OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 19

1 mark · Medium difficulty · Multiple Choice

Identify the incorrect statement regarding infrared radiation among the given options involving global warming, ozone depletion, bond vibrations, and breathpolysers.

Practise this question

Question

Multiple choice question 19 asking which statement about infrared radiation (IR) is not correct. Four options A, B, C, and D are provided, covering topics such as greenhouse gases and global warming, CFCs and ozone breakdown, bond vibrations, and IR breathpolysers. An answer box and mark allocation [1] are shown at the bottom.
Question text

19 Which statement about infrared radiation (IR) is not correct?

A Absorption of IR by molecules such as CO2, H2O and CH4 has been linked to global

warming.

B IR causes CFC molecules to produce chlorine radicals that initiate ozone breakdown in the

upper atmosphere.

C IR causes some covalent bonds to vibrate more and absorb energy.

D IR is used in modern breathalysers to detect ethanol.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 19 indicating the correct answer is B, with a mark allocation of 1.

19 B 1

How to answer it

Evaluating Statements About Infrared Radiation

What this question tests

This question assesses your understanding of the interaction between electromagnetic radiation (specifically infrared, IR, and ultraviolet, UV) and matter. It tests your knowledge of greenhouse gases, atmospheric chemistry (ozone depletion vs. global warming), the mechanism of IR spectroscopy (bond vibration), and real-world analytical applications like modern breath analysers.

Question 19: Multiple Choice Analysis

Identifying Incorrect Statements on Infrared Radiation

✅ Correct Answer: B

Statement B is not correct because the breakdown of CFC molecules to produce chlorine radicals in the upper atmosphere is initiated by high-energy ultraviolet (UV) radiation, not infrared (IR) radiation.

💡 Key Knowledge: UV vs. IR

  • UV radiation has shorter wavelengths and higher frequencies/energies, making it capable of breaking covalent bonds homolytically to form radicals (photodissociation).
  • IR radiation has lower energy; absorption causes covalent bonds to bend and stretch more (vibrate), but it does not have enough energy to break bonds or form radicals.

🧠 Exam Technique

Read the stem very carefully for negative phrasing like "not correct". Evaluate each option methodically and cross out true statements until only the false scientific claim remains.

❌ Common Errors

Students frequently confuse the roles of UV and IR radiation in atmospheric chemistry, incorrectly assuming that all atmospheric interactions and pollutant breakdowns are driven by heat/infrared rays.

Mark Allocation: [1 mark] awarded for selecting option B.

Why the Other Statements Are True

Option A (True)

Greenhouse gases such as carbon dioxide ( CO₂ ), water vapour ( H₂O ), and methane ( CH₄ ) absorb outgoing infrared radiation from the Earth's surface, trapping heat and contributing to global warming.

Option C (True)

The fundamental principle of IR spectroscopy is that absorption of IR radiation causes specific covalent bonds within molecules to increase their vibrational energy (stretching and bending).

Option D (True)

Modern roadside breath analysers use IR spectroscopy to shine an IR beam through a breath sample, measuring the characteristic absorption of the O-H bond in ethanol to determine blood alcohol concentration.

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.2 Alcohols, haloalkanes and analysis · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.