Acidic gases form acidic solutions when they dissolve in water. As an example, when nitrogen dioxide dissolve in water, nitric acid and nitrous acid mixture is given. In this tutorial, we are going to learn, what are the acidic gases and how they behave and form acidic solutions in water.
These acidic gases dissolve in water and form acidic solution. But, every gas does not dissolve in water in same amount and dissolved gases dissociate in different amounts to form the acidic solution. As an example, chlorine gas dissolve in water than bromine does. Also chorine gas dissociate than bromine gas to give acids.
Acidic gases are important to prepare different compounds such as production of nitric acid from nitrogen dioxide by ostwald process. Also, some acidic gases such as sulfur dioxide, nitrogen dioxide cause to acid rains which is a major environmental pollution incident in the world.
Most times (not always), when acidic gas dissolve in water, a weak acidic solution form. In following examples, you may understand this very well. Lets take carbon dioxide gas as an example. First gas dissolve in water to be in the aqueous phase.
CO2(aq) + H2O(l) → H2CO3(aq)
But, like nitrogen dioxide gases can form strong acidic solutions because nitrogen dioxide make nitric acid in water. Those kind of gases are introduced as strong acidic gases.
It is good to categorize acidic gases according to the elements. Therefore you can find which element can make acidic gases.
Remember, acidity of carbon dioxide gas is not enough to form acid rains.
Nitrogen form so many oxides. From them, oxides above +3 oxidation number of nitrogen, are acidic. So, dinitrogen trioxide, nitrogen dioxide, dinitrogen tetroxide and dinitrogen pentoxide are acidic. But, these gases form different acids when they dissolve in water.
Nitrous acid is given as the acid. Oxidation number of nitrogen is protected in this reaction.
N2O3(aq) + H2O(l) → HNO2(aq)
Nitrous acid and nitric acid are given as the acids. Nitric acid is a strong acid. Due to formation of nitric acid, acidic rains can be occurred. So nitrogen dioxide is a gas which can cause acid rains.
2NO2(aq) + H2O(l) → HNO2(aq) + HNO3(aq)
Nitrous acid is not stable and decomposes to nitric acid, nitric oxide and water. Then, above reaction can be written as below. In this reaction, nitrogen dioxide reacts with water and produce nitric acid and nitric oxide.
3NO2(aq) + H2O(l) → NO(aq) + 2HNO3(aq)
Sulfur dioxide (SO2) and sulfur trioxide (SO3) are acidic gases of sulfur. These acidic gases of sulfur can cause acid rains.
SO2(aq) + H2O(l) → H2SO3(aq)
Sulfur dioxide forms sulfuric acid in the water. Which is a strong acid and dibasic acid in the chemistry.
SO3(aq) + H2O(l) → H2SO4(aq)
Hydrogen sulfide (H2S) is a very toxic and an acidic gas. Solubility of hydrogen sulfide is very low and also it is a weak acid. Acidity of hydrogen sulfide is less than sulfurous acid and sulfuric acid.
When fluorine gas is sent to water, hydrogen fluoride (HF) is formed. Hydrogen fluoride is a weak acid. Products of fluorine and water reaction depend on how much fluorine gas is sent to the water.
2F2 + 2H2O → 4HF + O2
3F2 + 3H2O → 6HF + O3
When chlorine gas dissolve in water, hypochlorous acid (HOCl) and hydrochloric acid (HCl) are produced.
Cl2(aq) + H2O(l) → HCl(aq) + HCl(aq)
HOCl is a weak acid and HCl acid is a strong acid.
Carbon monoxide is a neutral gas.