Chemistry (AS Level)·Foundations of organic chemistry · NSSCAS 4.1.3
Infra-red Spectroscopy
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Infra-red spectroscopy lets you read a molecule like a map. Learn why bonds absorb infra-red light and vibrate, how to read a spectrum's axes and dips, and the key characteristic absorption ranges from the Data Booklet. Then work full examples that tell an alcohol, a carbonyl and a carboxylic acid apart from their dips — and see how infra-red is used to monitor air pollution.
What you'll learn in this lesson
By the end you should be able to (NSSCAS Chemistry (AS Level) 4.1.3):
- Explain that bonds absorb infra-red radiation and vibrate (stretch and bend), and that different bonds absorb at characteristic wavenumbers
- Read an infra-red spectrum: wavenumber (cm⁻¹) on the x-axis running high→low, transmittance on the y-axis, with dips showing absorptions
- Use characteristic absorption ranges from the Data Booklet: O–H (alcohol) broad ~3200–3600, O–H (acid) very broad ~2500–3300, C–H ~2850–3100, C=O ~1680–1750, C=C ~1620–1680, C–O ~1000–1300, N–H ~3300–3500
- Analyse a spectrum to identify functional groups and deduce whether a molecule is an alcohol, a carbonyl compound or a carboxylic acid
- Outline the use of infra-red spectroscopy in monitoring air pollution (CO₂, CO, NOx and unburnt hydrocarbons in the atmosphere and vehicle emissions)
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