How Do Carbonic Acid and Carbonate Salts Form and React?
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.
[Image will be uploaded soon]
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:
[Image will be uploaded soon]
This resonance may be summarized by a model with the delocalized charges and fractional bonds:
[Image will be uploaded soon]
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++ CO32-
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-⇌CO32- + 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 → CO32-
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.
CO32- + H2O ⇌ HCO3- + 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.
FAQs on Carbonic Acid and Carbonate Salts Explained
1. What is carbonic acid and how is it formed in nature?
Carbonic acid (H₂CO₃) is a weak, diprotic acid that forms when carbon dioxide (CO₂) gas dissolves in water (H₂O). It is highly unstable and exists in equilibrium with its dissolved components. This reaction is fundamental to many natural processes, including the formation of acid rain and the regulation of pH in natural water bodies.
2. What is the fundamental difference between carbonic acid and a carbonate salt?
The primary difference lies in their chemical nature. Carbonic acid (H₂CO₃) is a molecule and a weak acid. A carbonate salt is an ionic compound formed when carbonic acid reacts with a base. In this reaction, the hydrogen ions (H⁺) of the acid are replaced by a metal cation (like Na⁺ or Ca²⁺), resulting in a compound containing the carbonate ion (CO₃²⁻). For example, calcium carbonate (CaCO₃) is a salt of carbonic acid.
3. What are the key chemical characteristics of carbonate salts?
Carbonate salts exhibit several key characteristics:
Basic Nature: Most carbonate salts are basic because the carbonate ion (CO₃²⁻) reacts with water to produce hydroxide ions (OH⁻), raising the pH of the solution.
Reaction with Acids: They react vigorously with acids to produce carbon dioxide gas, a salt, and water. This is observed as a characteristic effervescence or fizzing.
Solubility: Carbonates of alkali metals (like sodium carbonate) are generally soluble in water, while most other carbonates (like calcium carbonate) are insoluble.
Decomposition: Most metal carbonates decompose upon heating to yield a metal oxide and carbon dioxide gas.
4. What happens when sodium carbonate reacts with a strong acid like hydrochloric acid?
When solid sodium carbonate (Na₂CO₃) reacts with dilute hydrochloric acid (HCl), a neutralization reaction occurs. The products are sodium chloride (NaCl), a salt; water (H₂O); and carbon dioxide (CO₂) gas. The release of carbon dioxide gas is observed as brisk effervescence. The balanced chemical equation for this reaction is: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g).
5. Where can we find common examples of carbonate and bicarbonate salts in our daily lives?
Carbonate and bicarbonate salts are very common. Some examples include:
Baking Soda (Sodium Bicarbonate, NaHCO₃): Used in baking as a leavening agent and as a household antacid.
Washing Soda (Sodium Carbonate, Na₂CO₃): Used as a water softener and in cleaning products.
Limestone/Marble (Calcium Carbonate, CaCO₃): Used as a building material and in the manufacturing of cement. It is also the main component in antacid tablets like Tums.
6. Why is pure carbonic acid considered highly unstable and difficult to isolate?
Carbonic acid is unstable because the equilibrium between dissolved carbon dioxide and water strongly favours the reactants. The activation energy required for carbonic acid to decompose back into CO₂ and H₂O is very low. This means that as soon as it is formed, it readily breaks down. Therefore, it cannot be isolated as a pure, stable substance under normal conditions and primarily exists as a transient species in aqueous solutions.
7. How does the equilibrium between carbonic acid, bicarbonate, and carbonate ions work?
Carbonic acid is a diprotic acid, meaning it can donate two protons (H⁺) in a stepwise manner. The equilibrium works as follows:
1. First Dissociation: Carbonic acid (H₂CO₃) first loses one proton to form a bicarbonate ion (HCO₃⁻).
H₂CO₃ ⇌ H⁺ + HCO₃⁻
2. Second Dissociation: The bicarbonate ion can then lose the second proton to form a carbonate ion (CO₃²⁻).
HCO₃⁻ ⇌ H⁺ + CO₃²⁻
The pH of the solution determines which species is dominant. In acidic conditions, H₂CO₃ is favoured, while in alkaline conditions, CO₃²⁻ is dominant. Bicarbonate is the major species at physiological pH (~7.4).
8. What is the importance of the carbonic acid-bicarbonate system in human blood?
This system acts as a crucial pH buffer in human blood, maintaining its pH within the narrow range of 7.35 to 7.45. If the blood becomes too acidic (excess H⁺), bicarbonate ions (HCO₃⁻) react with the excess H⁺ to form carbonic acid, which then breaks down into CO₂ and is exhaled. If the blood becomes too alkaline (low H⁺), carbonic acid dissociates to release H⁺ ions, thus lowering the pH. This rapid-response buffer system is vital for preventing acidosis or alkalosis.
9. How do carbonate salts and bicarbonate (hydrogen carbonate) salts differ chemically?
The main difference lies in the anion. Carbonate salts contain the carbonate ion (CO₃²⁻), which has a charge of -2. Bicarbonate salts contain the bicarbonate ion (HCO₃⁻), which has a charge of -1 and still contains a hydrogen atom. This structural difference leads to different properties. For instance, bicarbonate salts like NaHCO₃ are generally less basic than their carbonate counterparts like Na₂CO₃ because the HCO₃⁻ ion is amphoteric—it can act as both a weak acid and a weak base.
10. How can you test for the presence of carbonate ions in a salt sample?
The classic test for carbonate ions involves adding a dilute strong acid, such as hydrochloric acid (HCl), to the sample. If carbonate ions are present, they will react with the acid to produce carbon dioxide gas, which is observed as brisk fizzing or effervescence. To confirm the gas is CO₂, it can be bubbled through limewater (an aqueous solution of calcium hydroxide, Ca(OH)₂). The CO₂ will react with the limewater to form a milky white precipitate of calcium carbonate (CaCO₃).
