2 | FRQ (Short) | Practice Sessions | AP Chemistry
TLDRIn this informative AP Daily practice session, Kristen Cacciatore guides viewers through the process of tackling short free response questions on the AP Chemistry exam. The video focuses on the decomposition of N2O5, explaining how to determine the rate constant (k) for a first-order reaction using the half-life concept. It then identifies the rate-determining step in a proposed reaction mechanism by comparing it to the overall rate law. The session also explores the concept of concentration independence for the rate constant at constant temperature. Additionally, the video covers a titration problem involving oxalic acid and potassium permanganate, explaining how to identify the reduced species, calculate the volume of KMnO4 used, and assess the feasibility of a proposed titration concentration. Kristen concludes with study tips and references to relevant AP Daily topics for further review, emphasizing the availability of more practice sessions on their YouTube channel.
Takeaways
- 📚 The session focuses on preparing for short free response questions on the AP exam, specifically discussing the decomposition of N2O5 and a titration involving oxalic acid and KMnO4.
- 🔍 For the N2O5 decomposition, the rate law is rate = k[N2O5], indicating a first-order reaction where the rate is directly proportional to the concentration of N2O5.
- ⏱️ The half-life of a first-order reaction is the time it takes for the concentration of the reactant to decrease by half, which for N2O5 is determined to be 1.67 hours.
- 🔑 The rate constant (k) for the reaction can be calculated using the formula k = 0.693 / (half-life), resulting in k = 0.415 h⁻¹ for the N2O5 decomposition.
- 🧬 In the proposed mechanism for N2O5 decomposition, step one is identified as the rate-determining step because its rate law matches the overall reaction rate law.
- 🧠 The concept of k being concentration-independent is emphasized; it remains the same at a constant temperature, regardless of the initial concentration of N2O5.
- 🧪 An experiment involving the titration of H2C2O4 with KMnO4 is described, where the species reduced (gained electrons) is MnO4⁻, as indicated by a decrease in oxidation number from +7 to +2.
- 📊 The importance of accurate volume measurements in titration is highlighted, with the student using a 50.0 mL buret and reading to the nearest 0.01 mL.
- 📉 The volume of KMnO4 solution added during titration is calculated by subtracting the initial buret reading from the final reading, yielding a volume of 26.20 mL.
- 🧮 Moles of manganese ions reacted are calculated by multiplying the molarity of KMnO4 by the volume of KMnO4 added, resulting in 0.000616 moles.
- ❌ A proposed titration using a more dilute KMnO4 solution is deemed unreasonable due to the expected increase in titration volume, which would exceed the capacity of a 50 mL buret.
- 📈 The video concludes with advice on reviewing topics such as rate laws, half-life, reaction mechanisms, oxidation numbers, and titration concentration, with resources available on the AP Daily YouTube channel.
Q & A
What is the topic of the AP Daily practice session two?
-The topic of the AP Daily practice session two is about short free response questions for the AP exam.
How many short free response questions are typically on the AP exam?
-There are typically four short free response questions on the AP exam.
What is the rate law given for the decomposition of N2O5?
-The rate law given for the decomposition of N2O5 is rate equals k times the concentration of N2O5.
How is the rate constant 'k' for the reaction determined?
-The rate constant 'k' is determined by using the half-life of the reaction and the relationship k equals 0.693 divided by the half-life.
What is the half-life of N2O5 in the given data table?
-The half-life of N2O5 in the given data table is 1.67 hours.
How does the rate determining step relate to the overall rate law?
-The rate determining step should have the same rate law as the overall reaction because it determines the rate of the reaction.
Why is step one of the proposed mechanism considered the rate determining step?
-Step one is considered the rate determining step because it has the same rate law as the overall reaction, which involves only one molecule of N2O5 as the sole reactant.
What would happen to the rate constant 'k' if the experiment was repeated with twice the initial concentration of N2O5 at the same temperature?
-The rate constant 'k' would remain the same because 'k' is concentration independent and depends only on temperature, which is constant in this scenario.
What was the student's task in the titration experiment with oxalic acid and KMnO4?
-The student's task was to dissolve a 0.139 gram sample of oxalic acid in water and then titrate the solution with a KMnO4 solution until a faint lavender color was observed, indicating the end point of the titration.
How should the initial and final volume readings from the buret be recorded?
-The initial and final volume readings from the buret should be recorded with two decimal places, as the buret scale is in 0.1 milliliter increments.
What is the significance of identifying the species that was reduced in the titration reaction?
-Identifying the species that was reduced helps in understanding the redox reaction mechanism, as it is the species that gained electrons and had its oxidation number reduced.
Why is the proposed titration using a 0.00143 molar KMnO4 solution not reasonable?
-The proposed titration using a 0.00143 molar KMnO4 solution is not reasonable because the resulting titration volume would be more than 10 times greater than the capacity of a 50 milliliter buret, which is the instrument being used.
What are some AP Daily topics that could help with reviewing concepts related to this practice session?
-Topics that could help with reviewing include rate laws and half-life (5.2 and 5.3), reaction mechanism and rate law (5.8), oxidation numbers and redox reactions (4.9), and determining concentration through titration (4.6).
Outlines
🧪 AP Daily Practice: Short Free Response Questions
Kristen Cacciatore introduces the second AP Daily practice session, focusing on short free response questions, which are crucial for the AP exam. She provides a link for viewers to download a PDF of the problems discussed. The session begins with a first-order reaction problem involving the decomposition of N2O5, where the rate law is given by rate = k[N2O5]. Using the half-life concept and the provided data table, Kristen calculates the rate constant k. The session continues with identifying the rate-determining step in a proposed mechanism for N2O5 decomposition and discusses how the rate constant behaves with different initial concentrations at a constant temperature. The summary concludes with a different chemistry problem involving a titration and the identification of the reduced species through oxidation numbers.
📊 Titration Analysis and Molarity Calculation
The second paragraph delves into a titration problem where a student titrates oxalic acid with a KMnO4 solution. The balanced chemical equation for the reaction is provided, and the task is to identify the species that was reduced during the titration based on oxidation numbers. Kristen explains the process of determining oxidation numbers for each element and identifies MnO4- as the species reduced. The paragraph continues with a practical exercise involving buret diagrams to calculate the volume of KMnO4 added during the titration and to determine the moles of manganese ions that reacted. Lastly, the paragraph addresses whether a proposed lower molarity of KMnO4 would be suitable for a subsequent titration, concluding that it would not due to the increased titration volume exceeding the buret's capacity.
📚 Review Tips and Additional Resources for AP Exam Preparation
In the final paragraph, Kristen offers tips for continued exam review, suggesting specific AP Daily topics to focus on based on the content covered in the video. These topics include rate laws and half-life (5.2 and 5.3), reaction mechanism and rate law (5.8), oxidation numbers and redox reactions (4.9), and determining concentration through titration (4.6). She also informs viewers about additional resources available on their YouTube channel, including more AP Daily practice sessions from 2023 as well as from 2021 and 2022. The paragraph concludes with a thank you note and an anticipation for the next session.
Mindmap
Keywords
💡Short free response questions
💡Rate law
💡Half-life
💡Rate constant (k)
💡Decomposition reaction
💡Mechanism
💡Oxidation numbers
💡Titration
💡Significant figures
💡Molarity
💡Reduction
Highlights
The session focuses on preparing for short free response questions on the AP exam.
There will be four short free response questions on the AP exam.
Participants can download a PDF of the problems from the provided link.
The first problem involves determining the rate constant k for a decomposition reaction.
The rate law for a first-order reaction is related to the half-life of the reactant.
The half-life of N2O5 is identified as 1.67 hours from the data table.
The rate constant k is calculated using the half-life and is found to be 0.415 with units of inverse hours.
The proposed mechanism for the decomposition of N2O5 includes three steps.
Step one of the proposed mechanism is identified as the rate-determining step due to its matching rate law.
The rate constant k remains the same if the experiment is repeated at the same temperature, regardless of the initial concentration of N2O5.
The experiment involves titrating a solution of oxalic acid with a KMnO4 solution.
The species MnO4- is identified as the one being reduced during the titration reaction.
The volume of KMnO4 added during the titration is determined from the buret readings.
The number of moles of manganese ions that reacted is calculated using the molarity and volume of KMnO4.
A proposed titration using a more dilute KMnO4 solution is deemed unreasonable due to the expected large titration volume.
Additional resources for exam preparation include AP Daily topics and practice sessions from previous years.
Tips for further review are provided, covering topics like rate laws, half-life, reaction mechanisms, oxidation numbers, and titration.
Transcripts
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