AF

UV/Vis Spectroscopy and Related Concepts

Blood Drive Information

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  • Instructions: Follow the link on our Instagram to submit your donations.

  • Incentive: Donors receive a football.

  • Walk-ins are welcome.

  • Date and Time: September 24, 2025, from 10 AM to 3 PM

  • Location: Krach Leadership Center

Course Announcement Details

Lecture Information

  • Lecture: 8

  • Topic: UV/VIS Spectroscopy and Beer-Lambert Law

  • Date: September 23, 2025

  • Source Material: Pages 304-305 from "Spectrometry in Chemical Analysis"

  • Relevant Section: 4.1 Solution Concentration

General Announcements

  • Office Hours: Wednesday, September 24 from 10:00 – 11:30 AM in WTHR 261 or by appointment (Contact via email: mille201@purdue.edu with 3 suggested days and times).

  • Feasting with Faculty: Wednesday, September 24, around 12:50 PM – 1:50 PM at Wiley, near dessert.

  • Textbook Availability: 1 donated, used looseleaf copy of the textbook available in 3-ring binder. Email if financial challenges arise (first come, first serve).

  • Exam 1: Scheduled for Thursday, September 25 at 8:00 PM; arrive by 7:40 PM.

  • Final Exam: Scheduled for Thursday, December 18 from 10:30 AM – 12:30 PM at Elliott Hall of Music for West Lafayette students (location TBA for Indianapolis students).

Exam 1 Details

Coverage and Format

  • Exam Coverage:

    • Lectures 1-7 (approximately 75%)

    • Recitation Weeks 1-4 and associated quizzes (approximately 25%)

    • Homework (HW01-HW04)

    • All associated reading assignments and learning objectives

  • Exam Format:

    • 20 multiple choice questions

    • Conducted on paper

    • Requires filling in bubbles in exam booklet

    • Assigned seating structure: Level – Aisle – Row Seat

    • Students to check grades in Brightspace.

Exam Guidelines

  • No lining up in advance: Lining up will cause delays to the exam start time.

  • Venue: Elliott Hall of Music, located within Purdue Mall.

Exam I Useful Information

  • Useful information will be attached to the exam booklet and also posted on Brightspace.

  • Only simple scientific calculators are permitted. Advanced calculators with graphing capabilities, formula saving options, or solver functions are not allowed.

Allowed Materials

  • Supplementary Light Sources:

    • Clip-on desk light, book light, headlamp, etc.

    • Prohibited: Any lighting that would disturb other students.

Learning Objectives (as of 10/14/2025)

  • Define concentration of solute in solution in terms of molarity (refer to §4.1).

  • Calculate molarity of a solution (SP 4.1).

  • Calculate concentration of a solute in solution after dilution (SP 4.3).

  • Describe the relationship between absorbance and solute concentration (Beer-Lambert law, pp. 304-305).

  • Explain the purpose of a standard solution of a solute (§4.1; pp. 304-305).

  • Explain the use of the absorbance at the wavelength of maximum absorption (λmax, pp. 304-305).

  • Describe the process for determining solute concentration using calibration plots (standard curves; pp. 304-305).

  • Produce a calibration plot (standard curve; Problem B7.1, p. 305).

  • Determine the concentration of a solute using a calibration plot (Problem B7.1, p. 305).

Molarity Concepts

Definition and Related Terms

  • Molarity (M): a unit of concentration defined as the number of moles of solute per liter of solution.

  • Solution: A homogeneous mixture of two or more substances.

  • Solvent: The largest component of the solution.

  • Solute: The substance(s) that are dissolved in the solvent.

  • Concentration: A measure of the quantity of solute in a specific quantity of solvent or solution.

Example Calculation of Molarity

  • Problem: What is the molarity of 250 mL of solution containing 111 g of calcium chloride (CaCl₂, with a molar mass of 110.98 g/mol)?

  • Choices for Molarity Calculation:

    • 1.60 M

    • 2.00 M

    • 3.00 M

    • 4.00 M

    • 6.40 M

Practice Calculation for Molarity

  • Problem: What mass of Na₃PO₄ is contained in 100.0 mL of a solution with a concentration of 0.125 M?

  • Steps to Calculate:

    1. Break down the units of M.

    2. Convert volume to common units (Liters).

    3. Find moles of Na₃PO₄ in 100.0 mL of solution.

    4. Calculate the molar mass of Na₃PO₄.

    5. Convert moles to grams of Na₃PO₄.

Additional Molarity Practice

  • Exercise: What mass of Ca(NO₃)₂ is contained in 50.00 mL of a solution with a concentration of 2.75 M?

  • Steps for Calculation:

    1. Break down the units of M.

    2. Convert volume to common units (Liters).

    3. Identify moles of Ca(NO₃)₂ based on molarity.

    4. Calculate the molar mass of Ca(NO₃)₂.

    5. Convert moles to grams of Ca(NO₃)₂.

Color and Light in Solutions

  • Understanding Colored Solutions:

    • Wavelengths that are absorbed are not seen, while wavelengths that are reflected (or transmitted) can be seen.

Spectroscopy Overview

Definitions

  • Spectrum: A plot of electromagnetic radiation emitted or absorbed by a chemical element.

  • Spectroscopy (or Spectrometry or Spectrophotometry): An instrumental technique used to gather data on the energy levels of a substance at the atomic and molecular level.

Emission vs. Absorption Spectra

  • Emission Spectra: Result from light emitted by atoms after electron excitation (e.g., hydrogen atom line spectrum).

  • Absorption Spectra: Occur when atoms absorb photons of specific wavelengths and become excited, with colored substances typically absorbing visible light.

Practical Applications of UV-Vis Spectroscopy

  • Example: A leaf appears green as it absorbs blue and red wavelengths while reflecting green and yellow. Blue and red light are absorbed by chlorophyll, and the plot of absorbance versus wavelength is an absorption spectrum.

UV-Vis Spectrometer Functionality

  • The UV-Vis spectrometer measures the extent of light absorption across the ultraviolet and visible spectrum, typically spanning wavelengths of approximately 200 – 800 nm.

Beer-Lambert Law

Historical Context

  • Beer-Lambert Law: Defined by August Beer (physicist) and Johann Heinrich Lambert (mathematician); highlights the relationship between light transmission and solution concentration.

Mathematics of Absorbance

  • Absorbance Equation: A = log rac{I_0}{I} = ebc

    • Where:

    • A = absorbance

    • e = molar absorptivity (L·mol⁻¹·cm⁻¹)

    • b = path length (cm)

    • c = concentration (mol·L⁻¹)

    • Indicates a linear relationship between absorbance (A) and concentration (c) for a specific path length and compound.

    • Allows for calculation of unknown solution concentration based on measured absorbance.

Determining Chlorophyll Concentration Example

  • Proposal: To find chlorophyll concentration in a leaf sample by measuring absorbance at 663 nm (a spectral peak).

Calibration Curve Development

  • Goal: To determine chlorophyll concentrations at known levels (0.1 M, 0.2 M, etc.) and measure absorbance to calculate the best-fit line from this data.

Practice Questions on Beer-Lambert Law

  1. Task: Label the variables and units in the Beer-Lambert law: A = ext{ε} imes l imes c

  2. Inquiry: What is the relationship between concentration (c) and absorbance (A) in spectrophotometric analysis?

Experimental Steps in Spectrophotometry

Steps to Follow

  1. Determine the wavelength of maximum absorption (λmax) for the substance being analyzed.

  2. Zero the spectrophotometer with a blank sample.

  3. Measure absorbance for standard solutions of known concentration, then create a calibration plot (standard curve).

    • Utilize the equation: y = mx + b , where Abs = m × conc + b.

Practice Application Questions

  1. Standard solutions were prepared, and their absorbance was measured at a specific wavelength using a SpectraVis spectrophotometer, with a path length of 1 cm.

  • Tasks include determining molar absorptivity and calculating concentrations of unknown samples, including corrections for dilutions performed (e.g., diluting 10 mL to 50 mL).

Conclusion

  • End of UV-Vis Spectroscopy Lecture.

  • Refer to these notes for preparation for CHM 11520 Labs 4 and 5.

Orbital Diagrams and Electron Configuration

Hund’s Rule

  • Definition: Hun’s rule states that for orbitals of equal energy, the lowest energy configuration involves the maximum number of unpaired electrons with parallel spins.

  • Example with Nitrogen (Z = 7): Configuration 1s² 2s² 2p³

Detailed Overview of Orbital Diagrams

  • Structure of Orbital Diagrams: Comprise boxes (or lines) representing each orbital in a given energy level, grouped by sublevel (indicating nl designation), with arrows showing electrons and their respective spins.

Electron Configuration Principles

  1. Aufbau Principle: Electrons fill atomic orbitals in the order of increasing energy, starting with the lowest energy levels.

  2. Pauli Exclusion Principle: Each atomic orbital can hold a maximum of two electrons, provided they have opposite spins.

  3. Hund’s Rule Application: Each orbital within a given energy sublevel receives one electron before any orbital gets a second electron (all occupying parallel spins).

Orbital Filling and Periodic Table Relationship

  • The sequence of orbital filling can be tracked through the periodic table itself, revealing the connection between an element's position and its electron configuration.

PracticeElectron Configurations

  1. For Carbon (C with 6 electrons), the electron configuration is 1s² 2s² 2p².

  2. For Nickel (Ni with 28 electrons), write the electron configuration, illustrating the complete distribution across orbitals.

Depicting Electron Configurations

  • Illustration Importance: Using diagrams to visualize orbital occupancy and energy levels accentuates the understanding of electron configurations for the first ten elements.

Review of Electron Configurations in Period 3

Atomic Number

Element

Full Electron Configuration

Condensed Electron Configuration

11

Na

[Ne] 3s¹

[Ne] 3s¹

12

Mg

[Ne] 3s²

[Ne] 3s²

13

Al

[Ne] 3s² 3p¹

[Ne] 3s² 3p¹

14

Si

[Ne] 3s² 3p²

[Ne] 3s² 3p²

15

P

[Ne] 3s² 3p³

[Ne] 3s² 3p³

16

S

[Ne] 3s² 3p⁴

[Ne] 3s² 3p⁴

17

Cl

[Ne] 3s² 3p⁵

[Ne] 3s² 3p⁵

18

Ar

[Ne] 3s² 3p⁶

[Ne] 3s² 3p⁶

Practice Exercises

  1. Determine which atom is represented by the provided electronic energy-level diagram. (Options: beryllium, carbon, oxygen, sulfur)

  2. What is the electron configuration of potassium (K)? Choose from given configurations.