Ch5: Acid Rain and its Effects

Natural Acidity of Rain

  • Rainwater is inherently acidic.
    • Atmospheric CO2\text{CO}_2 dissolves in raindrops to form carbonic acid.
    • CO<em>2+H</em>2O⇌H<em>2CO</em>3\text{CO}<em>2 + \text{H}</em>2\text{O} \rightleftharpoons \text{H}<em>2\text{CO}</em>3
    • Carbonic acid partially dissociates, lowering pH slightly ("good" or expected acidity).

Extra Acidity (Acid Rain)

  • Modern rain is often more acidic than natural levels.
  • Primary culprits for additional acidity:
    • Nitrogen oxides (NO, NO2\text{NO}_2) — largely from vehicle exhaust and some industrial sources.
    • Sulfur oxides (SO$x$ = SO</em>2\text{SO}</em>2, SO3\text{SO}_3) — mostly from coal-burning power plants and other factories.
  • Common student misconception: CFCs harm the ozone layer but do not directly acidify rain.
  • Geographical patterns (maps discussed):
    • Highest acidity (red areas) clustered in U.S. East Coast & Ohio Valley where heavy industry and coal power dominate.
    • Smaller but noticeable hotspot around Los Angeles due to intense automobile traffic (high NO$_x$ emissions).

Emission Sources & Their Distribution

  • Coal-fired power plants: concentrated in the East & Midwest.
  • Natural-gas power plants: also more numerous in the East.
  • Petroleum-fired plants: relatively common in East & Midwest.
  • West Coast has comparatively few fossil-fuel plants → generally higher pH rain.

Atmospheric Chemistry Producing Strong Acids

  • Sulfuric acid pathway:
    • SO<em>2+12 O</em>2→SO3\text{SO}<em>2 + \tfrac12\,\text{O}</em>2 \rightarrow \text{SO}_3 (oxidation)
    • SO<em>3+H</em>2O→H<em>2SO</em>4\text{SO}<em>3 + \text{H}</em>2\text{O} \rightarrow \text{H}<em>2\text{SO}</em>4
    • H<em>2SO</em>4→2 H++SO42−\text{H}<em>2\text{SO}</em>4 \rightarrow 2\,\text{H}^+ + \text{SO}_4^{2-} (acidic dissociation)
  • Nitric acid pathway:
    • 2 NO<em>2+H</em>2O→HNO<em>2+HNO</em>32\,\text{NO}<em>2 + \text{H}</em>2\text{O} \rightarrow \text{HNO}<em>2 + \text{HNO}</em>3
    • HNO<em>3→H++NO</em>3−\text{HNO}<em>3 \rightarrow \text{H}^+ + \text{NO}</em>3^-
    • HNO<em>2→H++NO</em>2−\text{HNO}<em>2 \rightarrow \text{H}^+ + \text{NO}</em>2^-
  • Key takeaway: extra H+\text{H}^+ generation drives pH downward.

Human Accountability

  • Factories (especially coal facilities) and automobiles are main emitters.
  • Individuals & society contribute through energy consumption and transportation choices.

Ocean Chemistry & Acidification

  • Healthy oceans are slightly basic due to two weak-base ions:
    • Bicarbonate (HCO3−)\left(\text{HCO}_3^-\right)
    • Carbonate (CO32−)\left(\text{CO}_3^{2-}\right)
  • Simplified buffering reaction (natural state):
    • Ca2++2 HCO<em>3−→CaCO</em>3+CO<em>2+H</em>2O\text{Ca}^{2+} + 2\,\text{HCO}<em>3^- \rightarrow \text{CaCO}</em>3 + \text{CO}<em>2 + \text{H}</em>2\text{O}
    • Calcium carbonate (shell/skeleton material) forms.
  • Rising atmospheric CO2\text{CO}_2 (greenhouse gas) dissolves in ocean water, shifting equilibrium backward (Le Chatelier principle):
    • CaCO<em>3+CO</em>2+H<em>2O→Ca2++2 HCO</em>3−\text{CaCO}<em>3 + \text{CO}</em>2 + \text{H}<em>2\text{O} \rightarrow \text{Ca}^{2+} + 2\,\text{HCO}</em>3^-
    • Result: shells & coral dissolve → harms marine life.

Lakes & Rivers: pH Variability

  • Map shows greatest acidification in Northeast U.S.
    • Midwest streams contain abundant limestone (CaCO$_3$) — acts as natural neutralizer.
    • New England waters lack this buffering, so pH drops more sharply with acidic rain.
  • Biological thresholds:
    • pH=6.5−9.5\text{pH} = 6.5{-}9.5 → normal aquatic life survives.
    • \text{pH} < 5 → majority of fish & invertebrates die.
    • Lower pH drives ecosystem collapse.

Cultural & Structural Damage

  • Acid rain erodes limestone & marble statues/buildings (mainly CaCO$_3$):
    • CaCO<em>3+2 H+→Ca2++CO</em>2+H2O\text{CaCO}<em>3 + 2\,\text{H}^+ \rightarrow \text{Ca}^{2+} + \text{CO}</em>2 + \text{H}_2\text{O}
  • Visual outcome: formerly beautiful monuments become pitted, rough, and lose detail.

Ethical & Practical Implications

  • Environmental stewardship: industries, policy makers, and individuals share responsibility to curb SO$x$ & NO$x$ emissions.
  • Cleaner energy (renewables, scrubbers), fuel efficiency, and public transit reduce acid rain precursors.
  • Protecting aquatic life, cultural heritage, and human health depends on addressing root causes.

Connections to Future Topics

  • Chapter 7 will revisit power-generation data and energy policy.
  • Chemical equilibrium concepts (Chem 2) will quantify how shifting concentrations drive reactions (e.g., ocean buffering system).
  • Acid–base strength, dissociation constants, and titration curves (upcoming lectures) elaborate on why sulfuric & nitric acids are strong.