The AP Chemistry exam is a challenging but rewarding experience for high school students. The free response question (FRQ) 4 is particularly daunting, as it requires students to apply their knowledge of multiple concepts in a single question.
What is FRQ 4?
FRQ 4 is a long-answer question that typically involves a real-world scenario or experimental data. Students are expected to demonstrate their understanding of chemical principles, problem-solving skills, and critical thinking abilities. The question is divided into several parts, each of which requires students to provide a specific response.
How to Approach FRQ 4
To succeed on FRQ 4, students should follow these steps:
- Read the question carefully. Make sure you understand what is being asked and identify the key concepts involved.
- Plan your response. Brainstorm ideas and organize your thoughts before you start writing.
- Write clearly and concisely. Use precise language and avoid unnecessary details.
- Show your work. Include all calculations and equations that you use to arrive at your answers.
- Check your answer. Make sure your response is complete and accurate.
AP Chem 2023 FRQ 4 Question
The question on the 2023 AP Chemistry exam was as follows:
A chemist is investigating the reaction between hydrogen gas (H2) and iodine gas (I2) to form hydrogen iodide (HI). The reaction is represented by the following equation:
H2 + I2 → 2HI
The chemist conducts two experiments at different temperatures. In the first experiment, the initial concentrations of H2 and I2 are both 0.10 M. The reaction is allowed to proceed for 100 minutes, and the concentration of HI is measured to be 0.050 M.
In the second experiment, the initial concentrations of H2 and I2 are both 0.20 M. The reaction is allowed to proceed for 50 minutes, and the concentration of HI is measured to be 0.100 M.
a) Calculate the rate constant for the reaction at each temperature.
b) Explain how the change in temperature affects the rate of the reaction.
c) Predict the concentration of HI after 150 minutes in the first experiment.
Answers to FRQ 4
a) Rate Constant Calculations
The rate constant (k) for a chemical reaction can be calculated using the following equation:
k = (Δ[HI]/Δt)/([H2][I2])
where Δ[HI] is the change in the concentration of HI over the time interval Δt, and [H2] and [I2] are the initial concentrations of hydrogen gas and iodine gas, respectively.
For the first experiment:
k = (0.050 M - 0.000 M)/(100 min - 0 min)/((0.10 M)(0.10 M))
= 2.50 x 10^-4 M^-1 min^-1
For the second experiment:
k = (0.100 M - 0.000 M)/(50 min - 0 min)/((0.20 M)(0.20 M))
= 2.50 x 10^-4 M^-1 min^-1
b) Effect of Temperature on Reaction Rate
The rate of a chemical reaction increases with increasing temperature. This is because higher temperatures provide more energy to the reactants, which allows them to overcome the activation energy barrier and react more quickly.
c) Prediction of HI Concentration after 150 Minutes
To predict the concentration of HI after 150 minutes in the first experiment, we can use the integrated rate law for a second-order reaction:
[HI] = (2kt[H2][I2])/(1 + 2kt[H2][I2])
where t is the time in minutes.
Plugging in the values we know:
[HI] = ((2)(2.50 x 10^-4 M^-1 min^-1)(150 min)(0.10 M)(0.10 M))/(1 + (2)(2.50 x 10^-4 M^-1 min^-1)(150 min)(0.10 M)(0.10 M))
= 0.075 M
Conclusion
Students who follow these steps and practice answering FRQ 4 questions will be well-prepared for the AP Chemistry exam. By understanding the question, planning their response, writing clearly, showing their work, and checking their answer, students can increase their chances of success on FRQ 4.
