[Chemistry Class Notes] on Carbonic Acid and Carbonate Salts Pdf for Exam

In chemistry, carbonic acid is defined as a dibasic acid having the chemical formula H2CO3. This pure compound decomposes at a temperature greater than Ca, −80 °C. In biochemistry, the word “carbonic acid” is often applied to the aqueous solutions of carbon dioxide that play an important role in the bicarbonate buffer system, which is used to maintain acid-base homeostasis.

Carbonate salts are nonflammable materials. They act as weak bases and thus participate in acid-base reactions, which generate heat and release CO2.

Structure and Bonding

Let us see the structure and bonding of carbonate ions.

The simplest oxocarbon anion is the carbonate ion. It has a trigonal planar structure with one carbon atom surrounded by three oxygen atoms, with D3h molecular symmetry. It also has a gross formal charge of 2.01 and a molecular mass of 60.01 g/mol. It is the conjugate base of hydrogen carbonate (which is bicarbonate) ion, HCO−3, the conjugate base of H2CO3, carbonic acid.

The carbonate ion’s Lewis structure has two (long) single bonds to the negative oxygen atoms and one short double bond to the neutral oxygen.

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This arrangement is incompatible with the observed ion’s symmetry, which means that the three bonds are identical in length and the three oxygen atoms are considered similar. As in the isoelectronic nitrate ion case, the symmetry may be achieved by a resonance among the three structures given below:

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This resonance may be summarized by a model with the delocalized charges and fractional bonds:

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Formation

Carbonic acid and carbonate salts

Carbonic acid (with the chemical formula H2CO3) can be formed in small amounts when its carbon dioxide (CO2), anhydride, dissolves in water.

CO₂ + H₂O⇌ H₂CO₃

Simply, the predominant species can be loosely hydrated CO2 molecules. Carbonic acid may be considered to be the diprotic acid from which the two series of salts may be formed—namely, hydrogen carbonates, holding HCO3−, and carbonates, having CO32−.

H₂CO₃ + H₂O ⇌ H₃O⁺ + HCO₃^–

HCO₃^– + H₂O ⇌ H₃O⁺+ CO₃^2-

However, the behaviour of acid-base carbonic acid depends on varied rates of a few of the reactions involved and their dependence on the pH of the system as well. For example, at a pH of below 8, the principal reactions, including their relative speed, are given as follows:

CO₂ + H₂O⇌H₂CO₃ (slow)

H₂CO₃+OH^–⇌HCO₃^– + H₂O (fast)

Above pH 10, the reactions given below are important:

CO₂ +OH^– ⇌ HCO₃^– (slow)

HCO₃^– + OH^–⇌CO₃^2- + H₂O (fast)

Between the pH values of 8 & 10, all the above-given equilibrium reactions are significant.

Carbonate and Hydrogen Carbonate Salts

These specific salts may be prepared by the carbon dioxide reacting with metal oxides and the metal hydroxides, respectively.

CO₂ + O₂ → CO₃^2-

CO₂ + OH^– → HCO₃^–

For example, when sodium hydroxide (NaOH) aqueous solution is saturated with carbon dioxide, sodium hydrogen carbonate (NaHCO3) can be formed in solution.

Na⁺ + OH^– + CO₂ → Na⁺+ HCO₃^–

When the water is removed from it, the solid compound is also known as sodium bicarbonate or as baking soda. For example, when baking soda is used in the cooking and, causes either cake or bread to rise, this effect is because of the reaction of the basic hydrogen carbonate anion (HCO3−) with an added acid, such as KHC4H4O6, potassium hydrogen tartrate (cream of tartar), or calcium dihydrogen phosphate, Ca(H2PO4)2.

As long as the soda is dry, no reaction takes place. When the milk or water is added, acid-base neutralization occurs, producing the water and gaseous carbon dioxide. Then, the CO2 becomes trapped in the batter, and when heated, the gas expands to create the texture of biscuits and bread characteristics.

Carbonates are moderately strong bases. Aqueous solutions are the basic due to the reason the carbonate anion may accept a hydrogen ion from water.

CO₃^2- + H₂O ⇌ HCO
₃^– + OH^–

Reaction of Acids with Carbonates

Acids and Metal Carbonates

Water, salt, and carbon dioxide are produced when acids react with carbonates like calcium carbonate (found in limestone, chalk, and marble).

Acid + Metal Carbonate → Salt + Water + CO₂

Sulfuric Acid + Iron (II) Carbonate → Iron (II) Sulfate + Water + CO₂

H₂SO₄ + FeCO₃ → FeSO₄ + H₂O + CO₂

The CO2 causes are bubbling at the time of reaction that can be noticed as fizzing. It may be detected by passing the gas via lime water, which will go cloudy.

And, the reaction of metal carbonates with the acids is exothermic (it means heat energy is given out).

This type of reaction may be used to test the unknown solutions to observe if they are acidic. Just add a solution of sodium carbonate to the solution, and if the carbon dioxide gas is given off, the solution becomes acidic.

This type of reaction may also be used to test the unknown solutions for the presence of carbonate (which is CO3–) ions. Just add the acid to the solution, and if the bubbles of CO2 are given off, the solution has carbonate ions.

Metal Hydrogen Carbonates

Metal Hydrogen Carbonates are a kind of base that also produces water, salt, and CO2 when they react with an acid. These are also sometimes known as Metal Bicarbonates.