IB Chemistry Infrared Spectroscopy: Common Questions Answered

Most marks are lost because students name a peak from memory instead of checking the wavenumber range given in the data booklet, or they confuse fingerprint-region peaks with functional-group peaks. Examiners want you to reference the actual range (e.g. 1700–1750 cm⁻¹ for C=O) and link it to a specific bond, not a vague guess.

IR spectroscopy identifies functional groups in a molecule by measuring which infrared wavenumbers it absorbs. Different bonds — C=O, O-H, N-H, C-H — absorb at characteristic wavenumber ranges, so a spectrum's peak pattern lets you deduce which groups are present in an unknown organic compound, particularly useful in reaction monitoring and structure elucidation questions.

Read left to right: high wavenumber (4000 cm⁻¹) to low (500 cm⁻¹). Identify sharp, strong peaks first — a strong peak around 1700 cm⁻¹ suggests C=O; a broad peak near 3300 cm⁻¹ suggests O-H or N-H. Ignore the cluttered fingerprint region below 1500 cm⁻¹ unless the question asks about it directly.

No — infrared spectroscopy is identical content for SL and HL students under Structure 3.2 in the current DP Chemistry guide. The difference in difficulty comes from how it's combined with other techniques (mass spectrometry, NMR) in longer HL Paper 2 structure-elucidation questions, not from extra IR theory itself.

You don't need to memorise exact numbers — the data booklet provides them in the characteristic infrared absorption table. What you must know is which functional group each range corresponds to and how to describe peak shape (broad vs sharp, strong vs weak) in your answer, since examiners mark on correct interpretation, not recall.

Look at the shape and range of the O-H peak. Alcohols show a broad O-H peak around 3200–3550 cm⁻¹. Carboxylic acids show a much broader, lower O-H peak spanning roughly 2500–3300 cm⁻¹, often overlapping with C-H stretches, plus a separate strong C=O peak near 1700–1750 cm⁻¹ that alcohols lack.

The most common command terms are "identify", "deduce" and "suggest". "Identify" wants a named functional group only. "Deduce" wants the group named with a reasoned link to the spectrum data. "Suggest" often appears when the answer isn't fully certain from IR alone and needs support from mass spec or NMR data too.

Usually because you named the group without justifying it against the actual wavenumber range shown, or you missed noting peak shape (broad vs sharp). IB mark schemes for this topic typically award one mark for the correct group and a second mark for correctly citing the range or shape as evidence — skip the evidence and you lose half the marks available.

Not conceptually hard, but it's a topic where careless reading costs marks fast. The chemistry itself — bonds absorbing specific infrared frequencies — is straightforward. Where students struggle is applying it under exam pressure alongside mass spectrometry and NMR data in the same question, especially in HL Paper 2 structure-elucidation problems.

IB structure-elucidation questions usually give you an unknown compound's molecular formula, a mass spectrum fragmentation pattern, an IR spectrum, and sometimes ¹H NMR data together, then ask you to deduce the full structure. Each technique narrows down possibilities: IR identifies functional groups, MS identifies mass and fragments, NMR confirms hydrogen environments.

According to the IB, infrared spectroscopy sits under Structure 3.2 (Functional groups) in the current Chemistry guide, first examined in 2025, and is assessed through interpreting given spectra rather than memorising theory. Assessment objectives focus on AO2 (application) and AO3 (analysis), meaning you're marked on interpreting real data, not reciting facts.

Past paper spectra are the single best resource because IB exam spectra follow consistent formatting and the same data booklet table every year. Practising with real exam-style spectra, alongside worked mark schemes showing exactly how marks were awarded for range and shape justification, builds the pattern-recognition speed you need under time pressure.

The most useful thing you can do is check whether your child is using the data booklet correctly rather than relying on memory — that's the single biggest cause of lost marks on this topic. Ask them to show you a past paper answer and explain which range they used and why; if they can't point to the booklet, that's the gap to fix.