CCEA A Level Chemistry
CCEA A Level Chemistry 2.7 Infrared Spectroscopy Exam Questions
Free CCEA A Level Chemistry revision for 2.7 Infrared Spectroscopy: what you need to know, common mistakes and exam-style questions with answers. Covers molecular vibrations, characteristic absorption wavenumbers, identifying functional groups from infrared spectra and the data leaflet.
Practise 2.7 Infrared Spectroscopy with past papers
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About CCEA A Level Chemistry 2.7 Infrared Spectroscopy
Section 2.7 of Unit AS 2 introduces your first spectroscopic technique. You need to understand that infrared radiation causes bonds in a molecule to vibrate (stretch and bend), and that particular bonds absorb at characteristic wavenumbers. Using the absorption table in the CCEA data leaflet, you must deduce which functional groups are present from an infrared spectrum or a list of absorptions. The most useful absorptions are O-H in alcohols and in carboxylic acids, C=O and C-H. Infrared spectroscopy is often combined with chemical tests and reactions, for example showing that an alcohol has been oxidised to an aldehyde or acid.
What you need to know
- How IR works Covalent bonds vibrate at specific frequencies. When infrared radiation of the same frequency passes through a sample, energy is absorbed and the bond vibrates more strongly. Absorption is shown as a dip in transmittance at that wavenumber (cm-1).
- Key absorptions O-H in alcohols gives a broad band at roughly 3200 to 3550 cm-1; O-H in carboxylic acids gives a very broad band at roughly 2500 to 3300 cm-1; C=O gives a strong, sharp band at roughly 1650 to 1750 cm-1. Always use the exact ranges in the data leaflet.
- Fingerprint region Below about 1500 cm-1 the pattern is complex and unique to each compound. It can be compared with a database to identify a substance, but you are not expected to assign individual peaks there.
- Using IR in reactions Oxidation of an alcohol to an aldehyde or ketone shows loss of the alcohol O-H and appearance of C=O. Full oxidation to a carboxylic acid shows C=O plus a very broad acid O-H.
Common mistakes in 2.7 Infrared Spectroscopy
- Reading wavenumbers in the wrong direction. The x-axis usually decreases from left to right, from about 4000 to 500 cm-1.
- Identifying an O-H from a weak sharp peak. Alcohol and acid O-H bands are broad because of hydrogen bonding.
- Trying to assign peaks in the fingerprint region. Use it only to match a compound with a known spectrum.
- Saying a C=O peak proves the compound is an aldehyde. Ketones, acids and esters also have C=O; use other evidence too.
- Saying infrared breaks bonds. Infrared energy makes bonds vibrate; it does not break them.
Try three 2.7 Infrared Spectroscopy exam-style questions
1. A compound C3H6O shows a strong absorption at about 1715 cm-1 and no broad absorption above 3000 cm-1. Suggest a structure. (2 marks)
Show the mark scheme answer
C=O present and no O-H; propanone, CH3COCH3 (or propanal, CH3CH2CHO).
2. Explain how IR spectroscopy could show that ethanol has been fully oxidised to ethanoic acid. (3 marks)
Show the mark scheme answer
Broad alcohol O-H absorption at about 3200 to 3550 cm-1 disappears; strong C=O absorption appears at about 1700 cm-1; very broad acid O-H appears at about 2500 to 3300 cm-1.
3. Explain why bonds absorb infrared radiation. (2 marks)
Show the mark scheme answer
Bonds vibrate at particular frequencies; radiation with the same frequency is absorbed, increasing the vibration.
2.7 Infrared Spectroscopy FAQs
What does CCEA A Level Chemistry 2.7 Infrared Spectroscopy cover?
It covers why bonds absorb infrared radiation and how to identify functional groups from absorptions using the data leaflet.
Which unit is 2.7 Infrared Spectroscopy examined in?
It is examined in Unit AS 2: Further Physical and Inorganic Chemistry and an Introduction to Organic Chemistry, a 1 hour 30 minute paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit AS 3.
Do I need to memorise IR wavenumbers?
The ranges are given in the data leaflet, but you should know which bonds to look for and recognise the shapes of the alcohol O-H, acid O-H and C=O bands quickly.
