Chemistry of Natural Resources

Chapter 6 - Acid Rain

Chapter Overview:

Acids and Bases

acid - a compound that releases a hydrogen ion (H + , also called a proton)

when it dissolves in solution , usually water

e.g., HCl (g) + H 2 O (l) -----> H 3 O + (aq) + Cl - (aq)

note - HCl releases H + and transfers it to water (so HCl is an acid)

HCl is hydrochloric acid, the main form of acid in the stomach

a simpler equation is often written and easier to remember:

HCl -----> H + + Cl -

base - a compound that releases hydroxide ion (OH - ) in solution

e.g., NaOH (s) -----> Na + (aq) + OH - (aq)

acid-base neutralization - reaction of an acid with a base

H + + OH - -----> H 2 O

HCl + NaOH -----> NaCl + H 2 O

acid base salt water

water is itself a very weak acid and a very weak base

H 2 O -----> H + + OH - (but very little of this reaction occurs)

Solution Concentration

molarity (M) - the number of moles of a compound dissolved in one liter of solution

e.g., a 1 M solution of sodium chloride has 58.5 grams dissolved in 1 L of solution

(molar mass of NaCl is 23 (Na) + 35.5 (Cl) = 58.5 grams/mole)

sample calculation: medical saline solution is 0.9% NaCl

0.9% means 0.9 g NaCl / 100 g solution = 9 g/L = 9/58.5 moles/L = 0.15 M

pH

water always contains some H + (acid) and some OH - (base)

autoionization of water: H 2 O -----> H + + OH -

K w = [H + ] [OH - ] = 1 x 10 -14

neutral solution: [H + ] = [OH - ] = 1 x 10 -7 M

acidic solution: [H + ] > [OH - ] , e.g., [H + ] = 1 x 10 -5 M , [OH - ] = 1 x 10 -9 M

basic solution: [OH - ] > [H + ] , e.g., [OH - ] = 1 x 10 -1 M , [H + ] = 1 x 10 -13 M

pH = - log [H + ]

i.e., the exponent (almost always negative) on the concentration of H +

note that each pH unit corresponds to a 10 x change in acidity or basicity

(i.e., a logarithmic scale, like the Richter scale for earthquakes)

e.g., [H + ] = 1 x 10 -5 M , pH = 5 (acidic, because pH < 7)

[H + ] = 1 x 10 -13 M , pH = 13 (basic, because pH > 7)

pH of Rain

the natural pH of rain is slightly acidic (about 5.6)

when CO 2 dissolves in water, it generates a weak acid (carbonic acid)

CO 2 + H 2 O -----> H 2 CO 3 -----> H + + HCO 3 -

(Soda water has a much lower pH, about 1-2. Why?)

Acid Rain

in many locations around the U.S., the pH of rain is lower than 5.6 (see Fig. 6.6)

rain, snow, fog, and mists all collect air pollutants in water solutions

some pollutants deposit directly on surfaces (dry acid deposition)

the main factors are sulfur oxides (SO x ) and nitrogen oxides (NO x ):

SO 2 + H 2 O -----> H 2 SO 3 -----> H + + HSO 3 -

SO 2 + O 2 -----> SO 3

SO 3 + H 2 O -----> H 2 SO 4 (sulfuric acid)

NO 2 + OH -----> HNO 3 (nitric acid)

sulfur oxides and nitrogen oxides come from combustion processes:

coal contains some sulfur, which burns to SO 2

automobile catalytic converters convert some N 2 to NO x

conversion of ores to metals often evolves sulfur, which can oxidize to SO 2

(NiS -----> Ni + S , but some S contacts O 2 to form SO 2 )

Effects of Acid Deposition

material damage by acid deposition:

limestone slowly dissolves in acid

CaCO 3 + 2 H + -----> Ca +2 + H 2 CO 3 ( -----> CO 2 + H 2 O )

metals slowly corrode in acid, e.g., rusting of iron

Fe + 2 H + + 1/2 O 2 -----> Fe +2 + H 2 O

acidification of lakes and streams:

typical freshwater lakes and streams require pH around 6.5 for optimum habitat

lakes have some acid-neutralizing capacity (ANC)

some compounds act as buffers to destroy excess acid

e.g., bicarbonate (from limestone) HCO 3 - + H + -----> CO 2 + H 2 O

lakes without limestone available are more susceptible to acid rain

acid rain tends to dissolve aluminum from soils and wash it into lakes

some fish kills are attributed to excessive aluminum concentrations

dying forests:

still unclear what factors have contributed to unhealthy forests

Control of Acid Deposition

Clean Air Act (1990) mandates lower emissions of various pollutants

coal-burning power plants are the major source of SO x (see Table 6.2)

low-sulfur coal is becoming less available

coal cleaning can remove about half of sulfur in coal (but expensive)

scrubbing the combustion gases is most effective method (also expensive)

CaCO 3 + SO 2 + 1/2 O 2 -----> CaSO 4 + CO 2

pollution credits: industries are allowed a certain level of emissions and may sell to

others the rights to any amounts that they don't emit themselves

political / economic issues: who should pay for the controls