Edexcel A Level Chemistry
Edexcel A Level Chemistry Topic 7: Modern Analytical Techniques I Exam Questions
Free Edexcel A Level Chemistry exam questions for Topic 7: Modern Analytical Techniques I, with a mark scheme for every pack. Covers mass spectrometry of organic compounds, molecular ion peaks, fragmentation patterns, and infrared spectroscopy to identify functional groups and bonds.
Topic 7: Modern Analytical Techniques I 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 A Level Chemistry Topic 7: Modern Analytical Techniques I
Topic 7 of Edexcel A Level Chemistry, Modern Analytical Techniques I, introduces two instrumental methods for identifying organic compounds. Mass spectrometry is used to find the relative molecular mass from the molecular ion peak and to deduce structural features from fragmentation patterns, for example distinguishing isomers. Infrared spectroscopy identifies bonds and functional groups from characteristic absorptions, using the data booklet, and explains why some bonds absorb because they change polarity when they vibrate. The topic links closely to Organic Chemistry I and is extended in Topic 19 with NMR and chromatography. It is assessed in Paper 2. Examiners often combine both techniques with chemical tests to identify an unknown.
What you need to know
- Molecular ion The peak with the largest m/z (ignoring small M+1 peaks) is the molecular ion, M+, and gives the relative molecular mass.
- Fragmentation The molecular ion breaks into a positive ion and a radical; only the positive ion is detected. Common fragments include m/z 15 (CH3+), 29 (C2H5+), 43 (C3H7+ or CH3CO+) and 31 (CH2OH+).
- Distinguishing isomers Isomers have the same molecular ion but different fragments. For example, propan-1-ol gives a peak at 31 from CH2OH+, while propan-2-ol gives a large peak at 45 from CH3CHOH+.
- How IR works Bonds vibrate and absorb IR radiation at specific frequencies, measured as wavenumber in cm-1. Only bonds that change polarity (dipole) during vibration absorb.
- Key IR absorptions O-H alcohol broad around 3200 to 3600 cm-1, O-H acid very broad around 2500 to 3300 cm-1, C=O around 1650 to 1750 cm-1 and C-H around 2850 to 3000 cm-1. Always use the data booklet values.
- Using spectra together Combine Mr from mass spectrometry with functional groups from IR, plus chemical tests, to deduce a structure. The fingerprint region below 1500 cm-1 can be matched to known spectra.
Common mistakes in Topic 7: Modern Analytical Techniques I
- Writing fragment ions without a positive charge.
- Assuming the tallest peak (base peak) is the molecular ion.
- Identifying an alcohol from any peak near 3000 cm-1; the O-H peak must be broad and in the correct range.
- Forgetting that the radical formed during fragmentation is not detected.
- Quoting IR values from memory that differ from the data booklet ranges.
Try three Topic 7: Modern Analytical Techniques I exam-style questions
1. The mass spectrum of a compound has a molecular ion at m/z 58 and IR shows a strong absorption at 1715 cm-1 but no broad absorption above 3000 cm-1. Suggest an identity. (3 marks)
Show the mark scheme answer
C=O present from 1715 cm-1; no O-H so not an acid or alcohol; Mr 58 fits propanone (CH3COCH3) or propanal.
2. Write an equation for the formation of the fragment at m/z 29 from the molecular ion of propanal. (2 marks)
Show the mark scheme answer
[CH3CH2CHO]+• → CH3CH2+ + CHO• (or CHO+ and CH3CH2•); positive charge on the detected fragment.
3. Explain why carbon dioxide absorbs infrared radiation but nitrogen does not. (2 marks)
Show the mark scheme answer
C=O bonds are polar and their vibrations (asymmetric stretch or bend) change the dipole; N≡N is non-polar so its vibration causes no change in dipole.
Topic 7: Modern Analytical Techniques I FAQs
What does Edexcel A Level Chemistry Topic 7 cover?
It covers mass spectrometry of organic compounds, including molecular ions and fragmentation, and infrared spectroscopy for identifying bonds and functional groups.
Which paper is Topic 7 examined in?
Modern Analytical Techniques I is assessed in Paper 2, and spectra can appear in synoptic questions in Paper 3.
How do I revise spectra questions?
Practise reading data booklet IR ranges and labelling fragment peaks, then work through mixed questions where you combine Mr, IR and chemical tests.
