Chemistry in Context: Chapter 6:
Neutralizing The Threat of Acid Rain
Assignment: All the problems with blue
codes or answers on Page 529-530.
What is an acid?
Sour taste; found in vinegar, citrus
fruits, and some candies.
„ Changes litmus from blue to red.
„ Reacts with carbonates; dissolves
egg shells and limestone.
„ HF, HCl, H2SO4, HNO3 are
common acids.
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An Acid is…
Any substance that generates
hydrogen ions, H+, in aqueous
(H2O) solution.
„ HCl (g) → H+(aq) + Cl-(aq)
„ H+ + H2O → H3O+ (solvated
protons give hydronium ions).
„ HCl + H2O → H3O+ + Cl-(aq)
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What is a base?
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Bitter taste; ∴not used in food.
Slippery to touch; soapy feel.
Changes litmus from red to blue
Strong base can cause severe damage to
eyes, skin, & clothing.
Aqueous solutions of ammonia (NH3)
and sodium hydroxide (NaOH).
A base is…
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Any substance that produces hydroxide
ions (OH-) in aqueous solution.
NaOH(s) → Na+(aq) + OH-(aq)
Ca(OH)2(s) → Ca2+(aq) + 2 OH-(aq)
NH3 + H2O → NH4+(aq) + OH-(aq)
Acidic or Basic or Neutral?
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[H+]•[OH-] = 1 x 10-14 ; [ ] = Molarity
In an acidic solution;
[H+] > [OH-] or [H+] >10 –7 M
In a basic solution;
[H+] < [OH- ] or [H+] <10 –7 M
Neutral solution; H+ = OH- = 10 –7 M
Neutralization
Acid + Base → Salt + H2O
„ HCl (aq) + NaOH (aq) →
NaCl (aq) + H2O
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Net reaction: H+ + OH- → H2O
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In a neutralization reaction, the H+
from the acid combine with the OHfrom the base to form water.
pH Scale for Acidity or Basicity
„Numerical
scale between 0 and 14
„As the pH value increases, acidity decreases.
Acid Rain
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Fig. 6.8
Figure 6.8 page 252
National Acid Precipitation Assessment Program
(1980) created by Congress.
Clean Air Act Phase I (1995) – reduction of acid rain
precursors by power plants.
Acid deposition via rain, snow, fog & mist is a
regional issue (note: precipitation = rain + snow).
pH data collected weekly at >225 sites since 1970;
more sites located in Northeast U. S.
Pollutant transport from Midwest (Ohio River valley)
to Northeast and Southeast; from Germany, Poland,
U. K. to Sweden & Norway.
Figure 6.9 Page 254
Midwestern states
tend to have high
levels of SO2 and NOx
emissions that are
transported to the
Northeastern part of
the U.S. by winds and
thus result in acid
deposition there.
“Extra” Acidity in Rain
Importance of CO2 in Affecting pH
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Atmospheric CO2 is about 370 ppm.
CO2(g) + H2O(l) → “H2CO3”
→ H+(aq) + HCO3-(aq)
∴ CO2 is the source of slight acidity in
normal rainfall (i.e. pH of 5.6).
HCO3- in blood provides a pH buffer that
keeps the blood pH at ~7.2.
CO2 is used in beverage industry for
producing soda drinks with pH at ~ 4.5.
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Sulfur oxides, SOx and nitrogen oxides, NOx
(acid anhydrides).
Acid rain occurs in regions with the most SOx
and NOx emissions.
SOx is emitted from coal-fired power plants,
steel mills, and other coal-burning industries
(e.g. Allegheny County, PA).
NOx is found in urban areas with high population
densities and heavy automobile traffic (e.g. Los
Angeles, Houston).
Page 257
The world’s largest
smelter converts
nickel sulfide to
nickel with the
emission of SO2; the
smokestack of 1250
feet is the tallest in
the world.
•SO2(g) + H2O(l) → H2SO3(aq) → 2 H+(aq) + SO32-(aq)
•SO3(g) + H2O(l) → H2SO4(aq) → 2 H+(aq) + SO42-(aq)
•The production of SO3 is slow but is catalyzed by
fly ash; 20-25% of H2SO4 is produced by SO2 + •OH.
U. S. Sulfur Dioxide Emission in 1998
Figure 6.11
page 256
U. S. Nitrogen Oxide Emission in 1998
Figure 6.12
page 258
Figure 6.13 Page 259
Fig. 6.13
U. S. Nitrogen Oxide Emission 1940-1998
Table 6.1 page 261
Fig. 6.22 on Page 273
A low NOx burner at an
electrical utility plant
Table 6.2
Table 6.3
Fig. 6.16
Figures 6.15 on page 263: corrosion of
limestone statues due to acid rain.
CaCO3(s) + 2 H+(aq) → Ca2+(aq) + CO2(g) + H2O (l)
Page 265-Loss of scenic vistas due to haze at
national parks may impact tourist industry; haze is
composed of aerosol particles containing sulfuric
acid, ammonium sulfate, and ammonium
hydrogen sulfate.
Page 267
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Healthy lakes pH = 6.5
pH < 6 = trouble
pH < 4 = dead lake
Acidity of Lakes
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Midwestern lakes have limestone (CaCO3) to neutralize
acids and hence high acid neutralizing capacity (ANC) or
ability to resist pH change; but Northeastern lakes are
surrounded by granite with little ANC.
Granite releases Al3+ in acidic water to form mucus in
fish gills or worsen the acidity by:
Al3+(aq) + H2O(l) → H+(aq) + [Al(OH)]2+(aq)
Adirondack lakes and mid-Appalachian streams have
reached N saturation and are susceptible to episodic and
chronic acidification.
Unhealthy Forests
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Big problem in Europe; fir and spruce trees at high altitudes;
loss of leaves and needles; weakened trees killed by drought,
cold, wind, and insects.
North American Sugar Maple Decline Project -10 states & 4
provinces in New England states and eastern Canada; decline
of red spruce in Appalachian
Acid precipitation is a contributing factor besides pest and
ozone; tree ring & surface water acidity studies.
Ozone and nitrogen oxides attack the waxy coating on
leaves, allowing H+ to deplete nutrients; soil acidification
mobilizes metals that attack tree roots by preventing nutrient
uptake and leaches out key nutrients.
Page 269 –Liming lakes to
neutralize excessive acidity
involves the addition of
calcium hydroxide or lime.
Page 270
What can be done?
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Limit SOx emissions from industries; restriction of
grandfather clause; emission credit trading.
Clean Coal Technology; low NOx burner, “reburning”,
coal switching (anthracite), coal washing with water, and
scrubbing acidic gases with powdered limestone (2 SO2 +
O2 + CaCO3 → CaSO4 + 2 CO2)
60% of SO2 reduction in the U.S. comes from switching
to coal from Western states whereas Japan lowers SO2
production from 7 to 1 g/kWh via desulfurization
technology.
Public transportation and electric cars