Acid Rain
Atmospheric and Environmental Chemistry
- Air quality
Localized (in cities)
- Ozone Depletion
Global
- Greenhouse Effect
Global
- Acid Rain
Regional
Acid deposition:
- acid rain
- acid snow
- fog
- dry deposition (acidic solids)
Acids and Bases
ACID:
Ex:
Substance that releases hydrogen ions, H+,
in aqueous (aq) solution
HCl (aq)
hydrogen
chloride
H+ (aq) + Cl- (aq)
hydrogen
ion
chloride
ion
More rigorously:
HCl (aq) + H2O (l)
H3O+ (aq)
hydronium
ion
+ Cl- (aq)
Acids and Bases
BASE:
Substance that accepts hydrogen ions (and
produces hydroxide ions, OH-, in aqueous
solution)
NH3 (aq) + H2O (aq)
ammonia
hydroxide
Ex:
NH4+ (aq) + OH- (aq)
ammonium
ion
NaOH (aq)
sodium
hydroxide
ion
Na+ (aq) + OH- (aq)
sodium
ion
Neutralization: Reaction between and acid and a base to
form a salt and water
HCl (aq) + NaOH (aq)
NaCl (aq) + HOH (l)
Mass, Mole, and Avogadro
6
8
Carbon
Oxygen
C
O
12.011
15.999
Atomic mass: mass, in grams,
of the same number of atoms
that are found in exactly 12
grams of carbon-12
Avogadro’s number: number of atoms in exactly 12 g of
carbon-12 (NA= 6.02 x 1023)
Mole: an Avogadro’s number of anything
Molar mass: the mass, in grams, of one Avogadro’s number
of whatever particles are specified
Molar mass CO2 = Atomic mass C + 2 x Atomic mass O
= 12.011 + 2 x 15.999 = 44.009 g mol-1
Concentration and pH
Concentration: amount of a substance dissolved in a given
volume of a solvent
Molarity (M): number of moles of a substance present in one
liter of solution
In pure water, the molarities of H+ and OH- are equal:
MH+ = MOH- = 0.0000001 M = 1.0 x 10-7 M
The addition of an acid or a base to water, changes these
concentrations.
In acidic solutions,
In a neutral solution,
In basic solutions,
MH+ > 1.0 x 10-7 M
MH+ = 1.0 x 10-7 M
MH+ < 1.0 x 10-7 M
H+ molarities can range from more than 1.0 mol/L
to less than 0.00000000000001 moles/L
It’s more convenient to use a logarithmic scale
pH = -log MH+
If MH+ > 10-7 M
pH < 7.00
Acidic solution
If MH+ = 10-7 M
pH = 7.00
Neutral solution
If MH+ < 10-7 M
pH > 7.00
Basic solution
Ex: What is the pH of a 2.54 x 10-4 M solution of HCl?
pH = - log (2.54 x 10-4)
pH = 3.60
What is the pH of rain?
What is the pH of rain?
What is the pH of rain?
Pure rain
pH~5.5
Why is pure rain acidic?
carbon dioxide
CO2 ( g ) + H 2O(l ) " H (aq ) + HCO (aq )
+
!
3
acidic
Rain in some areas has a pH of 5.0 and below...
What is the origin of the extra acidity?
What is the origin of the extra acidity?
• Where is rain abnormally acidic?
• What does acidic rain contain? (besides H2O and CO2)
• What’s the origin of these other contaminants?
Where is rain abnormally acidic?
• Low pH values in heavily industrialized areas (Northeast and
Ohio River Valley).
What does acidic rain contain? (besides H2O and CO2)
• Analysis of acid rain shows presence of nitrogen and sulfur
oxides.
What’s the origin of these other contaminants?
• Emissions of sulfur dioxide (SO2) are highest in regions with many
coal-fired electric power plants, steel mills, etc. (Allegheny County,
PA : highest SO2 emissions)
• Nitrogen dioxide (NO2) emissions found in large urban areas, with
high population, and heavy automobile traffic. (Los Angeles, CA :
highest NO2 emissions)
NOx Emissions
SO2 Emissions
How do sulfur and nitrogen oxides affect the acidity of rain?
SO2 (g) + H2O (l)
H2SO3 (aq)
sulfurous acid
H+ (aq) + HSO3- (aq)
SO3 (g) + H2O (l)
H2SO4 (aq)
sulfuric acid
2H+ (aq) + SO42- (aq)
NO2 (g) + 2 H2O (l) + O2 (g)
4 HNO3 (aq)
nitric acid
4H+(aq) + 4NO3-(aq)
Where do sulfur oxides come from?
SO2
and
SO3
from combustion of coal
Coal is mostly carbon, but it also contains
hydrogen, oxygen, nitrogen, sulfur, and small
amounts of silicon, sodium, calcium, etc.
Burning coal: C(in coal) + O2 (g)
CO2 (g)
S(in coal) + O2 (g)
Once in the atmosphere:
2 SO2 (g) + O2 (g)
SO2 (g)
2 SO3 (g)
Review
ACID: Substance that releases hydrogen ions, H+, in aqueous (aq)
solution
BASE: Substance that accepts hydrogen ions (and produces hydroxide
ions, OH-, in aqueous solution)
Neutralization: Reaction between and acid and a base to form a salt
and water
HCl (aq) + NaOH (aq)
NaCl (aq) + HOH (l)
Molarity (M): number of moles of a substance present in one liter of
solution
pH = -log MH+
If MH+ > 10-7 M
pH < 7.00
Acidic solution
If MH+ = 10-7 M
pH = 7.00
Neutral solution
If MH+ < 10-7 M
pH > 7.00
Basic solution