E2: Acid Deposition E.2.1 Acid Deposition….huh?

Ohh, acid rain! Acid deposition, or acid rain as it’s commonly called, refers to the acidic particles and gasses that fall to earth. This includes both wet deposition of acidic gases and acidic particles brought down as precipitation by rain.

But….how? Rain is naturally acidic due to dissolved carbon dioxide, which reacts to form carbonic acid, a weak acid. CO2(g) + H2O ⇌ H2CO3(aq) H2CO3(aq) ⇌ H+(aq) + HCO3-(aq)

When should we worry? Well, you should worry about acid rain no matter what the pH because any decrease from normal results in an increase in erosion, plant decay, etc. To give you an example, rain with a pH of 4.0 is sufficient to kill fish life.

How does acid rain happen? Acid rain can be formed in two ways, from oxides of sulfur, or from oxides of nitrogen.

From oxides of sulfur Sulfur dioxide combines with the moisture in the air to form sulfurous acid, a weak acid: SO2(g) + H2O(aq)  H2SO3(aq) Also, sulfur dioxide can be oxidized in the presence of O2, O3, ·OH with sunlight to yield sulfur trioxide, which produces sulfuric acid, a strong acid in the presence of moisture. SO3(g) + H2O(l)  H2SO4(aq)

From oxides of nitrogen Nitrogen dioxide can combine with moisture in the air to form nitrous acid, a weak acid, and nitric acid, a strong acid. When in the presence of oxygen and moisture, nitrogen dioxide can form nitric acid.

E11: the HL Acid Deposition In the HL expansion of this topic, the mechanism of acid deposition caused by the oxides of nitrogen and the oxides of sulfur are explained. As well as the role of ammonia in acid deposition.

Mechanism caused by oxides of nitrogen First, nitrogen dioxide photo-dissociates into nitrogen monoxide and oxygen free radicals NO2 + uv light  NO + ·O The free radicals initiate a series of important reactions to produce secondary pollutants, like ozone and hydroxyl free radicals. Then the hydroxyl free radicals react with the nitrogen monoxide, and the nitrogen dioxide to produce nitrous acid and nitric acid respectively.

Mechanism caused by oxides of sulfur With sulfur, the hydroxyl free radicals react with sulfur dioxide to produce HSO3·. This compound reacts with oxygen to produce the anhydride of sulfuric acid which quickly reacts with the moisture in the air to produce sulfuric acid, lowering the pH of the rain.

Ammonia….really…? Ammonia can actually reduce acid rain!! YAY! It neutralizes the strong acids into weak ones. This is possible due to the lone pair of electrons on the top of the ammonia molecule allowing it to bond with sulfuric acid and nitric acid, forming ammonium salts, ammonium sulfate and ammonium nitrate respectively. These salts readily react with the moisture in the air releasing a gaseous proton, in the form of H3O+.