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.
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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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Unit 3 Free-response questions.
The FRQ questions on the AP exam often combine content from two or three different units. In today’s episode, we are focusing on questions that require knowledge and skills from Unit 3: Intermolecular Forces and Properties. This is THE BIG ONE! Unit 3 makes up 18 to 22% of the AP Exam! So it’s definitely worth zooming in! The questions we are using 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 3. The questions used are: 2019, 2018 and 2017 question 4 for each one of them (seeing a pattern here?) These are released FRQs from previous exams and copyright of the College Board.
Lets Zoom in:
Unit 3 starts with Intermolecular Forces and takes a closer look at the states of matter - solid, gases and liquids/solutions with regard to Intermolecular Forces. In our explanations, we are staying with a similar structure. Let’s start with the discussion of Question 4 from the 2018 exam.
In this question, you have two molecules, COS (carbonyl sulfide) and CS2 (carbon disulfide), with molecular structure and boiling point given. What makes this question interesting is that the data is already interpreted for you: the boiling point of CS2 is higher than the boiling point of COS. And why is this interesting? CS2 is a nonpolar molecule, which is experiencing London Dispersion Forces. (By the way, you are allowed to use LDFs as an abbreviation in your responses). COS is a polar molecule, which is experiencing London Dispersion forces as well as dipole-dipole interactions. Therefore, without the data, we would assume that the intermolecular forces for COS are greater and the boiling point is higher. But the question states the opposite. Don’t be confused by this and don’t try to “disprove” the statement. Just run with it. If CS2 has the higher boiling point, it seems that the London Dispersion Forces are stronger than the LDFs AND dipole-dipole interactions in COS. Stating the intermolecular forces for both molecules as well as the explanation comparing the strength earns you the points!
The second question of this short-answer FRQ leads us into gas laws and asks you to calculate the pressure once all of the liquid CS2 has vaporized. You are given a 10.0g sample, as well as 5.0L volume and a temperature of 325 K. In the first step, you have to convert the grams to moles using the Molar Mass of CS2. In the second step you use the Ideal Gas Law “PivNert” to solve for pressure. The nice thing here is: Temperature is already in Kelvin, no conversion is needed. Keep in mind, all gas law problems use Kelvin! Another piece of advice: Make sure you choose the correct R constant. In this case the question didn’t specify the unit, so you could have calculated it in atm, torr, mmHg… just make sure you specify the unit and use the correct R value.
Let’s stick with Gas Laws and move to question 4 from the 2019 exam. In question part a) we are asked to describe the effect of raising the temperature on the motion of the CO2 particles. When raising the temperature, the average speed of the particle increases. This short sentence already earns you one point. Test taking tip: Don’t overdo it. This is all they want you to write here. How do you know? Take a look at the “action verb”: describe. It doesn’t say “explain”. If it would ask for an explanation or a justification, you would certainly have to use more than a sentence and refer to the relationship between temperature and kinetic energy. But it “only” asks you to describe and that can be done in a sentence.
Part b) prompts you to calculate the new pressure. Since we have a rigid container, the volume stays the same and we can use P1 over T1 equals P2 over T2 - also known as Gay-Lussacs Law. You do NOT have to know the scientists' names connected to the Gas Laws, just wanted to add it for the bigger picture. You know your original pressure P1 and your original temperature T1 as well as the new temperature T2, so you can solve for the new pressure P2, which is 0.99 atm. Don’t forget the unit!
In part c) you are using Kinetic Molecular Theory to explain the changes to the pressure after heating the sample. Pressure is a measurement of how often or how forceful particles collide with the wall of the container. By increasing the temperature, the particles will move faster and therefore hit the container wall more often. You would also get the point if you state that they hit the container wall with greater force. Either more frequently or more forceful gets you the point. Here the focus is on a BRIEF explanation using a theory. In comparison to part a) the description was already given, you have to explain it by mentioning scientific principles and theories.
Last, but not least for this short response question is a prompt referring to the difference between ideal and real gas. A student observes a lower pressure than predicted and the question asks you to explain this. In your response, you have to indicate that the attractive forces, in this case LDFs, between the CO2 molecules lower the gas pressure, because the particles of a real gas will hit the wall less frequently or forcefully. This is a question that refers to the Deviations from the Ideal Gas Law. If you need a refresher, check out episode 52!
Final FRQ for today, we are moving on from gases and taking a closer look at solutions and in the case of question 4 from the 2017 AP Exam, solutions and chromatography. The question prompt shows an experimental setup where a student determines the identity of an unknown dye by comparing its chromatogram to three pure dyes. He is using a polar chromatography paper and a nonpolar solvent. The prompt shows us the set up as well as the result.
Part a) asks us to state which of the three pure dyes is the least polar and to justify our answer. We can see that dye B travelled the least and dye C travelled the furthest. Therefore, dye C is the least polar, because it moved the farthest with the nonpolar solvent. Your justification should state that the nonpolar dye has more attraction with the nonpolar solvent and therefore travels the farthest. You could also argue it the other way round and refer to the polar paper-dye interaction being the weakest.
Part b) of this question asks you to identify the unknown dye and justify your answer. At a quick first glance you might have been confused: it doesn’t line up with any of the three dyes! Huh. So what now? Taking a closer look, you can see that the solvent front is also different. This is how far the solvent travelled before the student removed the chromatography paper from the chamber. How far the dye travelled has to be determined in relation to the solvent front. We can see, or even measure, that the dye has travelled half the distance between origin and solvent front - and so has dye A. Therefore, the unknown dye has to be A. You can, but you don’t have to, even support that claim by measuring and calculating the retention factor Rf, which is the distance the dye travelled divided by the distance the solvent front travelled. Both, the unknown dye as well as dye A have an Rf value close to 0.50.
To recap…
Unit 3 is an important unit, as 18 to 22% of the exam refer to it. When a claim is given and it asks you to support it, don’t try to disprove it. Take a close look at the action verb used: describe is different than briefly explain, explain or justify. Most FRQ answers are short and concise, so don't lose valuable time by providing information that is outside of the question. You may use common abbreviations like LDFs or IMFs. In gas law calculations, make sure you choose the correct R constant and use Kelvin temperature. Don’t forget to show your work and units. For solutions, be prepared to describe the interaction between solvent and solute in terms of polarity and intermolecular forces.
Coming up next on the Apsolute RecAP Chemistry Edition: Unit 4 selected free-response questions.
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.