Dhristi JEE 2022-24

What is Carbonic Acid?

Carbonic acid can be defined as a carbon-containing compound having the chemical formula H2CO3. The carbon dioxide solutions in water comprise small amounts of this compound. The carbonic acid's chemical formula can also be written as OC(OH)2 because, in this compound, there is one carbon-oxygen double bond.


Carbonic acid can often be described as a respiratory acid due to the reason it is the only acid exhaled by the human lungs in the gaseous state. It forms bicarbonate and carbonate salts, and it is a weak acid.


Note: H2CO3 is capable of dissolving limestone, which leads to the calcium bicarbonate (Ca(HCO3)2 formation. This is the primary reason for various features of limestone, including stalactites and stalagmites.


H2CO3 Structure

The carbonic acid or H2CO3 structure can be illustrated below.


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From the illustration above, we can understand that the carbonic acid structure consists of one carbon-oxygen double bond and two carbon-oxygen single bonds. The oxygen atoms that participate in a single bond with the carbon each contain one hydrogen atom attached to them.


Carbonic acid, formed by the hydrolysis and dissolution of CO2 in water, is the primary natural leaching agent in various temperate ecosystems. Carbonic acid is both unstable, weak and dissociates quickly into the bicarbonate ions (HCO3) and hydrogen ions (H+). When CO2 is dissolved in water, it participates in the chemical equilibrium as given below.

CO2 + H2O ⇌ H2CO3

However, in the chemical equilibrium described above, only a small amount of carbon dioxide is converted into carbonic acid.


Properties of H2CO3

A few major physical and chemical properties of carbonic acid are listed in this subsection:

Physical Properties

  • The carbonic acid molar mass can be given as 62.024 grams per mole.

  • The H2CO3 compound holds a pKa value of 3.6.

  • It has a density of 1.668 grams per cubic centimetre in a standard state.

  • Generally, this compound exists as a solution. But, it has been reported that NASA scientists have prepared solid H2CO3 samples.

  • The conjugate base corresponding to the carbonic acid is bicarbonate.


Chemical Properties 

  • H2CO3 is unstable in nature, and it is a weak acid.

  • In the presence of water, it undergoes partial dissociation to yield HCO3 (bicarbonate) and H+ ions.

  • Carbonic acid is a diprotic acid, and hence, it can form two types of salts: carbonates and bicarbonates.

  • Adding a small quantity of base to H2CO3 yields bicarbonate salts, whereas the addition of a base in excess amounts yields carbonate salts.

  • It should also note that carbonic acid can be obtained as the burning of fossil fuels at an industrial scale or a by-product of industrial fermentation processes.


Carbonic Acid Uses

H2CO3 is an essential compound with a wide range of applications. A few of the carbonic acid uses are listed below:

  • The preparation of sparkling wine, carbonated water, and other aerated drinks involves carbonic acid use.

  • H2CO3 can be used in the precipitation of various ammonium salts like ammonium persulfate.

  • It helps in carbon dioxide transportation out of the body.

  • H2CO3 protonates many bases that contain nitrogen in blood serum

  • Ringworm and other dermatitides are treated through the carbonic acid application over the affected area.

  • Solutions that contain this compound are much more effective in contact lens cleaning.

  • It can be orally consumed to induce vomiting whenever required (such as, in the case of a drug overdose).


Carbonic Acid Importance in Blood

The bicarbonate ion or molecule is an intermediate for the carbon dioxide out of the human body through the respiratory gas exchange process. The hydration reactions undergone by carbon dioxide are very slow, especially in the absence of a suitable catalyst. The presence of the enzyme family called carbonic anhydrases in the red blood cells will result in an increase in reaction rate. 


The carbonic anhydrase enzymes' enzymes work to catalyze the conversion of water and carbon dioxide into dissociated carbonic acid ions. This forms the bicarbonate anions that get dissolved in the plasma of the blood. This catalyzed reaction will get reversed in the lungs, resulting in CO2 formation, and then exhaled.


A Comment on the Carbonic Acid's Acidity

Carbonic acid is described as a carboxylic acid which holds a substituted hydroxyl group. It is also called polyprotic acid. Actually, this compound is diprotic and, hence, it has two protons which dissociate from the primary parent molecule. Thus, two dissociation constants exist, where the first one is for the dissociation into the bicarbonate ion.


Natural Acidity of Rainwater

All students know that pure water is neutral in nature, that is, it has a pH of 7. But, unpolluted rainwater is slightly acidic in nature, that is, it has a pH below 7. The natural acidity of rainwater is around 5.6 which makes it acidic in nature. 


Students must recall that pH in simple terms is nothing but the measure of hydrogen ion concentration in the solution being tested. When natural, unpolluted rainwater is tested, it is observed that the acidic nature is because of three gases present in the air. 


These gases named carbon dioxide, nitric oxide and sulphur dioxide are found in the lowest layer of the earth's atmosphere called troposphere. A natural source of carbon dioxide in the atmosphere is when carbon dioxide is made upon the decomposition of organic materials on earth. Compared to the other two gases, carbon dioxide is the biggest contributor to the acidic nature in unpolluted rainwater because it is present in the highest concentration. The concentration of carbon dioxide is 355 ppm. Parts per million is what 'ppm' stands for. It is used as a measure for concentration in environmental chemistry. The ppm of nitric oxide and sulphur dioxide is almost similar to each other and very low compared to carbon dioxide. The concentration of nitric oxide and sulphur dioxide in the atmosphere is just 0.01 ppm each. Natural source of nitric oxide is lightning thunders that lead to an electric discharge from the reaction between 2 atmospheric gases called Nitrogen and Oxygen. Natural source of sulphur dioxide is being one of the volcanic gases on earth.


Carbon dioxide in the air reacts with water in the clouds to form carbonic acid. Then, carbonic acid dissociates to give the hydrogen carbonate ion and the hydrogen ion. The ability of carbonic acid to give the hydrogen ion is what makes it acidic in nature and hence, lowers the pH of natural unpolluted rainwater.


Also, nitric oxide in the air gets oxidised to nitrogen dioxide during lightning storms and then, the nitrogen dioxide in the air reacts with water in the clouds to form nitric acid. Nitric acid dissociates in water to give nitrate ions and the hydrogen ions similar to the dissociation reaction of carbonic acid, which lowers the pH of rainwater and makes it naturally acidic.


Students must know how concerning is the issue of polluted rainwater called acid rain. Polluted rainwater's acidity is much higher than natural acidity of rainwater due to increased air pollution. Air pollution is caused by an increase of more and more acid forming compounds in the air that are a result of high industrial activity and vehicular traffic. This results in the pH of polluted rainwater to being as low as 3 or even worse in many cases which makes acid rain way more acidic than natural levels. Acid rain is an environmental threat as they change the pH of freshwater lakes on earth upon downpour. Many fish species die as they can survive only in fresh water as their natural habitat. These fishes are sensitive to slight changes in pH levels of freshwater lakes and cannot survive if it turns acidic or basic in nature due to any reason like industrial pollution, acid rain and so on. This affects the entire lake health as the whole aquatic food chain gets affected.


Not only this, acid rain poses more risks like damaging crops and reducing the expected crop yield and eroding buildings and monuments as well.


Students can learn more about real life implications of increasing pollution, elevated carbon dioxide concentration and global warming due to the greenhouse effect on Vedantu. Study material for exam preparation and revision is available to students to download for free from the Vedantu website and app.

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FAQs (Frequently Asked Questions)

1. What is the importance of carbonic acid in oceans?

The absorption of the excess amount of carbon dioxide in the atmosphere (primarily because of human activities) by oceans is believed to have caused a pH shift of the ocean's water approximately by -0.1. Now, the absorbed carbon dioxide reacts with ocean water and produces H₂CO₃. Commonly, this process is referred to as ocean acidification.

2. Mention the Role of Carbonic Acid in the Blood?

Bicarbonate acts as an intermediate in the respiratory gas exchange for conveying CO⁻₂ out of the body. Generally, the hydration reaction of CO₂ is too slow in the catalyst absence, but the red blood cells hold a substance called carbonic anhydrase, which increases the rate of reaction by creating dissolved bicarbonate (HCO₃⁻) in the blood plasma.

3. Can Carbonic Acid be Said as a Strong Acid?

No, H₂CO₃ does not belong to a strong acid. It is also a weak acid that dissociates into a bicarbonate ion (which is HCO₃⁻ anion) and proton (H⁺ cation). This compound dissociates only in aqueous solutions partly. Moreover, the conjugate base of the carbonic acid - the bicarbonate ion, is relatively a good base. 

4. Mention if carbonic acid is dangerous.

Carbonic acid is not considered dangerous or toxic to human health because it is naturally present in the human body. But, it is important to make a note that exposure to high concentrations of H₂CO₃ may irritate the eyes and respiratory tract.

5. Is  carbonic acid organic or inorganic?

The chemical structure of the molecule makes carbonic acid to be referred to as an inorganic acid even if it has carbon and hydrogen in it. Carbonic acid has two hydrogen atoms, one carbon atom and three oxygen atoms. Compounds are only considered organic if they have carbon and hydrogen and at least one hydrogen is bonded to carbon. But in the case of carbonic acid, no hydrogen bonds to the central carbon atom. Therefore, carbonic acid is classified as being inorganic. 


Carbonic acid is a weak, inorganic acid that can form two different kinds of inorganic salts: carbonate and bicarbonate. For example, carbonic acid dissolves limestone and it leads to the formation of calcium bicarbonate.


Students can learn more about such carbon-containing compounds and their unique chemical and physical properties along with their industrial use on Vedantu. Lessons on different topics in Organic Chemistry for exam preparation and revision are easily available to students for free download on the Vedantu website and app.  

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