Acid Rain

Rain is indespensible for life on Earth.Unpolluted rain is normally slightly acidic, with a pH of 5.6. Carbon dioxide (CO₂) from the atmosphere dissolves   in water to form carbonic acid.

However , pollutants  like sulfur and nitrogen oxides   combine  with the rain, the acidity of rain  increases greatly. These gases are oxidized in the air before been  captured by raindrops to form nitric and sulfuric acids  .  We commonly know this as acid rain, but we can have acidic snow or hail and even acidic dust particles falling from the sky.

  

Acid rain is a human-related phenomena. Since our industries are so fond of burning fossil fuels (coal and oil) they tend to release a lot of sulfur into the air. This sulfur combines with the oxygen already present in the air to form sulfur dioxide (SO₂).

Also, since we like to drive big fancy cars rather than ride bikes or walk, we cause the formation of nitrogen oxides (NO or NO₂ or NO ₃, etc) in air from burning gasoline.

CHEMICAL REACTIONS:

Formation of SO₂ and nitrogen oxides (NO & NO_x):

S(in fuel) + O₂ -------> SO₂
N₂ + O₂ -------> 2NO
NO + .5O₂ -----> NO₂
Formation of hydrogen peroxide

VOC + sunlight + HO₂ (in air) -------> H₂O₂
***VOC = Volatile Organic Compound ***
Formation of acids :

SO₂ + H₂O₂ and O₃ (in clouds) -------> H₂SO₄ SO₂+ OH + O₂ (in air) --------------> H₂SO₄
SO₂ + Oxidants (from wet surfaces) --> H₂SO₄ 
NO_x + sunlight + OH (air) ------------> HNO₃

EFFECTS OF ACID RAIN

 

 
 The acids can cause a lot of harm to things that we care about and enjoy. For example. some animals, like frogs and fish, have difficult time adapting to and reproducing in an acidic environment.

 Also, the leaves of many plants and trees can be severely damaged by acidic precipitation. It is believed that acid rain leaches calcium and magnesium from the soil. This causes a decrease in  the ratio of calcium to aluminum in the soil, which stimulates the uptake of aluminum by roots. The uptake of aluminum by trees and plants can be destructive. Finally, in cites and towns all over the world, stone structures, such as buildings, ancient ruins, etc are being deteriorated by the corrosive effects of acidic rainfall.

Autoionozation of water for Grade 10

The self-ionization of water (also autoionization of water, and autodissociation of water) is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H₃O⁺ and hydroxide, OH⁻.




Chemically pure water has an electrical conductivity of 0.055 µS·cm⁻¹. According to the theories of Svante Arrhenius, this must be due to the presence of ions. The ions are produced by the self-ionization reaction

H₂O + H₂O H₃O⁺ + OH⁻

This equilibrium applies to pure water and any aqueous solution.

The value of K_w decreases as temperature increases and it decreases with increasing pressure.

The equilibirum expression for the above reaction is written below and is treated mathematically like all equilibrium expressions.

K_w = [H₃O⁺][OH^-]
At 25^oC, the value of K_w has been determined to be 1 x 10^-14. 

If one knows the concentration of either the hydronium ions or of the hydroxide ions in a water solution,. the other ion concentration can be determined.

Example:  What is the hydronium ion concentration in a water solution that is 0.050 M NaOH?
The NaOH is a strong base and will 100% dissociate into its component ions.  Therefore, the concentration of the hydroxide ions will be 0.050 M.  The hydronium ion concentration is then calculated.

[H₃O⁺] = (1 x 10^-14)/(0.050) = 2.0 x 10^-13 M
Example:  What is the hydroxide ion concentration in a water solution that is 4.0 x 10^-5 M H₃O⁺?

[OH^-] = (1 x 10^-14)/(4.0 x 10^-5) = 2.5 x 10^-10 M .