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• SCH4U — Grade 12 Chemistry (Ontario)
• General Chemistry I
  • 1. Stoichiometry
  • 2. Aqueous Reactions
  • 3. Chemical Equations
    • 3.1 Balancing Equations
    • 3.2 Synthesis Reactions
    • 3.3 Decomposition Reactions
    • 3.4 Combustion Reactions
  • 4. Thermochemistry
  • 5. Atomic Structure
  • 6. Electronic Structure & Periodic Trends
  • 7. Chemical Bonding & Molecular Geometry
  • 8. Gases
  • 9. Liquids, Solids & Intermolecular Forces
  • 10. Solutions & Colligative Properties
  • 11. Gases & Atmospheric Chemistry
• General Chemistry II
  • 1. Reaction Kinetics & Rate Laws
  • 2. Chemical Equilibrium
  • 3. Acids, Bases, and Buffers
  • 4. Electrochemistry & Redox Cells
  • 5. Thermodynamics
  • 6. Solubility & the Solubility Product
  • 7. Nuclear Chemistry
  • 8. Coordination Chemistry
• Organic Chemistry I
  • 1. Nomenclature & Functional Groups
  • 2. Stereochemistry & Isomerism
  • 3. Substitution and Elimination
  • 4. Structure & Bonding in Organic Molecules
  • 5. Alkenes
  • 6. Alkynes
  • 7. Alcohols & Ethers
  • 8. Spectroscopy
  • 9. Polymers
• Organic Chemistry II
  • 1. Aromatic Chemistry
  • 2. Aldehydes & Ketones
  • 3. Carboxylic Acids & Derivatives
  • 4. Enolates & the Aldol Reaction
  • 5. Amines
  • 6. Conjugated Systems & Diels-Alder
  • 7. Biomolecules
  • 8. Synthesis Strategy & Retrosynthesis

11. Gases & Atmospheric Chemistry

The atmosphere is a chemical system: 78% nitrogen, 21% oxygen, and a handful of trace gases that decide the climate, the ozone layer, and the air we breathe. This page applies gas chemistry to the air itself — the SCH3U "Gases and Atmospheric Chemistry" unit.

The Ozone Layer

Ozone forms in the stratosphere when UV light splits $\text{O}_2$ and the free oxygen atom joins another $\text{O}_2$: $$\text{O}_2 \xrightarrow{\text{UV}} 2\,\text{O} \qquad \text{O} + \text{O}_2 \rightarrow \text{O}_3$$ Ozone then absorbs the same UV, shielding the surface. Chlorofluorocarbons (CFCs) release chlorine radicals that catalytically destroy ozone — one Cl atom can destroy thousands of $\text{O}_3$ molecules before being removed.

Photochemical Smog

Sunlight drives reactions between nitrogen oxides ($\text{NO}_x$) and volatile organic compounds from vehicles, producing ground-level ozone and PAN — the brown haze over cities on hot, still days.

The Greenhouse Effect

$\text{CO}_2$, $\text{CH}_4$, and $\text{N}_2\text{O}$ absorb infrared radiation re-emitted by Earth and re-radiate it, warming the surface. The natural effect keeps Earth habitable; rising $\text{CO}_2$ from fossil-fuel combustion intensifies it. Methane is a stronger greenhouse gas per molecule than $\text{CO}_2$, though far less abundant.

Acid Rain

$\text{SO}_2$ and $\text{NO}_x$ from combustion oxidize in the atmosphere to sulfuric and nitric acids, which fall as rain with pH below 5.6, damaging lakes, forests, and stone buildings.

Practice Problems

1. Write the two-step sequence by which ozone forms in the stratosphere.
2. Why does one chlorine atom from a CFC destroy many ozone molecules?
3. Name three greenhouse gases and their main human sources.
4. Why is rain with pH 4.5 called acid rain? What are the two main acids involved?

Answers: 1. O₂ + UV → 2 O; O + O₂ → O₃   2. Chlorine acts as a catalyst — it is regenerated in the destruction cycle   3. CO₂ (fossil fuels), CH₄ (agriculture, natural gas), N₂O (fertilizers)   4. Natural rain is ~pH 5.6 from dissolved CO₂; below that, sulfuric and nitric acids dominate