October 17, 2001

Acid Rain (Chapter 6)

Quick Overview

• (much has been covered already)
• acids-- what are they
  • why be concerned?
• bases and neutralization
• pH -- a way to measure acidity
• acid rain-- what makes rain acidic?
  • NOx and SOx (and CO₂)
  • is this a new or enhanced problem?
• what can/should be done
• what has been done? (how effective?)

Acid-- A Class of Chemical Compounds

• taste-- tart (citrus, vinegar)
• color reactions --
  • with acid/base indicators (litmus)
• react with bicarbonate-- bubbles of CO₂
• may react with metals
  • (depends on strength and concentration)
• Working definition of an acid
• compound that produces H+ in water
• (H+ actually reacts with water = H₃O⁺)
• simplest cases-- the H comes from the compound
  • HCl H₂SO₄ HNO₃
  • hydrochloric, sulfuric and nitric acids
  • HCl (aq) ----100% -----> H+ (aq) +Cl- (aq)
    • aq = dissolved in water
  • we say HCl is a strong acid (100%)
    • if very dilute, we still say "strong"
• CO₂ is an acid also, but has no H
  • CO₂ (aq) + H₂O------ few % -----> HCO₃- + H⁺
  • water is both solvent and reactant
  • weak acid , few % produces H+

H⁺ + H₂O ----> H₃O⁺ hydronium ion

• we may say either hydronium or hydrogen ion
• role of water is implied
• (we won't discuss harder cases without water)

Probably need to discuss bases now

• word "base" has many meanings
• here we really mean alkaline material,
  • opposite of an acid
  • taste--
  • slippery feel (dissolved skin protein perhaps)
  • color reactions with indicators
  • reacts with acids

• working definition of a base --
  • a species that produces OH^- ions in water
  • again, the OH- might be already there
    • NaOH, Ca(OH)₂
  • might come from reaction with water
    • NH₃ + H₂O ----> NH₄+ + OH-
  • some strong
  • NaOH (ionic solid) -----100% ---> Na+ (aq) + OH-(aq)
• some weak
  • NH₃ + H₂O ---few % ---> NH₄+ + OH-

Neutralization:

• H⁺ and OH- can't coexist
• react H⁺ + OH^- ----nearly 100%--> H₂O
• so solution could be acidic or basic but not both
• could also be neutral (if neither acidic or basic)
• if solution has OH^- ions it has much less H⁺
• [OH-] x [H+] = 1 x10^-14

• H+ concentration ---
  • pure water, 1x10^-7 Molar (quite tiny)
  • vinegar 1 x 10^-4 Molar
  • stomach acid 0.1-0.5 Molar
  • lab bottle of HCl 6 - 12 Molar

• wide range... like to avoid 0 .000 000 25
  • prefer to think.... nearly 10^-7
  • talk about log₁₀ H⁺ (logarithm, see book)
    • pH 0 is 100 = 1.0 Molar
    • pH 5 is 10^-5 Molar = 0.000 01 M
    • pH 8 is 10^-8 Molar = 0.000 000 01 M
    • (less H+ than pure water?) (must mean [OH-] = 10^-6M ; a weak base)

Plants live, proper from pH 5 to pH 8

• varies,
• blueberries, azaleas, pine favor pH 4.5- 6.5
• tomato and maple have problems at pH 4.5-6
• Some waterways (bogs, swamps)
  • may be quite acidic
  • may limit both plant and animal life

Now, Acid Rain

• Culprits-- limited to things in atmosphere
  • limited to acids
  • CO₂ already mentioned
  • SO₂ and SO₃
  • NO, NO₂, NOx
• rain dissolves gases as it falls to earth
  • certainly not saturated solutions
  • stronger if gas concentration is higher
• water ("pure") is pH 7
  • simple rain about pH 5-6
  • mostly dissolved carbon dioxide
• acid rain is perhaps pH 2-4
  • can damage leaves
  • can slowly dissolve marble and limestone
  • can speed up rusting of steel
• on the ground, two possible cases
  • Midwest, limestone, reacts with acid
    • neutralizes much of the acid
  • regions with volcanic soils
    • mountains in Rockies, NY-NH-Maine
    • nothing to neutralize
    • evaporation of water, concentrated acids

Sources of SO₂ and SO₃

• SO₂ from burning S rich fuels
• SO₂ from metal refining, smelting
  • world wide-- 50% not man made
• SO₃ from SO₂ gas
  • 2SO₂ + O₂ ---(ash, droplet, sunlight)---->2 SO₃
  • SO₂ + H₂O ---> H₂SO₃ --few%---> H+ + HSO₃-
  • SO₃ + H₂O ---> H₂SO₄ ---nearly 100%---> 2 H⁺ + SO₄2-

• where?
  • down wind of SO₂ sources
  • several 100 miles
  • generally to the east in US
    • often Northeast US and Canada
    • sometimes Southeast (they get Texas SO₂ )

• How to avoid?
  • Reduce SO₂ production
  • Capture SO₂ where produced
  • (NIMBY-- taller smokestacks)
• Remediation?
  • add limestone to affected water (lakes)
  • costly, only affects lake itself

• NOx Quite Similar
  • Different sources
  • Somewhat different approach

Regional Politics

• high S vs low S coal
  • where is it mined?
• what does it cost
• what are the alternatives
  • beyond reduced consumption
• my SO₂ doesn't affect my rain
• acid rain is not the sole cause of
  • dying forests
  • deteriorating high altitude fauna/flora

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