Infrared (IR) Spectroscopy

Active learning helps students move from passive spectators to active analysts of IR spectra, where subtle differences in peak shape and position reveal molecular structures. Immediate feedback and peer discussion during these activities strengthen their ability to link theoretical absorption values with real spectral features, building both confidence and accuracy.

• Year 13
• Chemistry
• 4 activities
• 25 min–50 min

Learning Objectives

1. Analyze IR spectra to identify characteristic absorption bands corresponding to specific functional groups.
2. Compare the IR spectra of alcohols and carboxylic acids, explaining the differences in their O-H and C=O stretching regions.
3. Deduce the presence or absence of key functional groups (e.g., C=O, O-H, C-H) within an unknown organic molecule based on its IR spectrum.
4. Explain the relationship between bond strength, bond type, and the wavenumber of infrared absorption for common organic functional groups.

Stations Rotation

⏱ 45 min·Small Groups

Stations Rotation: Spectrum Matching Stations

Prepare four stations with IR spectra and compound cards for alcohols, aldehydes, ketones, and carboxylic acids. Groups rotate every 10 minutes, annotating peaks and justifying matches. Conclude with a class gallery walk to compare annotations.

Prepare & details

Explain how different functional groups absorb infrared radiation at specific frequencies.

Facilitation Tip: During Spectrum Matching Stations, circulate and ask each pair to explain why they placed a peak in a specific region, prompting them to use the wavenumber tables provided.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Case Study Analysis

⏱ 30 min·Pairs

Pairs: Unknown Spectrum Challenge

Provide pairs with five unknown IR spectra and a functional group table. They identify groups step-by-step: locate major peaks, assign bonds, eliminate possibilities. Pairs present one spectrum to the class for feedback.

Prepare & details

Analyze an IR spectrum to identify the presence or absence of key functional groups.

Facilitation Tip: For the Unknown Spectrum Challenge, provide spectra without labels and require students to draft a short report with their identification before discussing with their partner.

Setup: Groups at tables with case materials

Materials: Case study packet (3-5 pages), Analysis framework worksheet, Presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management

Case Study Analysis

⏱ 50 min·Small Groups

Whole Class: Jigsaw Expert Groups

Assign expert groups one functional group and its IR features. Experts teach their peers via mini-presentations with spectra examples. Everyone then quizzes on mixed spectra.

Prepare & details

Differentiate between the IR spectra of an alcohol and a carboxylic acid.

Facilitation Tip: In Jigsaw Expert Groups, assign each group a distinct functional group to research, then rotate so every student teaches their peers the absorption patterns they studied.

Setup: Groups at tables with case materials

Materials: Case study packet (3-5 pages), Analysis framework worksheet, Presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management

Case Study Analysis

⏱ 25 min·Individual

Individual: Digital Spectrum Simulator

Students use online IR simulators to generate and interpret spectra for custom molecules. They screenshot annotations and note challenges in peak overlaps.

Prepare & details

Explain how different functional groups absorb infrared radiation at specific frequencies.

Setup: Groups at tables with case materials

Materials: Case study packet (3-5 pages), Analysis framework worksheet, Presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management

Teaching This Topic

Teach this topic by starting with the most distinct peaks, like the broad O-H in alcohols and the sharp C=O in carbonyls, before introducing subtler features in the fingerprint region. Avoid overwhelming students with too many peaks at once; focus on patterns they can generalize. Research shows that students grasp IR spectroscopy best when they compare multiple spectra side-by-side, as this builds their visual literacy for identifying functional groups.

What to Expect

Students will confidently identify key functional groups by matching peaks to wavenumber ranges and justify their choices using precise language. They will also recognize the limitations of IR spectroscopy by comparing spectra and discussing when additional data, like NMR, is necessary.

Watch Out for These Misconceptions

Common Misconception

During Spectrum Matching Stations, watch for students who assume all O-H peaks look the same across compounds.

What to Teach Instead

Have students compare the broad O-H peak around 3200-3600 cm⁻¹ in ethanol with the very broad, often dimeric O-H peak in ethanoic acid alongside its sharp C=O peak. Ask them to articulate the differences in shape and position, then adjust their placement of peaks accordingly.

Common Misconception

During the Unknown Spectrum Challenge, watch for students who dismiss the fingerprint region as unimportant.

What to Teach Instead

Require students to highlight at least two peaks in the fingerprint region and explain why these patterns are unique to the molecule. Use guiding questions like, 'Could another molecule have the exact same peaks in this region? Why or why not?' to refocus their attention.

Common Misconception

During Jigsaw Expert Groups, watch for students who think peak intensity directly indicates the quantity of a functional group.

What to Teach Instead

Use the spectra in the expert groups to show how bond polarity and dipole changes affect intensity. Provide examples where identical functional groups have varying peak heights and ask students to debate why this occurs, linking back to bond properties.

Assessment Ideas

Quick Check

After Spectrum Matching Stations, provide three spectra (ethanol, ethanoic acid, propanal) and ask students to label the key absorption bands and identify the molecule. Collect their responses to check for correct peak assignments and functional group identifications.

Exit Ticket

During the Unknown Spectrum Challenge, give each student a simplified IR spectrum with a strong peak around 1720 cm⁻¹ and a broad peak around 3000 cm⁻¹. Ask them to identify the functional groups and explain in one sentence why both peaks are present, then collect responses to assess their understanding.

Discussion Prompt

After Jigsaw Expert Groups, pose the question: 'How could IR spectroscopy help a chemist distinguish between pure ethanol and pure ethanoic acid?' Encourage students to refer to specific wavenumber ranges and functional group absorptions in their answers, then use their responses to guide a whole-class discussion.

Extensions & Scaffolding

• Challenge early finishers to predict the IR spectrum of a molecule like ethyl ethanoate by combining the absorption patterns of esters and ethers.
• Scaffolding for struggling students: provide a checklist of key wavenumber ranges and a word bank of functional groups to guide their identifications.
• Deeper exploration: ask students to research how IR spectroscopy is used in industry, such as in quality control for pharmaceuticals or food safety testing.

Key Vocabulary

Wavenumber	A unit of measurement for IR absorption, typically expressed in cm⁻¹, representing the number of waves per centimeter. Higher wavenumbers indicate higher energy absorption.
Functional Group	A specific group of atoms within a molecule that is responsible for characteristic chemical reactions and spectral properties, such as an alcohol (-OH) or a carbonyl (C=O).
Infrared (IR) Spectrum	A plot showing the intensity of infrared radiation absorbed by a sample as a function of wavenumber, used to identify functional groups.
Absorption Band	A region on an IR spectrum where the sample absorbs infrared radiation at a specific wavenumber, indicating the presence of a particular bond vibration.
Fingerprint Region	The complex region of an IR spectrum below 1500 cm⁻¹, which contains many absorption bands unique to a specific molecule and can be used for identification.