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Updated Mar 21, 2026

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2 pages

A-Level Chemistry: Organic Analysis with Infrared Spectroscopy (AQA)

Maya A

@maya.ah

Infrared spectroscopy is like giving molecules a health check-up using... Show more

# Infrared Spectroscopy

Is a useful analytical technique to identify unknown chemicals by observing the prescence or absence of certain fun

How Infrared Spectroscopy Works

Think of covalent bonds as tiny springs that are constantly vibrating. When you shine infrared light through a compound, these bonds absorb energy at specific frequencies and vibrate even more. It's a bit like how different guitar strings vibrate at different pitches.

The vibration frequency depends on three key factors: how heavy the atoms are, how strong the bond is, and what type of vibration occurs. Stronger bonds vibrate faster (higher frequency), whilst heavier atoms make bonds vibrate more slowly (lower frequency).

There are three main types of bond vibrations you need to know: symmetric stretch (both ends moving outward together), asymmetric stretch (one end moving out whilst the other moves in), and bending (the bond angle changing).

Quick Tip: Remember that stronger = faster vibrations, heavier atoms = slower vibrations.

Scientists measure these vibrations using wavenumbers (cm⁻¹) on the x-axis, which is proportional to frequency. Most useful IR absorption happens between 4,000-400 cm⁻¹.

Reading IR Spectra and Functional Groups

IR spectra are split into two main regions that serve different purposes. Above 1500 cm⁻¹ is where you'll identify functional groups - this is the money zone for working out what's in your compound.

Here are the key peaks you absolutely must memorise: O-H bonds in alcohols appear at 3,230-3,550 cm⁻¹, C=O bonds show up at 1,680-1,750 cm⁻¹, and C=C bonds appear at 1,620-1,680 cm⁻¹. N-H bonds in amines create peaks around 3,300-3,500 cm⁻¹.

Below 1500 cm⁻¹ is the fingerprint region - it's incredibly complex with loads of peaks caused by the whole molecule vibrating. This region is unique for every compound, so chemists use computers to match it against databases to identify unknown substances.

Remember: Strongly polar bonds give intense peaks, and hydrogen bonding creates broad, spread-out bands.

IR spectroscopy also explains global warming. Greenhouse gases like CO₂ and methane have bonds that absorb infrared radiation really effectively, trapping heat that Earth tries to radiate back to space.

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Chemistry

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Updated Mar 21, 2026

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2 pages

A-Level Chemistry: Organic Analysis with Infrared Spectroscopy (AQA)

Maya A

@maya.ah

Infrared spectroscopy is like giving molecules a health check-up using light waves. By shining infrared radiation through a sample, chemists can identify what functional groups are present and even work out what unknown compounds are.

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How Infrared Spectroscopy Works

Quick Tip: Remember that stronger = faster vibrations, heavier atoms = slower vibrations.

Scientists measure these vibrations using wavenumbers (cm⁻¹) on the x-axis, which is proportional to frequency. Most useful IR absorption happens between 4,000-400 cm⁻¹.

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Reading IR Spectra and Functional Groups

Remember: Strongly polar bonds give intense peaks, and hydrogen bonding creates broad, spread-out bands.