OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 12
1 mark · Easy difficulty · Multiple Choice
Identify which process does not involve infrared radiation from a list of environmental and analytical processes.
Practise this questionQuestion
Question text
12 Which process does not involve infrared radiation?
A Catalytic ozone depletion
B Monitoring toxic gases in the air
C Measuring ethanol in breathalyser tests
D Global warming
Your answer
[1]
Mark scheme
Show the mark scheme
12 A 1
How to answer it
Infrared Radiation: Applications & Atmospheric Chemistry
Core specification coverage: Module 4 – Core Organic Chemistry (Spectroscopy & Atmospheric Chemistry)
- Practical uses of IR absorption: Monitoring vehicle/air pollution and roadside breathalysers for drink-driving enforcement.
- Atmospheric mechanisms: Understanding which greenhouse gas bonds absorb infrared radiation to drive global warming.
- Electromagnetic spectrum differentiation: Distinguishing between processes driven by ultraviolet (UV) radiation (homolytic fission in ozone depletion) and infrared (IR) radiation (covalent bond vibration).
Question 12
Multiple Choice: Which process does not involve infrared radiation? [1 Mark]
✅ Correct Answer
A — Catalytic ozone depletion
Reasoning: Ozone depletion in the stratosphere is initiated by ultraviolet (UV) radiation, which provides the high energy required to break the C–Cl covalent bond in chlorofluorocarbons (CFCs) via homolytic fission, creating chlorine radicals (Cl•).
• [1 Mark] for selecting A.
Note on Mark Scheme snippet: The provided clip displays the entry for question 13 (B). For Question 12, the correct specification response is unambiguously A.
💡 Key Knowledge (OCR Specification)
The OCR A-Level specification explicitly details three major environmental and analytical uses of infrared absorption:
- Breathalysers: Measures ethanol concentration by detecting the characteristic absorption of infrared by C–H bonds (at ~2950 cm⁻¹).
- Monitoring air pollution: Remote sensors use IR spectroscopy to detect toxic pollutants (e.g. CO, NO, and unburnt hydrocarbons) in vehicle exhausts.
- Global warming: Earth absorbs UV/visible light from the Sun and re-emits it as IR radiation. Greenhouse gases (CO₂, H₂O, CH₄) absorb this IR, causing their polar covalent bonds to vibrate more vigorously and re-emit heat back to Earth.
📐 Option Elimination Breakdown
| Option | Process | Radiation Involved | Status |
|---|---|---|---|
| A | Catalytic ozone depletion | Ultraviolet (UV) light breaks C–Cl bonds: CF₂Cl₂ + UV → CF₂Cl• + Cl• | Correct (Does NOT use IR) |
| B | Monitoring toxic air pollutants | Infrared (IR) absorption detects specific bond vibrations of gases like CO and NO. | Incorrect (Uses IR) |
| C | Breathalyser tests | Infrared (IR) radiation is absorbed by the C–H / C–O bonds in ethanol vapor. | Incorrect (Uses IR) |
| D | Global warming | Infrared (IR) emitted by Earth is trapped by greenhouse gas bond vibrations. | Incorrect (Uses IR) |
❌ Common Errors & Traps
- Confusing UV with IR: Many candidates confuse the greenhouse effect (IR absorption causing covalent bond vibration) with ozone depletion (high-energy UV absorption causing bond dissociation into radicals).
- Overlooking the word "NOT": Missing the negative trigger word in MCQ stems is one of the most frequent causes of lost marks across Section A.
- Assuming breathalysers are purely chemical: Older tests used potassium dichromate reduction, but modern roadside and station breathalysers specifically rely on IR absorption spectrometry.
🧠 Exam Technique: The "Bond Energy" Rule
Always compare the energy of electromagnetic radiation to the chemical outcome:
- Infrared (IR): Lower energy. It can only cause covalent bonds to stretch, bend, and vibrate. It is never energetic enough to break bonds homolytically.
- Ultraviolet (UV): Much higher energy. It possesses sufficient energy to break covalent bonds and produce free radicals (photodissociation).
- Takeaway: Whenever free-radical initiation or bond breaking in the atmosphere is mentioned, think UV, not IR!
Topics
Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis
Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.