HSC Chemistry — Module 8
Spectroscopy — Flashcards & Quiz
Spectroscopy lets chemists identify unknown compounds from the way they interact with electromagnetic radiation, and HSC Chemistry Module 8 tests your fluency in mass spectrometry, infrared, proton NMR and UV-Vis. You need to read characteristic peaks, link IR stretches to functional groups, count proton environments in NMR, and combine multiple techniques to propose a structure — the flagship Module 8 extended-response question.
Key Points
- Mass spectrometry ionises the sample and separates fragments by mass/charge; the molecular ion peak gives the molar mass.
- Infrared (IR) spectroscopy identifies functional groups by their characteristic bond-vibration wavenumbers — e.g. O-H broad at ~3300 cm⁻¹, C=O sharp at ~1720 cm⁻¹.
- Proton NMR shows the chemical environments of hydrogen atoms; the number of peaks = number of H environments, and the integral ratios give relative H counts.
- NMR chemical shift (ppm) depends on the electronegativity of neighbouring atoms — alcohols, aromatics, and aldehydes each have characteristic ranges.
- UV-Vis spectroscopy detects electronic transitions; used to determine concentration via the Beer-Lambert law A = εcl.
- Combined technique: HSC exam questions often give mass spec + IR + NMR data and ask you to propose a structure by systematically eliminating possibilities.
Common Mistakes to Avoid
- Confusing MS (mass), IR (bonds), NMR (H environments), and UV-Vis (electronic transitions) — each gives different info.
- Reading IR peaks without their wavenumber — 3300 cm⁻¹ is O-H, 1720 cm⁻¹ is C=O, etc.
- Misinterpreting NMR integration ratios — they give relative H counts, not absolute.
- Using the M+1 peak in mass spec without understanding it comes from ¹³C isotope abundance.
- Applying UV-Vis to substances with no conjugation — it needs π-electron systems to absorb.
Exam Strategy
HSC Module 8 spectroscopy questions give you a spectrum or spectra and ask you to propose a structure. Method: (1) get molar mass from MS, (2) identify functional groups from IR, (3) count H environments from NMR peaks, (4) combine into a candidate structure, (5) verify against all data.
Sample Flashcards
Q1: How does UV-visible spectrophotometry differ from colourimetry?
A UV-visible spectrophotometer uses a monochromator to select a single, precise wavelength anywhere in the ultraviolet and visible range, rather than a broad filter band. Scanning the sample shows the wavelength of maximum absorbance (λmax), which is then used for measurements because it gives the greatest sensitivity. As in colourimetry, absorbance is proportional to concentration and a calibration curve is used. It can also measure species that absorb only in the UV and look colourless.
Q2: How does atomic absorption spectroscopy (AAS) measure metal ions?
The sample is sprayed into a flame, where the metal ions are converted to free atoms. A hollow cathode lamp made of the element being measured emits that element's characteristic wavelengths, which only atoms of that element absorb. The detector measures how much light is absorbed; absorbance is proportional to the concentration of the element, and a calibration curve of standards gives the unknown's concentration. AAS was developed in Australia by Alan Walsh at CSIRO in the 1950s.
Q3: What information does a mass spectrum give about an organic compound?
Molecules are ionised and fragmented, and the ions are separated by their mass-to-charge ratio (m/z). The molecular ion peak (M⁺), usually the peak with the highest m/z (ignoring small isotope peaks), gives the molar mass. The base peak is the tallest peak (relative intensity 100%) and comes from the most abundant ion, which is often a fragment. Fragment peaks and the mass differences between them suggest which groups are present.
Q4: How is infrared (IR) spectroscopy used to identify functional groups?
Covalent bonds absorb infrared radiation at characteristic wavenumbers (cm⁻¹) that make them stretch or bend. Approximate ranges: O–H in alcohols 3230–3550 (broad); O–H in carboxylic acids 2500–3000 (very broad); N–H 3300–3500; C–H 2850–3300; C=O 1680–1750; C=C 1620–1680; C–O 1000–1300. The region below about 1500 cm⁻¹ (the fingerprint region) is unique to each compound.
Q5: What does a carbon-13 NMR spectrum show?
Each peak corresponds to a different carbon environment, so the number of peaks equals the number of non-equivalent carbon atoms (symmetry reduces the count). The chemical shift (δ, in ppm) indicates the environment: roughly 5–40 ppm for alkyl carbons, 50–90 ppm for carbon bonded to oxygen (C–O), about 100–150 ppm for C=C, 160–185 ppm for C=O in acids and esters, and 190–220 ppm for C=O in aldehydes and ketones. ¹³C spectra are normally shown without splitting.
Sample Quiz Questions
Q1: Within the range of a calibration curve, absorbance is directly proportional to the concentration of the absorbing species.
Answer: TRUE
This is the Beer–Lambert relationship used in colourimetry, UV-visible spectrophotometry and AAS.
Q2: In AAS, the hollow cathode lamp is made of the element being measured.
Answer: TRUE
The lamp emits the characteristic wavelengths of that element, which only its atoms in the flame can absorb — this makes AAS specific to one element.
Q3: AAS can only measure metal ions at high concentrations, such as several grams per litre.
Answer: FALSE
AAS is very sensitive and is used for trace analysis, often at parts-per-million levels or lower, e.g. lead in drinking water.
Revision Tip
Spectroscopy interpretation is practice-heavy — use Revizi flashcards with 5-6 unknown compounds and their combined MS/IR/NMR data to practise the deduction.
Related Concepts
Last updated: 9 October 2026 · 7 sample flashcards · 7 sample quiz questions