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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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The FRQs discussed in this episode are the question 4s from the 2019, 2018 and 2017 AP Exam. These are released FRQs from previous exams and copyright of the College Board (0:23).
Question 4 of the 2018 exam focuses on comparing the IMFs of CS2 and COS, asking you to explain why CS2 has a higher boiling point (1:48). The second part has you calculate the pressure of CS2 using the ideal gas law (3:11). In question 4 of the 2019 exam you describe the effect of raising the temperature on the motion of the CO2 particles connecting temperature and particle speed (4:09). In part b you calculate the new pressure under constant volume (4:59), in part c) you describe why the pressure increases (6:35) and in part d you compare an ideal and a real gas (7:19). Question 4 of the 2017 exam focuses in Chromatography, identifying the least polar dye by discussing the interactions between dye and solvent/paper (8:29) as well as identifying an unknown by comparing how far the dye travelled (9:05).
Today’s Question of the day is about FRQ Questions.
True or false: The term “van-der-Waals” forces can be used instead of London Dispersion Forces.
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Coffee needs to be “just right” and we can tell how strong it is by the color (0:30). We can use the color to determine concentration also in Chemistry (1:09). The color we see is determined by the wavelength an object reflects, while absorbing all other colors (1:23).
The Beer-Lambert Law absorbance to molar absorptivity, path length and concentration (2:08). In AP Chemistry, the molar absorptivity and path length are held constant, therefore absorbance is directly proportional to concentration (2:28).
Experimentally, we measure absorbance using spectrophotometers (4:41). To determine the concentration of, for example, blue dye in a sports drink, we have to create a calibration curve using solutions with known concentration and measuring the absorbance (5:35). We can then use the graph and a measurement of absorbance of the sports drink to determine the concentration (6:16).
In which of the following examples could you use spectroscopy and the Beer-Lambert law to determine the concentration?
A. Determination of bilirubin in blood plasma samples. B. Determination of colorless zinc(II) nitrate in a sample. C. Determination of ethanol (drinking alcohol) in an alcoholic beverage. D. Determination of isopropyl in hand sanitizer.
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Teachers wish students would always behave ideal, but in reality life happens and it gets messy (0:32). The same is true for gases: There is a difference between real and ideal gases (1:12). The Ideal Gas Law assumes that particles don’t experience intermolecular forces nor that they take up volume (1:34). But real gases do and this leads to deviations from the Ideal Gas Law, especially under high pressure and close to condensing (2:00). Under low temperatures, intermolecular forces become more and more significant (2:41), leading to a lower pressure of a real gas compared to an ideal gas (3:30). This is enhanced by the strength of intermolecular forces and therefore their polarity (4:36). Because particles do have volume, the usable space is less in a real gas than an ideal gas (5:12). Therefore at high pressures the volume of a real gas is larger than the volume of ideal gases (5:32).
Which molecule would have the least deviation from the Ideal Gas Law?
A. CH4 B. Ne C. H2O D. Cl2
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Sitting in your room, you might have salted peanuts as a snack, a pencil, tea with sugar and you listen to this episode with earphones in (0:31). Sodium chloride, sugar, graphite and copper represent four types of solids. Sodium chloride is an ionic solid with low vapor pressure, high melting points and high boiling points, because of the strong attractive forces. Sugar is a molecular compound with low melting points due to weak IMFs (1:22). Graphite is an example of a network covalent solid (5:29), which generally have high melting points. Copper is representative of a metallic solid, which are good conductors of heat (6:44), ductile and malleable (7:06). Homogenous mixtures of metals are alloys (7:21).
Which of the following could be the identity of a solid that exhibits the following properties: it melts at 2973°C; it doesn't conduct electricity as a solid nor as a liquid.
A. ionic B. covalent C. covalent network D. metallic
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The FRQs discussed in this episode are 2008 - Question 2 and 5 as well as 2019 question 5. These are released FRQs from previous exams and copyright of the College Board (0:22).
Question 2 of the 2008 focuses on the experimental approaches, first the determination of the formula of a hydrate (2:06) and then gravimetric analysis (3:33). Question 5 of the 2008 exam starts with questions about ionization energies for fluorine in comparison with oxygen and xenon. The second part of the question asks for Lewis Diagrams of xenon compounds (7:59), the geometric shape (8:25) as well as hybridization (8:35) and polarity (8:55).
Question 5 of the 2019 exam provides a PES spectrum, which you will use to determine the electron configuration and identity of the element (9:08) and asks you to calculate the wavelength needed to remove an electron from the valence shell (10:01).
True or false: I have to answer the FRQ Questions from 1 to 7 as well as a through last letter in order.
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Lewis Diagrams give us information about the arrangement and bond order of compounds (0:23). There are four steps to write Lewis Structure: 1) Sum up available valence electrons (1:16), 2) write symbols and connect them with a single bond (1:57), 3) complete the octet for surrounding atoms (2:27), and 4) add remaining electrons as lone pairs on central atom (2:48).
Some elements, like hydrogen, boron and beryllium as well as elements in Period 3 and beyond are exceptions to the octet rule (3:24). Formal charges can be used to determine the best Lewis Structure (5:15). In our episode we discuss calculating the formal charge for two possible structures of CO2 (6:07).
Which two elements will never be in the center of a Lewis Structure?
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Do you have your own bathroom? Then you might be familiar with soap scum, which is the result of a combination of having hard water and soap (0:44). To determine how hard water is, we can use gravimetric analysis, which is an experimental procedure used to determine the amount of a substance, for example an ion, by precipitating it from an aqueous solution (1:25). Basing off of the solubility rules (1:45), the episode describes the steps to precipitate the calcium carbonate with sodium carbonate (3:05) and briefly discusses how to calculate the water hardness (5:08) and calcium ion concentration (5:38). Experimental errors described in the episode discuss how the yield can be higher or lower than expected (6:01).
Question: Why does the sodium carbonate have to be in excess?
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Switching glucose and formaldehyde would be really bad (0:42)! So let’s make sure we know the difference between empirical and molecular formulas! Our episode starts with a definition of empirical formulas (1:20) and the comparison of ionic and molecular compounds (1:49). It briefly recaps the significance of a chemical formula (2:16) and then recaps the steps to calculate an empirical formula using glucose as an example (2:50). But what if you do not have a whole-number ratio right away (5:30) and how do you go from Empirical to Molecular Formula (6:14)? One specific type of calculation is the combustion analysis (7:11).
Question: What is the first step, if I have given grams of elements as part of the substance instead of % composition?
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The episode discusses and compares the structure of the two exam formats: the paper exams in May and the digital exam in June (0:50). Differences between the administration of the digital exam and paper exam are described (1:11), especially the calculator policies (2:16). We are taking a quick look at timing, which is the same for the paper exam and the digital exam (4:26). Your last episode of the season ends with a quick recap of test taking tips for multiple-choice questions (4:57) as well as free-response questions (5:33). Good luck everyone!
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The FRQ section has 3 long and 4 short question sets which you will have 105 minutes to complete (0:52). Past free response questions are available on the College Board website. (1:32) Make sure you carefully read the question and answer directly with precise language. (2:59) For calculations, make sure you show all your work, use units throughout and report your answer with the correct number of significant figures. (6:16)
True or False: You are not allowed to bring a highlighter to mark information in the exam booklet.
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You’ve made it through the year! Let’s hone our Multiple Choice skills to make sure we can show off our knowledge and skills! Our episode starts by discussing pacing: The multiple choice section consists of 60 questions within 90 minutes (0:54). Then we are taking a closer look at strategies: using the process of elimination (2:09), carefully reading and circling key information (2:47), mental math and estimations (4:20), and most importantly: practice, practice, practice (5:04).
How many Multiple-Choice questions will be on the 2021 digital exam?
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Our listener’s choice recaps three topics: (R)ICE tables, titrations and buffers. We start with the RICE table - what does it actually stand for? (0:49). RICE tables are best explained with an example. In our case: The reaction of gaseous hydrogen with gaseous chlorine to form gaseous hydrogen chloride - a reaction that has a K = 49. And we are calculating the concentrations at equilibrium (1:44)! Two important tips for RICE tables: (1) practice, practice, practice; (2) make assumptions (4:09).
Our next two topics are connected: titrations and buffers. We start with the titration of a strong acid/strong base and discuss the calculation of the pH at several points along the titration curve (6:01). Our second example is the titration of a weak acid with a strong base. Again, we discuss the pH calculations along the titration curve, but then focus on buffers, which are formed when the weak acid is partially neutralized (7:29). The episode defines buffers and describes how they work (8:00). Moving along the titration curve, we discuss the calculations for buffers and the midway point of the titration (9:18), the calculations at equivalence point (10:03) and beyond the equivalence point.
Question: What will be the approximate pH of an equimolar solution of NH3 and HCl?
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- Se mer
