Carbonic acid
A metastable acid central to respiration and water chemistry.
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Carbonic acid is a chemical compound with the formula H2CO3. It is a diprotic Brønsted acid that rapidly converts to water and carbon dioxide in the presence of water, yet in pure anhydrous form it exhibits surprising kinetic stability. Its interconversion with carbon dioxide is central to the breathing process of aerobic organisms and to the acidification of natural waters.
- chemical formula
- H2CO3
- molar mass
- 62.03 g/mol
- half-life at 37°C in water
- ~20 ms
- conformational isomers
- cis–cis, cis–trans, trans–trans
Lore & Background
Later, chemists reconceptualized 'acid' and no longer accepted CO2 as an acid, so 'carbonic acid' became the systematic name of H2CO3. It was long thought that H2CO3 could not exist independently, only in solution. In the 1960s, experimenters produced adducts of H2CO3, such as the diethyl ether adduct at -78°C and the dimethyl ether adduct at -30°C. However, α−H2CO3 was later argued to be a monomethyl ester.
Reader's Guide
Carbonic acid's significance lies in its role as an intermediate in the carbon dioxide–bicarbonate equilibrium that governs respiration and ocean acidification. This stability, explained by transition state theory, arises because water catalyzes its decomposition. The molecule has been isolated in solid form and its structure studied under high pressure: neutron diffraction of D2CO3 at 1.85 GPa shows planar molecules forming dimers with unusually short O—O distances (2.13 Å) and nearly equidistant C-O bonds (1.34 Å), attributed to delocalized π bonding and strong hydrogen bonds. This stability has led to suggestions that solid carbonic acid could be used for carbon sequestration. In aqueous solution, the hydration equilibrium constant is low ([H2CO3]/[CO2] ≈ 1.7×10−3), so most dissolved CO2 remains as gas, but the enzyme carbonic anhydrase rapidly equilibrates the system.
Did You Know?
- Neutron diffraction of D2CO3 at 1.85 GPa shows all three C-O bonds are nearly equidistant at 1.34 Å, intermediate between typical C-O and C=O bond lengths.
- The hydration equilibrium constant for carbonic acid in pure water at 25°C is [H2CO3]/[CO2] ≈ 1.7×10−3.
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Frequently Asked Questions
Who is Carbonic acid?
Carbonic acid (H₂CO₃, molar mass 62.03 g/mol) is a diprotic Brønsted acid that exists in three conformational isomers—cis–cis, cis–trans, and trans–trans. Fans often call it a 'metastable workhorse' because it sits at the crossroads of respiration, ocean chemistry, and atmospheric CO₂ cycling.
What are Carbonic acid's powers or role?
Its signature ability is rapid, reversible interconversion with carbon dioxide and water, which lets it shuttle CO₂ between the blood, lungs, gills, and the environment. As a diprotic acid it can also release two protons, making it a key pH regulator in natural waters.
Why is Carbonic acid important to the canon?
It is the chemical linchpin of aerobic gas exchange: without its equilibrium with CO₂, lungs and gills could not efficiently load and unload oxygen. It also governs the acidity of rain, rivers, and oceans, so virtually every aquatic ecosystem depends on its behavior.
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