A scientist is studying the chemistry of ozone in the stratosphere. Explain how ozone is both formed and broken down naturally in the stratosphere, and describe the role of ultraviolet radiation in these processes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ozone (O₃) exists in a layer of the stratosphere where it absorbs harmful ultraviolet radiation. The concentration of ozone in this layer is maintained by a natural cycle of formation and breakdown reactions.
Model answer (5 marks)
Ozone is formed when UV‑B radiation (λ≈ 200–280 nm) is absorbed by an O₂ molecule, causing it to dissociate into two free oxygen atoms:
1. O₂ + hν → 2 O
The free oxygen atom then reacts with another O₂ molecule, in the presence of a third body (M) to carry away excess energy, to give ozone:
2. O + O₂ + M → O₃ + M
Ozone itself absorbs UV‑C and UV‑B radiation. When it absorbs a photon it dissociates back into an O₂ molecule and a free oxygen atom:
3. O₃ + hν → O₂ + O
The free oxygen atom can again react with O₂ to reform O₃, completing a continuous, dynamic cycle that keeps the stratospheric ozone concentration roughly constant.
1. O₂ + hν → 2 O
The free oxygen atom then reacts with another O₂ molecule, in the presence of a third body (M) to carry away excess energy, to give ozone:
2. O + O₂ + M → O₃ + M
Ozone itself absorbs UV‑C and UV‑B radiation. When it absorbs a photon it dissociates back into an O₂ molecule and a free oxygen atom:
3. O₃ + hν → O₂ + O
The free oxygen atom can again react with O₂ to reform O₃, completing a continuous, dynamic cycle that keeps the stratospheric ozone concentration roughly constant.
Examiner tips
- Use the exact reaction equations and mention UV wavelengths; show the cycle is continuous; include the role of a third body in the formation step.
Common mistakes
- Omitting the third body (M) in the ozone formation reaction; confusing UV‑C with UV‑B wavelengths; failing to state that the cycle is continuous and keeps concentration constant.
Mark scheme (5 marks)
- Oxygen molecules (O₂) absorb ultraviolet radiation (UV) and split into individual oxygen atoms / free oxygen atoms (O)
- The free oxygen atom (O) reacts with an oxygen molecule (O₂) to form ozone (O₃)
- Ozone (O₃) absorbs ultraviolet radiation and breaks down / is destroyed
- The breakdown of ozone produces an oxygen molecule (O₂) and a free oxygen atom (O) / the oxygen atom can reform ozone
- The formation and breakdown of ozone is a continuous / dynamic cycle that keeps ozone concentration roughly constant / ozone is constantly being made and destroyed naturally
Key terms in this question
ozone · ultraviolet radiation · stratosphere
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More The ozone story questions
- A manufacturer of fire extinguishers previously used halon gases in their produc…
- A student is researching the history of ozone depletion and the international re…
- Ultraviolet (UV) radiation from the Sun reaches Earth's atmosphere. The ozone la…
- Chlorofluorocarbons (CFCs) were once widely used in aerosol sprays and refrigera…