Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Mass Spectra and IR Exam Questions
Free Edexcel International A Level Chemistry exam questions for Mass Spectra and IR, with a mark scheme for every pack. Covers mass spectra of organic Molecules, molecular ion peaks, fragmentation, infrared absorption, bond vibrations, functional group identification and greenhouse effect.
Mass Spectra and IR exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel International A Level Chemistry Mass Spectra and IR
Mass spectra and IR in Unit 2 of Edexcel International A Level Chemistry introduces the instrumental techniques chemists use to identify organic compounds. Mass spectrometry gives the relative molecular mass from the molecular ion peak and structural clues from fragment ions. Infrared spectroscopy identifies functional groups because particular bonds absorb at characteristic wavenumbers. These skills are used with Alcohols and Halogenoalkanes, for example to show that an oxidation has worked, and are extended in Unit 4 with NMR in Spectroscopy and chromatography. Examiners provide spectra and data tables and ask you to identify peaks, fragments and an unknown compound.
What you need to know
- Molecular ion peak The M+ peak at the highest m/z (ignoring small M+1 peaks) gives the relative molecular mass of the compound.
- Fragmentation The molecular ion can break into a positive fragment ion and a radical; only the ion is detected. Common fragments: m/z 15 CH3+, 29 C2H5+ or CHO+, 43 C3H7+ or CH3CO+.
- Distinguishing isomers Isomers have the same M+ peak but different fragmentation patterns, for example propan-1-ol gives a strong peak at m/z 31 (CH2OH+), while propan-2-ol gives a strong peak at m/z 45 (CH3CHOH+).
- IR absorption Bonds absorb infrared radiation at specific frequencies, causing them to vibrate (stretch or bend). Peaks are reported as wavenumbers in cm-1 and the bond is identified from a data table.
- Key IR peaks O to H in alcohols gives a broad peak around 3200 to 3550 cm-1. O to H in carboxylic acids is very broad around 2500 to 3300 cm-1. C=O gives a strong sharp peak around 1700 cm-1.
- Greenhouse gases Molecules such as CO2, H2O and CH4 absorb infrared radiation emitted by the Earth because their bonds vibrate, which links IR absorption to the greenhouse effect.
Common mistakes in Mass Spectra and IR
- Writing a fragment as a neutral species. Fragments detected in a mass spectrum must be positive ions, for example [C2H5]+.
- Choosing the tallest peak as the molecular ion. The tallest is the base peak; M+ is the peak with the highest m/z.
- Using the fingerprint region alone to identify functional groups. Use it for matching a whole spectrum with a known sample.
- Identifying an alcohol from any peak near 3000 cm-1. C to H absorbs near 2900 cm-1; look for the broad O to H peak.
- Forgetting that the absence of a peak is also evidence, such as no broad O to H peak showing that an alcohol has been fully oxidised to a ketone.
Try three Mass Spectra and IR exam-style questions
1. The mass spectrum of a compound has a molecular ion peak at m/z = 58 and a large peak at m/z = 43. The IR spectrum shows a strong absorption at 1715 cm-1 and no broad peak at 3200 to 3550 cm-1. Identify the compound. (3 marks)
Show the mark scheme answer
Mr = 58; C=O present and no O to H; peak at 43 is CH3CO+; compound is propanone, CH3COCH3 (propanal would give a large peak at 29 for CHO+ or C2H5+ rather than 43).
2. Explain how IR spectroscopy could show that ethanol has been oxidised to ethanoic acid. (2 marks)
Show the mark scheme answer
The product spectrum shows a C=O absorption at about 1700 cm-1; and a very broad O to H absorption at about 2500 to 3300 cm-1 typical of a carboxylic acid, rather than the alcohol O to H at 3200 to 3550 cm-1.
3. Write an equation for the fragmentation of the molecular ion of ethanol to give a peak at m/z = 31. (2 marks)
Show the mark scheme answer
[CH3CH2OH]+ gives [CH2OH]+ + CH3 radical; ion correctly identified with positive charge and radical shown.
Mass Spectra and IR FAQs
What does Edexcel International A Level Chemistry Mass spectra and IR cover?
It covers using molecular ion peaks and fragmentation in mass spectra, using infrared absorptions to identify bonds and functional groups, and the link between IR absorption and greenhouse gases.
Which unit is Mass spectra and IR examined in?
It is part of Unit 2, and these techniques are combined with NMR and chromatography in Unit 4.
Do I need to memorise IR wavenumbers?
A data table is provided in the exam, but knowing the main O to H, C=O and C to H regions saves time and helps you spot the key peaks quickly.
