Carbon dioxide and global warming: the greenhouse effect
2 marks · HardExplain why carbon dioxide, but not nitrogen, can act as a greenhouse gas.
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Also accept: CO2 is able to absorb IR radiation due to a change in polarity when it vibrates; Nitrogen does not have changing polarity when vibrating, so it does not absorb IR
Common mistakes: Stating both are non-polar but not explaining vibrational changes; Confusing greenhouse gases with general pollutant gases
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Marking points
B1 · 1 mark · Explains that CO2 can absorb infrared radiation because its polarity changes as it vibrates
B2 · 1 mark · Explains that N2 molecules do not change polarity as they vibrate and so cannot absorb IR radiation
Worked solution
Carbon dioxide can act as a greenhouse gas because, although it is a non-polar molecule, its vibrations cause a change in polarity, allowing it to absorb infrared radiation. Nitrogen cannot act as a greenhouse gas because its vibrations do not produce a change in polarity, so it cannot absorb infrared radiation.