AQA A-Level Chemistry AS Paper 2, June 2025: Question 17
1 mark · Easy difficulty · Multiple Choice
Identify which gas is not linked to global warming caused by the absorption of infrared radiation.
Practise this questionQuestion
Question text
17 Which of these gases is not linked to global warming caused by the absorption of
infrared radiation?
[1 mark]
A CO2
B C3H8
C H2O
D N2
Mark scheme
Show the mark scheme
17 D 1 (AO1) N2
How to answer it
Greenhouse Gases & Infrared Radiation Absorption
This question assesses your understanding of the mechanism of global warming and infrared (IR) spectroscopy principles from AQA A-Level Chemistry Section 3.3.6 (Organic Analysis):
- Understanding how greenhouse gases absorb infrared radiation re-emitted by the Earth's surface.
- Knowing the molecular criteria for IR absorption: bonds must undergo a change in dipole moment during vibration.
- Distinguishing between greenhouse-active molecules (polar or polarisable asymmetrical vibrators) and non-absorbing homonuclear diatomic gases (e.g. N₂, O₂).
Question 17 Analysis
Multiple Choice — 1 Mark (AO1)
✅ Correct Answer
D — N₂
Nitrogen (N₂) is a homonuclear diatomic molecule. Because both atoms have identical electronegativities, the triple bond has no dipole moment, and stretching the bond produces no change in dipole moment. Therefore, N₂ cannot absorb infrared radiation.
💡 Key Knowledge
- The Greenhouse Mechanism: The Earth absorbs short-wavelength UV/visible radiation from the Sun and re-emits it as longer-wavelength infrared radiation.
- Why do bonds absorb IR? Infrared photons excite covalent bonds into higher vibrational energy states (stretching and bending). This absorption can only happen if the vibration causes a change in the dipole moment of the molecule.
- IR Inactive: Homonuclear diatomics like N₂ and O₂ cannot change their dipole moment when vibrating.
- IR Active: Molecules like CO₂, H₂O, and hydrocarbons (such as propane, C₃H₈) possess bonds (C=O, O–H, C–H, C–C) whose vibrations alter dipole moments, making them absorb IR strongly.
🧠 Exam Technique & Logic
- Watch the negative: The question asks which gas is NOT linked to global warming. Highlighting negative keywords prevents giving an answer for the opposite question.
- Deductive atmospheric reasoning: N₂ makes up roughly 78% of Earth's atmosphere and O₂ makes up 21%. If either gas absorbed IR radiation effectively, the greenhouse effect would be so intense that life on Earth could not survive.
- Evaluate each option:
- CO₂: Well-known major greenhouse gas (absorbs IR via asymmetric C=O stretch and bends).
- C₃H₈: Alkane; C–H bonds absorb IR strongly (~2850–3000 cm⁻¹). Hydrocarbons are potent greenhouse gases.
- H₂O: Water vapour is actually the single largest contributor to Earth's natural greenhouse effect.
❌ Common Errors & Traps
- Confusing H₂O as "harmless": Many students incorrectly pick H₂O, thinking that because water is naturally occurring and non-toxic, it cannot be a greenhouse gas. Water vapour is a powerful absorber of IR.
- Overlooking unburnt hydrocarbons: Students often recognize methane (CH₄) as a greenhouse gas but fail to realise that other alkanes like propane (C₃H₈) absorb IR in the exact same manner.
- Rushing past the prompt: Picking A (CO₂) purely because it is the most famous greenhouse gas, having missed the word "not" in the stem.
Topics
Organic Chemistry · 3.3.6 Organic Analysis
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.