PASCO Spectrometer: Kinetics

Summary

Tom Loschiavo, Chemistry Education Manager at PASCO, demonstrates how to conduct kinetics experiments using the PASCO Spectrometer. He walks through the calibration process, using a blank cuvette to set a dark and light reference. The experiment involves crystal violet and sodium hydroxide, where absorbance over time is measured. Loschiavo explains how to select a wavelength, track changes in absorbance, and interpret data to determine reaction kinetics. He highlights key chemistry concepts like reaction orders, half-lives, and Beer's Law, and demonstrates applying linear fits to analyze the reaction's rate constant.

Takeaways

• 🔹 The PASCO Spectrometer is used to conduct kinetics experiments, including the reaction of crystal violet with sodium hydroxide.
• 🔸 The spectrometer is Bluetooth-connected to a computer with the PASCO Spectrometry software ready for use.
• 🔹 Before starting, the spectrometer requires calibration with both a dark reference (no light) and a light reference (with light through the solvent).
• 🔸 The solvent used for calibration is water, and a blank cuvette is placed in the spectrometer for this process.
• 🔹 After calibration, a sample of crystal violet is placed in the spectrometer, and the absorbance spectrum is measured, particularly in the yellow range.
• 🔸 For kinetics analysis, the user selects an appropriate wavelength (around 577 nm) using the Coordinates tool in the software.
• 🔹 The experiment then tracks absorbance versus time at a rate of 1 Hz, showing the decrease in absorbance as the reaction progresses.
• 🔸 Adding sodium hydroxide to the crystal violet sample initiates a reaction that causes the absorbance to decline over time, which is monitored in real-time.
• 🔹 The software allows for quick calculation and visualization of reaction order by displaying absorbance, natural log of absorbance, and inverse absorbance.
• 🔸 The straightest linear fit indicates the reaction order, and in this case, the natural log of absorbance provided the best fit, suggesting a pseudo-first-order reaction.

Q & A

What is the purpose of the experiment in the video?

The experiment is designed to study kinetics using crystal violet and sodium hydroxide with the PASCO Spectrometer.

How does the speaker prepare the spectrometer for the experiment?

The speaker calibrates the spectrometer by using a blank cuvette of water for both a dark reference and a light reference to ensure the solvent's influence is neutralized.

Why does the crystal violet solution appear violet in color?

Crystal violet appears violet because it absorbs light in the yellow range of the spectrum.

What is the significance of selecting a wavelength for the kinetics experiment?

The selected wavelength (around 577 nm) is crucial for tracking absorbance changes over time, which is necessary for analyzing the kinetics of the reaction.

What happens when sodium hydroxide is added to the crystal violet solution?

When sodium hydroxide is added, the reaction begins, and the absorbance decreases over time as the reaction proceeds.

How is the time page used in the spectrometer software?

The time page tracks absorbance versus time, allowing the user to monitor how absorbance changes during the reaction at a set sample rate (e.g., 1 Hz).

What is the role of absorbance in determining reaction kinetics?

Absorbance is directly related to concentration, and its decline over time helps determine the reaction order and rate constant.

How can the reaction order be determined from the data?

The reaction order is determined by finding the straightest line between absorbance, natural log of absorbance, and inverse absorbance. In this case, the natural log of absorbance gave the best fit.

What is the significance of the slope in the linear fit of the data?

The slope of the straightest line represents the rate constant (or pseudo rate constant) of the reaction.

How does the software aid in understanding reaction kinetics?

The software allows users to visualize absorbance changes over time, apply linear fits, and calculate the reaction order, making it easier to study kinetics.