You oxides are an inorganic function whose compounds are binary, that is, they have only two constituent elements, the most electronegative of which is oxygen. You acid oxides are those oxides that when they react with water produce an acid.
The emission of acid oxides into the atmosphere has been growing more and more in recent decades. They can be emitted through natural sources such as plant and animal respiration, volcanic eruptions and decomposition of plant and animal debris. However, the aggravating factor is that some acid oxides are also released in the combustion of fossil fuels, such as petroleum derivatives (gasoline, diesel oil, etc.).
When these oxides meet in the atmosphere, they react with rainwater and make it acidic. The normal pH of water is 7.0 (neutral). Values smaller than this indicate acidic solutions.
It is true that all rain carries with it impurities present in the atmosphere, and it is not totally pure. Furthermore, it is normal for a certain amount of these oxides to be in the atmosphere. However, with the increasing use of fossil fuels, the concentrations of these substances have become alarming.
Carbon dioxide (carbon dioxide - CO2) is an example of an acid oxide that reacts with rainwater, as per the reaction below, and forms the carbonic acid:
1 CO2(g) + 1 hour2O(1)→ 1 H2CO3(aq)
This acid is weak and therefore this type of rain is not considered harmful.
Something similar happens with nitrogen oxides (AT THEx), among which the main one is nitrogen dioxide (NO2). It reacts with water forming the nitrous acid (HNO2) and nitric acid (HNO3):
2 NO2(g) + 1 hour2O(1)→ 1 HNO2(aq) + 1 HNO3(aq)
Although this type of acid rain is not considered harmful, in the long term, it can cause a certain environmental impact.
Mind Map: Acid Rain
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However, the most dangerous acid oxides, as they form a stronger acid rain, are sulfur oxides (SO2 and SO3). These compounds react with water and form the sulfuric acid (H2ONLY4).
1 S(s) + 1 O2(g) → 1 OS2(g)
1 SO2(g) + 1 hour2O(1)→ 1 H2ONLY3(aq) (sulfur acid)
1 SO2(g)+ ½ the2(g) → 1 OS3(g)
1 SO3(g) + 1 hour2O(1)→ 1 H2ONLY4(aq) (Sulfuric acid)
Sulfuric acid is the main villain of acid rain, as it is a very strong acid.
Technically, those with a pH lower than 5.6 are called acid rain.
Acid rain degrades artistic monuments, statues, constructions, metallic structures of houses, buildings and bridges, destroys crops and trees, poisons soils, rivers and groundwater, causing death in many animals.
By Jennifer Fogaça
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/Oxidos-chuva-Acida.htm