CHEMICAL CONCEPT

What is Acidity ?

This chemical concept will be explained in the context of acid rain study, consequently only acidity in aqueous solutions will be covered.

Acid and Sour

In many languages the word acid is also used to mean sour and this is what sour looks like. Indeed one of the most well-known property of acids is their acid taste. But how can we characterize an acid from a chemical point of view? Acidity is defined in opposition and comparison with basicity. The two notions are intrinsically connected and acid and bases react together.

Definitions

Acidity can be defined according to 2 different theories :

Brønsted-Lowry Theory – Established in 1923 by the two physical chemists : the Danish Johannes Nicolaus Brønsted and the British Martin Lowry. They described the reaction between a specie A and a specie B consisting in the loss of a proton (H+) by A and the gain of this same particle by B as an acid-base reaction, A being the acid and B the base.

Lewis Theory – The same year Gilbert Lewis, an American physical chemist, proposed a different concept in which a specie A accepting a pair of electrons is called an acid and the specie which donated this pair of electron is called a base.

Brønsted-Lowry Lewis
Particle Exchanged Proton Pair of electrons
Acid Donates Accepts
Base Accepts Donates

Reactions involving at least one proton can easily be described according to both models since the acceptation of a pair of electron by a Lewis acid will automatically result in break of a hydrogen bond to maintain the shell configuration (octet rule) but only the Lewis model can be used to explain acid-base reaction in other cases.

Polyprotic Acids

An acid is a specie that donates 1 proton according to Brønsted-Lowry theory. Some species have more than 1 proton and can donate successively several protons. Only 1 proton is donated for each reaction.

For instance phosphoric acid is a polyacid :

H3PO4 + H2O  →  H2PO4− + H3O+       (1)

H2PO4− + H2O  →  HPO42− + H3O+        (2)

HPO42− + H2O  →  PO43− + H3O+      (3)

In the above example of the decomposition of phosphoric acid we can notice that H2PO4− (dihydrogen phosphate ion) is the conjugated base of H3PO4 in (1) and the conjugated acid of HPO42−  in (2).

Some species can behave as acids as well as bases depending on the other species present in the reaction. Therefore it is more rightful to talk about the acidic character of a compound and in order to measure it we use logarithmic scales as the pH or the pK.

Measuring Acidity

Ka : acid dissociation constant, or constant of acidity

In order to rank acids and quantify their ability to dissociate, donate a proton, we are using a scale which is known as the Ka, it measures the strength of an acid in solution. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/L):

Ka = [A-][H+] / [HA]

Very often the Ka value is expressed by using the pKa. The larger the value of pKa is, the smaller the extent of dissociation is.

pKa = -log(Ka)

Strongest base existing in water HO- : pKa = 14

Strongest acid existing in water H+ : pKa = 0

pH : standing for potential of hydrogen, is a numerically defined as:

pH = -log(αH+)

with : αH+ : activity of the hydrogen ions in the solution

In the case of an aqueous solution αH+ = [H+] / C° with C° the reference molarity. The pH depends on the concentration of hydrogen ions at a specific moment and is a characteristic of a given chemical system at a precise time.

What is Acid Rain?

Acid rain, or acid deposition, is any form of precipitation (rain, snow, fog, hail or even dust) with acidic components, such as sulfuric or nitric acid that fall to the ground from the atmosphere. Normal rain has a pH of about 5.6; it is slightly acidic because carbon dioxide (CO2) dissolves into it forming weak carbonic acid.  Acid rain usually has a pH between 4.2 and 4.4.

Formation of Acid Rain

Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents.  The SO2 and NOX react with water, oxygen and other chemicals to form sulfuric and nitric acids.  These then mix with water and other materials before falling to the ground.

Carbon dioxide (CO2) reacts with water to form carbonic acid (H2CO3) (1). H2CO3 dissociates to give hydrogen ion (H+) and the hydrogen carbonate ion (HCO3-) (2).

(1)

(2)

Nitrogen (N2) and oxygen (O2) will react with high temperature to form Nitric Oxide (NO) (3). NO is further oxidized to nitrogen dioxide (NO2) (4), which reacts with water to give nitric acid (HNO3) (5). HNO3 then dissociates to give H+ and the nitrate ion (NO3-). (6)

(3)

(4)

(5)

(6)

Sulfur dioxide (SO2) in the air will be oxidized to sulfur trioxide (SO3) which will react with water to form sulfuric acid (H2SO4) (7)

H2SO4 is a strong acid, so it readily dissociates in water, to give H+ ion and HSO4- ion (8). The HSO4- ion may further dissociate to give H+ and SO42- (9).

(7)

(8)

(9)

The formation of carbonic acid, nitric acid and sulfuric acid causes the concentration of H+ ions to increase dramatically, and so the pH of the rainwater drops to harmful levels. Thus, forming acid rain.

Sources of Acid Rain

Natural Sources

Lightning strikes contains enough energy to cause nitrogen and oxygen in the atmosphere to react and create nitrogen oxide and nitrogen dioxide.

Decaying vegetation, decomposition of organisms and wild fire causes carbon dioxide to form which is one of the acid rain forming gases.

Volcanic eruptions release gases such as carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen chloride, hydrogen sulfide, hydrogen fluoride, ammonia, methane and silicon tetrafluoride. These gases react with water vapor to precipitate as acid rain.

Anthropogenic Sources

Power plants use fuel to generate energy producing sulfur dioxide and nitrogen oxide gases.

Coal and oil are burned to produce energy to power machinery and are also used for heating, cooking, and lighting. This produces oxides of nitrogen and sulfur dioxide.

Automobiles such as trucks, cars, airplanes, and buses uses fuel to generate energy and this produces high levels of sulfur and nitrogen gaseous emissions into the atmosphere.

Sources

https://www.utdallas.edu/~scortes/ochem/OChem1_Lecture/Class_Materials/12_lewis_ac_bases.pdf

https://fr.wikipedia.org/wiki/Acide_phosphorique

https://www.boundless.com/chemistry/definition/solution/

https://www.epa.gov/acidrain/what-acid-rain

http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/FreshWater/acidrain.html

http://education.seattlepi.com/natural-causes-acid-rain-4613.html