Environmental Chemistry

Learning Objectives

Describe the chemical reactions involved in the formation of acid rain.

Explain the catalytic cycle of ozone depletion by chlorofluorocarbons (CFCs).

Describe the basic steps of municipal water treatment.

Chemistry and the Environment

Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It is a cross-disciplinary science that applies chemical principles to understanding and solving environmental problems.

Acid Rain

Acid rain is precipitation with a pH lower than about 5.6. It is caused by the emission of sulfur oxides (SOx) and nitrogen oxides (NOx) into the atmosphere.

Sulfur Source: Primarily from burning sulfur-containing coal.

S (in coal) + O₂ → SO₂(g)

2SO₂(g) + O₂(g) → 2SO₃(g)

SO₃(g) + H₂O(l) → H₂SO₄(aq) (Sulfuric acid)

Nitrogen Source: Primarily from high-temperature combustion in vehicle engines.

N₂(g) + O₂(g) → 2NO(g)

2NO(g) + O₂(g) → 2NO₂(g)

2NO₂(g) + H₂O(l) → HNO₃(aq) (Nitric acid) + HNO₂(aq)

Stratospheric Ozone Depletion

The stratospheric ozone layer protects life on Earth by absorbing harmful UV-B radiation.

Cause: Chlorofluorocarbons (CFCs), once used as refrigerants and propellants.

Catalytic Cycle:

1.UV radiation breaks a chlorine atom off a CFC molecule: CCl₃F → •CCl₂F + •Cl

2.The free chlorine radical (•Cl) acts as a catalyst to destroy ozone (O₃).

3.•Cl + O₃ → ClO• + O₂

4.ClO• + O → •Cl + O₂

Net Result: O₃ + O → 2O₂. The chlorine radical is regenerated in step 4, allowing a single chlorine atom to destroy thousands of ozone molecules.

Water Treatment

Municipal water treatment makes water potable (safe to drink) through several steps:

1.Coagulation and Flocculation: Chemicals (like alum) are added to the water, causing small dirt particles to clump together into larger particles called 'floc'.

2.Sedimentation: The heavy floc particles settle to the bottom of the tank.

3.Filtration: The clear water on top passes through filters of varying composition (sand, gravel, charcoal) to remove dissolved particles, such as dust, parasites, bacteria, viruses, and chemicals.

4.Disinfection: A disinfectant (usually chlorine) is added to kill any remaining bacteria or viruses.

Key Terms

Acid Rain: Rain or any other form of precipitation that is unusually acidic, caused by atmospheric pollution from sulfur and nitrogen oxides.
Ozone Layer: A layer in the earth's stratosphere at an altitude of about 10–50 km that contains a high concentration of ozone, which absorbs most of the ultraviolet radiation reaching the earth from the sun.
Chlorofluorocarbons (CFCs): A class of compounds of carbon, hydrogen, chlorine, and fluorine, which are harmful to the ozone layer.
Potable Water: Water that is safe to drink or to use for food preparation.
Coagulation: A process in water treatment where chemicals are added to cause small, suspended particles to clump together into larger aggregates (floc).