You effects of acid rain about the historical monuments are caused by the occurrence of inorganic reactions, called double exchange reactions. It is worth mentioning that all rain has acid content.
In addition to the physical wear (small breaks) caused by the contact of the raindrop with buildings, there is also a chemical phenomenon, with processes that lead to the loss of building mass.
THE acid rain causes chemical effects on historic monuments because in it there is the presence of two inorganic acids considered strong, the nitric and the sulfuric, represented, respectively, by the formulas HNO3 and H2ONLY4.
The acids present in acid rain originate in the atmosphere with the chemical reaction of nitric oxides (NO2) and sulfuric (SO3), according to the following reactions:
AT THE2 + H2O → HNO3
and
ONLY3 + H2O → H2ONLY4
Because they have strong acid characteristics, the acids present in the acid rain are corrosive, however, like the historical monuments usually consist of rocks, ie minerals, corrosion does not always happen.
What actually happens is the ability of these acids to react with some components of the monument, such as the marble (calcium carbonate-CaCO3) and the soapstone (Na2CO3), that are inorganic salts.
When acids from acid rain come into contact with calcium carbonate, a double exchange reaction occurs, forming a new acid and a new salt.
Reaction between nitric acid and marble
HNO3 + CaCO3 → Ca (NO3)2 + H2CO3
or
HNO3 + CaCO3 → Ca (NO3)2 + H2O + CO2
In this reaction, there is the formation of calcium nitrate salt, a brittle salt, and carbonic acid, which is converted into water and carbon dioxide.
Part of a Cambodian temple degraded by acid rain
Reaction between sulfuric acid and marble
H2ONLY4 + CaCO3 → Case4 + H2CO3
or
H2ONLY4 + CaCO3 → Case4 + H2O + CO2
In this reaction, calcium sulfate, which is gypsum, and carbonic acid are formed, which is converted into water and carbon dioxide.
Marble sculpture degraded by acid rain
As observed in the reactions described, acid rain promotes double exchange reactions, with the formation of salts with a brittle characteristic, which causes the monument to fall apart over time. time.
The same can occur in monuments built with soapstone, a material that is composed of sodium carbonate (Na2CO3).
Reaction between sulfuric acid and soapstone
H2ONLY4 + In2CO3 → In2ONLY4 + H2CO3
or
H2ONLY4 + In2CO3 → In2ONLY4 + H2O + CO2
In this reaction, sodium sulfate, which is a salt, and carbonic acid, which converts into water and carbon dioxide, are formed. The salt formed is quite soluble in water, so when it rains, the monument is further degraded.
By Me. Diogo Lopes Dias
Source: Brazil School - https://brasilescola.uol.com.br/quimica/efeitos-chuva-acida-nos-monumentos-historicos.htm