Carbonic Acid
Carbon dioxide in the atmosphere dissolves to a slight extent in water and reacts with it to produce a slightly acidic solution of carbonic acid:
CO2 (g) + H2O(l) ————–> H2CO3 (aq) carbonic acid
H2CO3 (aq)—————-> H+ (aq) + HCO3 – (aq)
The carbonic acid dissociates slightly leading to rain with a pH around 5.3.
Sulfuric Acid
This is what happens when coal is burned. Coal contains 1-6% sulfur.
S(s) + O2 (g) ——-> SO2 (g)
2 SO2 (g) + O2 (g) ——-> 2 SO3 (g)
Burning of sulfur with oxygen produces sulfur dioxide gas, which is poisonous.
S(s) + O2 (g) ———-> SO2 (g)
Once in the air, the SO2 can react with oxygen molecules to form sulfur trioxide, which acts in the formation of aerosols.
2 SO2 (g) + O2 (g) ——-> 2 SO3 (g)
Sulfur dioxide emissions are highest in regions with many coal-fired electric power plants, steel mills, and other heavy industries that rely on coal.
SOx react with water to form acids:
SO2 (g) + H2O(l) —–> H2SO3 (aq) sulfurous acid
SO3 (g) + H2O(l) —–> H2SO4 (aq) sulfuric acid
Nitric Acid
The main contributions of NO, which give rise to NO2 that cause the acid rain is by automobiles. Due to high temperatures in the car engine, Nitrogen gas and Oxygen gas reacts by the following equation:
N2 (g) + O2 (g) —–> 2NO(g) at high temp
Once formed, NO is highly reactive and reacts with hydroxyl radical , oxygen and volatile organic compounds (VOC) to form NO2.
VOC + OH · (radical) —–> A + O2 —–> A’ + NO —–> A” + NO2
A, A’ and A’’ are reactive intermediate species present in trace amounts. The production of acid rain requires trace amounts of VOC in the atmosphere. The nitric acid that is formed contributes to acid rain.
Overall: 4 NO2 (g) + 2 H2O(l) + O2 (g) —–> 4 HNO3 (aq) nitric acid
Like sulfuric acid, nitric acid also dissociates to release the H+ ion:
HNO3 (aq) —–> H+ (aq) + NO3 – (aq)