Unit 8, acids and bases as well as parts of Unit 9, Application of Thermodynamics. It also links back to concepts from earlier units, like gas laws. The questions we are using today are online accessible. Our suggestion: Answer the questions yourself and then listen to this episode to hear the explanations, as well as do’s and don'ts for answering questions of Unit 7. We will review 2016 - Question 6, 2015 - Question 4 and 2014 - Question 4. These are released FRQs from previous exams and copyright of the College Board.

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This episode is an application of earlier discussed concepts. In episode 33 we talked about disruptions affecting equilibrium: concentration, temperature and pressure. In episode 34, solubility equilibria, we talked about the dissolution of a salt. In this episode we set our focus a bit differently: We look at environmental factors, like pH or other dissolved ions and how they affect the dissolution of a salt. pH was a concept introduced in Unit 4 with more in depth coverage in unit 8 It's all connected. So, let’s recap the recaps that lay the foundation for this episode, then introduce the new concepts and dive into how these factors affect the dissolution of a salt.

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Unit 6 is all about the big idea Energy (0:46). Episode 59 discusses the questions 2021 - Question 4, 2017 - Question 5 and 2013 - Question 3. These are released FRQs from previous exams and copyright of the College Board.

Question 4 of the 2021 exam starts with our favorite equation: mcAT (1:34). In part b) you calculate the mass of iron and in part (c) discuss the effect of doubling the mass of iron on the maximum temperature. Question 5 of the 2017 looks at similar concepts, but it is a combustion reaction (3:40). It starts again with calculating the magnitude of heat energy and using mcAT. In part (b) we calculate how much energy 1 mole of 2-propanol would release. Part (c) asks about the effect of having a water/propanol mixture on the final temperature. Question 3 of the 2013 exam starts with stoichiometry - identifying the limiting reactant (5:30) and in part (b) the inconsistent trial. In (c) we are using again mCAT for our calculations. (D) and (e) have us calculate the enthalpy - in d) using experimental data, in (e) as enthalpy of formation. In (f) we are provided with an explanation for the discrepancy between (d) and (e) and are asked to explain if that could be the reason.

Today’s Question of the day is about Enthalpy. How do we calculate the enthalpy using bond enthalpies?

A. bonds broken - bonds formed B. bonds formed - bonds broken C. bonds broken + bonds formed D. bonds formed + bonds broken

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We are taking a closer look at what happens when hot water is cooled down or ice is heating up (0:30).

After a brief review of the terminology of phase changes (1:11) we are taking a closer look at the particles in solid, liquids and gases (1:29). Focusing on the energy component of phase changes, we are describing the five segments of a heating curve (2:22). In segments where the temperature increases with increasing heat added, we are increasing the particle motion and can calculate the heat added using mCΔT (3:26). In segments where the temperature is not increased, the energy added is used to overcome the intermolecular forces. For these segments we use the enthalpies of fusion and vaporization to calculate the heat added (5:54). The episode closes with three important reminders about the heat of vaporization, the magnitude of enthalpies for a cooling curve and a reminder about the units for the calculations (6:26).

Question: Which of the following substances would have the greatest enthalpy of fusion?

A. methane (CH4) B. acetic acid (CH3COOH) C. ethanol (C2H5OH)

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Unit 5 is often a stand alone unit and offers a unique perspective on reactions. Episode 57 discusses the questions 2010B - Question 6 and 2019 - Question 6. These are released FRQs from previous exams and copyright of the College Board (0:44).

Question 6 of the 2010B exam focuses on the synthesis of hydrogen chloride from its elements (1:33). We are asked to determine the order of reaction for both reactants and justify the answer.

Question 6 of the 2019 exam looks at a decomposition reaction and the data collected is graphed in three graphs. We can use the graphs to identify the reaction as second order with respect to nitrogen dioxide and write the rate law. In part c we have two possible mechanisms, and have to determine if they are both consistent with the rate law. The first mechanism has a slow first step, the second mechanism has a slow second step and therefore requires substitution in the justification (7:43).

Question: What is the order of ALL half-life reactions on the AP Chemistry exam? A. zeroth order B. first order C. second order

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An AP Chemistry classic experiment for concentration changes over time is determining the rate law of fading Crystal Violet. (1:07) If the reaction is zero order, the plot of our concentration of our reactant vs time is linear. (2:51) If the reaction is first order with respect to our crystal violet, a plot of the natural log of the concentration of the reactant vs time will be linear. (3:17) If the reaction is second order with respect to crystal violet, a plot of 1 over concentration vs time will be linear. (3:45) We can use the graph and determine k as follows: for zeroth and first order k equals - slope and for second order reactions k = slope. (5:03) Integrated rate laws can be used to calculate the concentration after a specific amount of time or to determine how long a reaction has to run to get a specific concentration. A specific application of the rate laws is half-life. In AP Chemistry, half-life is always a first-order reaction. (6:15)

Question: If you have a reaction with a half-life of 4 days and an initial concentration of 0.1M, what fraction will be left after 20 days? (8:30)

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Unit for is a mix of foundational skills, like stoichiometry, and an introduction to later topics (0:32). The questions in this episode are: 2018 - Question 3 d) through i) and 2014 - Question 1 a) - f).

These are released FRQs from previous exams and copyright of the College Board (1:32). 2018 starts with three questions from a different unit. Part d) asks for a redox reaction (2:05). In part e) you are asked to calculate the concentration of iron(ii)plus in the solution (2:36) and in f) to discuss lab equipment (3:58). G) and i) refer to a second experiment involving iron impurities (4:39) and the effect of incomplete oxidation (5:34). Question 1 from 2014 is a gravimetric analysis with the goal of determining the iodide content in a potassium iodide tablet (7:00). Therefore we write the net-ionic equation (7:22), reflect on our data (7:52) and ion concentrations (8:06) before we calculate the number of moles of precipitate (8:23) and percentage iodide (8:53). Part f) concludes with a question about a potential error (9:38).

Question: True or false: You MUST indicate the state of matter in parentheses in your balanced chemical equation, even if not asked for it.

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