HookThe machine that convicts drink-drivers is an infrared spectrometer
Fail a roadside breath screening in England or Wales and you are taken to a police station to blow into the evidential instrument — the reading that stands up in court. The legal limit is 35 micrograms of ethanol per 100 millilitres of breath (Scotland cut its own limit to 22 in 2014), and the machine that measures it is, at heart, an infrared spectrometer. Ethanol's C–H bonds absorb infrared radiation strongly near 2,950 cm⁻¹; shine IR through the breath sample, measure how much is absorbed at wavelengths ethanol absorbs and water vapour does not, and the absorbance reports the concentration. A conviction that changes someone's life rests on the idea at the centre of this section: bonds absorb infrared radiation at characteristic frequencies, so spectra identify molecules.
Section 3.3.6 is the analyst's toolkit, three instruments deep. Test-tube chemistry — four cheap, fast reactions that flag alkenes, alcohols, aldehydes and carboxylic acids by colour changes, mirrors and fizzing (Required practical 6 is you running them properly). Mass spectrometry — the molecular ion hands you the molecular mass, the fragments hand you the skeleton, and precise masses separate molecules that nominal masses cannot. Infrared spectroscopy — absorptions above 1,500 cm⁻¹ name the bonds present, and the fingerprint region below matches the whole molecule against a database. AQA's favourite question hands you an unknown and data from all three at once.
ModelFour test tubes, four functional groups
Each test is a reagent, a condition and a pair of observations — what happens when the group is present, and what happens when it is not. Alkene: shake with orange bromine water; a C=C bond consumes the bromine by addition and the orange colour is decolourised, while a saturated compound leaves it orange (this is 3.3.4's electrophilic addition working as a diagnostic).
Aldehyde versus ketone is the classic pairing, because both contain C=O but only aldehydes are easily oxidised further. Tollens' reagent — ammoniacal silver nitrate, warmed gently in a hot-water bath — is reduced by an aldehyde to metallic silver, plating the inside of the tube as a silver mirror; a ketone gives no change. Fehling's solution does the same job with copper: warm the deep-blue solution with an aldehyde and a brick-red precipitate of copper(I) oxide forms; a ketone leaves it blue. Either test earns the marks, but the observation must be complete — 'silver mirror forms' or 'blue solution gives a brick-red precipitate'.
Carboxylic acid: add sodium carbonate or sodium hydrogencarbonate solution. Effervescence — and the gas turning limewater milky confirms CO₂. Alcohols, aldehydes and ketones are not acidic enough to do this, which makes the fizz wonderfully unambiguous. Alcohol (primary or secondary): warm with acidified potassium dichromate(VI); the solution turns from orange to green as the alcohol is oxidised. A tertiary alcohol leaves it orange — a negative result that is itself diagnostic, provided the other tests have ruled the alternatives out.
CaseRequired practical 6 — sequence the tests so they cannot lie
RP6 assesses whether you can run those tests on genuine unknowns, and the intellectual content is ordering. The dichromate test has a blind spot: aldehydes also turn it orange to green, because they too are oxidised. So a green result alone cannot distinguish 'alcohol' from 'aldehyde' — you must run Tollens' first, and only read the dichromate result once you know whether an aldehyde is present. A sensible sequence for a set of unknowns: carbonate first (the fizz instantly isolates the acid), bromine water next (the alkene), then Tollens' (the aldehyde), and dichromate last to split the remaining alcohols — green for primary or secondary, unmoved orange for tertiary.
The practical marks are in technique and safety. Use small volumes in clean test tubes — a greasy tube ruins a silver mirror. Tollens' reagent must be freshly prepared and rinsed away immediately after use: stored mixtures can deposit explosive silver compounds. Warm with a hot-water bath, never a naked flame — the unknowns are volatile, flammable organic liquids. And record observations for every tube, including the negatives: 'no effervescence' is data, and AQA mark schemes frequently award the negative observation its own mark.
Four unlabelled liquids, W–Z, known to be hex-1-ene, butanal, butan-1-ol and propanoic acid. W fizzes with sodium hydrogencarbonate and the gas turns limewater milky — propanoic acid. X decolourises bromine water in seconds — hex-1-ene. Y gives no fizz and leaves bromine water orange, but produces a silver mirror with freshly prepared Tollens' in a water bath — butanal. Z is negative in all three, then turns acidified dichromate from orange to green when warmed — butan-1-ol. Run the dichromate test first instead and both Y and Z go green: the tests have not changed, but the sequence has destroyed the information.
DataMass spectrometry — the molecular ion and the useful wreckage
Ionise an organic compound in a mass spectrometer and some molecules lose one electron and survive intact: the molecular ion, M⁺. It is the peak at the highest m/z (bar tiny isotope peaks beyond it), and its m/z equals the compound's relative molecular mass — the single most valuable number on the spectrum. But the ionisation also smashes molecules apart: fragmentation. A covalent bond in the ion breaks, producing a cation, which the machine detects, and a radical, which it does not: \(\text{CH}_3\text{COCH}_3^{+\bullet} \rightarrow \text{CH}_3\text{CO}^+ + \bullet\text{CH}_3\). Only charged species reach the detector, so every fragment peak is a cation, written with its + charge.
The fragments are a fingerprint of the skeleton. Learn the small vocabulary: m/z 15 is CH₃⁺, 29 is C₂H₅⁺ or CHO⁺, 43 is C₃H₇⁺ or CH₃CO⁺, and a gap matters as much as a peak — a fragment 15 below M⁺ means a methyl radical was lost. Isotopes add a second signature: a compound containing one chlorine shows M and M+2 peaks in a 3:1 ratio (³⁵Cl : ³⁷Cl), while one bromine gives M and M+2 at roughly 1:1 — instantly visible in the spectra of the halogenoalkanes from 3.3.3.
Fragmentation is how mass spectrometry tells structural isomers apart when their molecular ions are identical. Propanal and propanone both show M⁺ at 58; propanone fragments overwhelmingly by losing a methyl radical to give the stable CH₃CO⁺ at 43, while propanal's spectrum is dominated by m/z 29. Same mass, different wreckage, different molecule.
A carbonyl compound has M⁺ = 58, a dominant fragment at m/z 43 and a visible peak at 15. The 58 fits C₃H₆O — propanal or propanone. The 43 peak is loss of 15 (a methyl radical), giving CH₃CO⁺: \(\text{CH}_3\text{COCH}_3^{+\bullet} \rightarrow \text{CH}_3\text{CO}^+ + \bullet\text{CH}_3\), and 15 is CH₃⁺ itself. A strong 43 with no strong 29 says propanone — propanal would shed its CHO or C₂H₅ groups to pile intensity at 29. Write fragmentation equations exactly like that one: mass and charge must both balance, the detected fragment carries the +, and the neutral partner is a radical with its dot.
DataPrecise mass — when 58 is not 58
Nominal masses collide. Butane and propanone both 'weigh' 58; nitrogen, carbon monoxide and ethene all 'weigh' 28. High-resolution mass spectrometry breaks the tie, because atomic masses are not actually whole numbers: ¹H = 1.0078, ¹²C = 12.0000 exactly, ¹⁶O = 15.9949. Sum them and every molecular formula acquires a unique decimal signature. A high-resolution instrument reading M⁺ to four decimal places is therefore reading the molecular formula directly — no combustion analysis, no guesswork.
The exam skill is honest arithmetic: compute the precise mass of each candidate formula, compare with the measured value, and state the match. It is also the quiet fix for the isomer problem in reverse — precise mass identifies the formula, but it can never separate propanal from propanone, because isomers share a formula exactly. Formula from the precise mass; structure from the fragments and the infrared. Each tool answers its own question, and full-credit answers say which tool answered what.
A high-resolution spectrometer records M⁺ = 58.0417. Candidates: butane, C₄H₁₀, precise mass \(4 \times 12.0000 + 10 \times 1.0078 = 58.0780\); propanone (or propanal), C₃H₆O, precise mass \(3 \times 12.0000 + 6 \times 1.0078 + 15.9949 = 58.0417\). The measurement matches C₃H₆O and rules butane out completely — a separation no low-resolution spectrum could make. Show the working to four decimal places for both candidates; the comparison is the answer.
ModelInfrared — bonds vibrate, and the data sheet does the memorising
Covalent bonds behave like springs: they stretch and bend at natural frequencies, and they absorb infrared radiation at exactly those frequencies. Which frequency depends on the bond — its stiffness and the masses on each end — so an infrared spectrum is a census of the bonds in the molecule, plotted as transmittance against wavenumber in cm⁻¹. AQA gives you the absorption table on the data sheet, so the skill is reading it, not reciting it. The workhorse windows: a strong, sharp dip at 1680–1750 cm⁻¹ is C=O; a broad absorption at 3230–3550 cm⁻¹ is the O–H of an alcohol; a very broad smear across 2500–3000 cm⁻¹, overlapping the C–H peaks near 2850–3300, is the hydrogen-bonded O–H of a carboxylic acid. Combinations diagnose: C=O plus acid O–H is a carboxylic acid; C=O with no O–H at all points to an aldehyde, ketone or ester.
Below about 1500 cm⁻¹ lies the fingerprint region — a dense thicket of absorptions from the whole skeleton flexing at once. It is too complicated to assign bond by bond, and that is precisely its value: the pattern is unique to each compound, so matching it against a database identifies the exact molecule, not just its functional groups. This is what the evidential breath instrument exploits — and the same physics runs the greenhouse effect, which AQA links here explicitly: the C=O bonds of CO₂, the O–H of water vapour and the C–H of methane absorb infrared radiated from the Earth's surface, trapping energy in the atmosphere. One idea, three appearances: forensic, analytical, planetary.
An unknown liquid has molecular formula C₃H₆O₂ (M = 74). The candidates include propanoic acid and the ester methyl ethanoate. Its IR spectrum shows a strong absorption at 1720 cm⁻¹ and a very broad absorption from 2500 to 3000 cm⁻¹. The 1720 cm⁻¹ dip is C=O — but both candidates have one, so it decides nothing alone. The broad 2500–3000 cm⁻¹ smear is the O–H of a carboxylic acid, which the ester lacks: the unknown is propanoic acid. Confirm for a mark you can bank: it will fizz with sodium hydrogencarbonate. The mass spectrum agrees — a fragment at m/z 45 (COOH⁺) and one at 57 (loss of OH, 74 − 17) fit the acid, where the ester would instead lose a methyl radical to give m/z 59.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Describe-a-test questions are marked as reagent, condition, and observation both ways — 'add freshly prepared Tollens' reagent, warm in a water bath: aldehyde gives a silver mirror, ketone gives no change'. Observations must be before-and-after and colour-precise: orange to colourless for bromine water (never 'clear'), orange to green for dichromate, blue solution to brick-red precipitate for Fehling's. In any identify-the-unknown question, state which test result eliminates which candidate — AQA's mark schemes credit the logic, not just the final name — and remember negative results carry marks of their own.
Mass-spectrometry marks divide three ways. Reading: the molecular ion sits at the highest m/z, so quote Mr from it directly. Writing: fragmentation equations must conserve mass and charge, with the + on the detected cation and a dot on the neutral radical — omitting the radical is the most common dropped mark in the topic. Calculating: precise-mass questions want the arithmetic for every candidate formula shown to four decimal places, then an explicit comparison sentence. In infrared questions, always give the trio — wavenumber range, bond, and functional group ('absorption at 1680–1750 cm⁻¹ shows C=O, so the compound is a carbonyl') — because any one alone is worth little. The data sheet supplies the numbers, so uncited ranges suggest you never opened it.
The synoptic six-marker hands you two or three data sources for one unknown. Structure the answer as a funnel: molecular formula first (precise mass or Mr), functional group second (IR plus a confirming test-tube result), exact structure last (fragments distinguishing the isomers). For RP6 itself, be ready to justify the sequence — Tollens' before dichromate, because aldehydes also turn dichromate green — and to quote the safety reasoning: water bath for flammable liquids, Tollens' made fresh and rinsed away because stored residues can become explosive. Practical assessment is at least 15% of the qualification, and this practical's marks are almost entirely in the reasoning.