Chapter 18: Light Absorption | CHM 214 | 153

Jacob Stewart
16 Apr 202107:27
EducationalLearning
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TLDRThe video script delves into the principles of light absorption and emission in analytical chemistry, explaining how molecules transition from ground to excited states through light interaction. It outlines the process of absorption spectroscopy, detailing the use of a spectrometer and the significance of transmission and absorbance in quantifying light absorption. The Beer-Lambert law is highlighted, linking absorbance to molar absorptivity, path length, and concentration, which is crucial for analytical measurements.

Takeaways
  • šŸŒŸ Light absorption is a fundamental concept in analytical measurements, particularly in spectroscopy.
  • šŸ“ˆ Molecules can transition from the ground state to an excited state by absorbing light, which can cause various physical changes.
  • šŸ”µ Analytical chemistry often focuses on the light absorption process, where the goal is to detect a strong signal of light being absorbed by a sample.
  • šŸ’” Emission spectroscopy involves the observation of light emitted by a molecule as it transitions from an excited state to a lower energy state.
  • šŸ”„ The basic setup of a spectrometer includes a light source, a monochromator to select specific wavelengths, a sample container (like a cuvette), and a detector to measure light intensity.
  • šŸ“Š Transmission (T) is the ratio of the intensity of light after passing through the sample (P) to the incident light intensity (Pā‚€), and it ranges from 0 to 1.
  • šŸ“Œ Absorbance (A) is a quantitative measure related to the concentration of a sample, calculated as the negative base 10 logarithm of the transmission (T).
  • šŸ” The Beer-Lambert Law states that the absorbance of a sample is directly proportional to its concentration, path length, and molar absorptivity.
  • šŸŒ Molar absorptivity (Īµ) is a measure of how strongly a molecule absorbs light and is typically expressed in units of Mā»Ā¹cmā»Ā¹.
  • šŸ“ Path length (b) refers to the distance light travels through the sample and is usually measured in centimeters.
  • šŸ§Ŗ In practice, a blank sample containing only the solvent or reagents is used for calibration, allowing for accurate measurement of light absorption by the actual sample.
Q & A

  • What is the primary method discussed for using light in analytical measurements?

    -The primary method discussed for using light in analytical measurements is light absorption.

  • What is the term for the lowest energy state of a molecule?

    -The lowest energy state of a molecule is referred to as the ground state.

  • How can a molecule transition from the ground state to an excited state?

    -A molecule can transition from the ground state to an excited state by absorbing light, which can cause various processes such as electron promotion, molecular vibration, or molecular spinning.

  • What is absorption spectroscopy?

    -Absorption spectroscopy is a technique where light is shone on a sample and the amount of light absorbed by the sample is measured to analyze its properties.

  • What is the purpose of a monochromator in spectroscopy?

    -A monochromator is used to select a specific wavelength or color of light for the spectroscopy experiment.

  • What is a cuvette in the context of spectroscopy?

    -A cuvette is a container, often used in spectroscopy, that holds the sample through which the light passes for analysis.

  • How is the amount of light detected in spectroscopy measured?

    -The amount of light is detected using a detector, which measures the power of the light before and after passing through the sample.

  • What is transmission in the context of spectroscopy?

    -Transmission in spectroscopy is the percentage of light that makes it through the sample, defined as the ratio of the light power after the sample (p) to the incident light power (pā‚€).

  • What is absorbance and how is it related to transmission?

    -Absorbance is a measure used to quantify the amount of light absorbed by a sample. It is calculated as the negative base 10 logarithm of the transmission.

  • What is the Beer-Lambert Law and how does it relate to absorbance?

    -The Beer-Lambert Law states that the absorbance of a sample is equal to the product of its molar absorptivity (epsilon), path length (b), and concentration (c). This law is fundamental for quantitative analysis in spectroscopy.

  • What are the units for molar absorptivity and path length in the context of the Beer-Lambert Law?

    -Molar absorptivity (epsilon) is typically expressed in units of Mā»Ā¹cmā»Ā¹ (molar to the minus one, centimeters to the minus one), while the path length (b) is usually given in centimeters.

Outlines
00:00
šŸŒŸ Light Absorption and Analytical Measurements

This paragraph discusses the concept of using light absorption for analytical measurements. It explains that when a molecule is in its ground state, light can be absorbed to promote an electron to a higher energy orbital, cause a molecule to vibrate, or induce molecular spin. These processes result in physical changes within the molecule, which can be measured. The focus in analytical chemistry is on the light absorption event rather than the specific states achieved. The paragraph introduces the concepts of emission spectroscopy and absorption spectroscopy, with an emphasis on the latter. It outlines the basic components of a spectrometer or spectrophotometer, including a light source, monochromator for selecting specific wavelengths, sample container (cuvette), and a detector to measure light intensity. The process of measuring the amount of light absorbed by comparing the intensity of light before and after passing through the sample is described.

05:02
šŸ“ˆ Understanding Transmission and Absorbance

This paragraph delves into the quantitative aspect of light absorption measurements by introducing the concept of transmission (T) and absorbance (A). Transmission is defined as the ratio of light intensity after passing through the sample (p) to the incident light intensity (p0), ranging from 0 to 1, where 0 indicates all light is absorbed and 1 indicates no absorption. Absorbance is then introduced as a more useful variable for quantitative analysis, calculated as the negative base 10 logarithm of the transmission (or p0/p). The Beer-Lambert law is mentioned, stating that absorbance is directly proportional to the molar absorptivity (Īµ), path length (b), and concentration (c) of the sample. The units for these variables are also explained, with Īµ typically expressed in Mā»Ā¹cmā»Ā¹ and b in centimeters.

Mindmap
Keywords
šŸ’”Light Absorption
Light absorption refers to the process where a molecule absorbs energy from light, causing an electron to move from a lower energy orbital to a higher one. In the context of the video, this is a fundamental concept for analytical measurements, as it is the basis for absorption spectroscopy. The script explains that light absorption can result in various physical changes within a molecule, such as starting a bond vibration or causing the molecule to spin.
šŸ’”Excited State
The excited state is a higher energy state that a molecule or atom can reach when it absorbs energy, such as from light. In the video, the excited state is contrasted with the ground state, which is the lowest energy state a molecule can occupy. The transition from the ground state to the excited state through light absorption is central to the analytical techniques discussed.
šŸ’”Analytical Measurements
Analytical measurements are techniques used to quantify the properties of substances, such as their concentration or molecular structure. The video emphasizes the use of light absorption and emission for these measurements, which are crucial in fields like chemistry and physics. Analytical measurements are essential for understanding the interactions between light and matter and for developing methods to analyze samples in a laboratory setting.
šŸ’”Emission Spectroscopy
Emission spectroscopy is a technique where a substance is studied based on the light it emits as it returns from an excited state to a lower energy state. Unlike absorption spectroscopy, which focuses on the light absorbed by a sample, emission spectroscopy observes the light released. The video briefly mentions this technique as an alternative to absorption spectroscopy, highlighting that it involves the detection of photons emitted by a molecule in its return to the ground state.
šŸ’”Monochromator
A monochromator is a device that selects a specific wavelength or color of light from a broader spectrum. In the context of the video, it is used in spectrophotometers to ensure that only one type of light interacts with the sample being analyzed. This allows for precise measurements of how that particular wavelength is absorbed or emitted by the sample, which is crucial for spectroscopic analysis.
šŸ’”Cuvette
A cuvette is a small, usually rectangular or cylindrical, container that holds a sample for the purpose of spectroscopic analysis. In the video, it is mentioned as the container in which the sample is placed before the light passes through it for analysis. Cuvettes are designed to be transparent and typically made of materials like glass or quartz, ensuring that the light can pass through the sample for accurate measurement.
šŸ’”Detector
A detector in the context of spectroscopy is a device that measures the intensity of light. It is used to record the amount of light that passes through the sample after absorption or emission has occurred. The video explains that the detector is essential for capturing the change in light intensity, which is then used to calculate the transmission and absorbance of the sample.
šŸ’”Transmission
Transmission refers to the percentage of light that passes through a sample. In spectroscopy, transmission is calculated by taking the ratio of the light intensity after passing through the sample (p) to the initial light intensity before the sample (p0). The video emphasizes that transmission is a crucial step in determining how much light is absorbed by the sample, as it directly relates to the concentration of the substance being analyzed.
šŸ’”Absorbance
Absorbance is a measure used to quantify the amount of light absorbed by a sample and is directly proportional to the concentration of the absorbing species. It is calculated as the negative base 10 logarithm of the transmission. In the video, absorbance is highlighted as a key variable for quantitative measurements, as it allows for the analysis of a sample's concentration based on the Beer-Lambert law.
šŸ’”Beer-Lambert Law
The Beer-Lambert law, also known as Beer's law, states that the absorbance of a sample is directly proportional to its concentration, the path length that the light travels through the sample, and the molar absorptivity of the substance. This law is fundamental in spectroscopy as it provides a quantitative relationship between the physical properties of light absorption and the chemical concentration of the sample.
šŸ’”Molar Absorptivity
Molar absorptivity, denoted by the symbol epsilon (Īµ), is a measure of how strongly a substance absorbs light at a specific wavelength. It is an intrinsic property of the molecule and is related to the ability of the molecule's electrons to transition between energy levels when they absorb light. In the context of the video, molar absorptivity is a crucial parameter in the Beer-Lambert law, which is used to calculate the concentration of a sample based on its absorbance.
Highlights

Exploring the use of light for analytical measurements, particularly through light absorption.

Molecules can transition from the ground state to an excited state by absorbing light, leading to various processes such as electron promotion, molecular vibration, and molecular spin.

Analytical chemistry focuses on light absorption as a signal for measurement, rather than the specific states achieved.

Emission spectroscopy involves a molecule emitting light as it transitions from an excited state to a lower energy state.

The basic setup of a spectrometer or spectrophotometer is outlined, including the need for a light source, monochromator, sample container, and detector.

Monochromators select a specific wavelength or color of light for the absorption experiment.

The process of measuring light absorption involves comparing the amount of light before and after it passes through the sample.

Transmission (T) is defined as the ratio of the detected light power (P) to the incident light power (P0), representing the percentage of light that makes it through the sample.

Absorbance (A) is quantified using the negative base 10 logarithm of the transmission (T), which is related to the concentration of the sample.

The Beer-Lambert Law states that the absorbance of a sample is equal to the product of molar absorptivity (Īµ), path length (b), and concentration (c).

Molar absorptivity (Īµ) is a measure of how strongly a substance absorbs light and is typically given in units of M^-1cm^-1.

Path length (b) refers to the distance that light travels through the sample and is usually measured in centimeters.

Concentration (c) is typically expressed in molarity and is a key factor in determining the absorbance of a sample.

The use of a blank sample containing only the solvent or reagents allows for accurate comparison of light absorption between the sample and a non-absorbing medium.

Quantitative measurements rely on the relationship between absorbance and concentration, enabling the analysis of sample composition.

The practical applications of light absorption and emission spectroscopy extend to various fields, including chemistry, physics, and materials science.

Understanding the principles of light interaction with matter is crucial for the development of new analytical techniques and technologies.

The transcript provides a comprehensive overview of the fundamental concepts and experimental setups in spectroscopy, which is essential for students and researchers in the field.

Transcripts

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SpectroscopyLightAbsorptionMolecularAnalysisChemicalProcessesAnalyticalChemistryEmissionSpectroscopyBeerLambertLawScienceEducationMolecularPhysics