carbonic acid
Summary
Carbonic acid (H₂CO₃) is a weak diprotic acid: a carbonyl carbon bearing two OH groups, formed when CO₂ dissolves in water. It loses a proton to give bicarbonate. Most testable: it is the acid member of the CO₂/bicarbonate buffer, so more CO₂ lowers pH.
Detail
Meaning: Carbonic acid, HO–C(=O)–OH, is the hydrated form of carbon dioxide. It is a weak diprotic acid. The first deprotonation gives bicarbonate (HCO₃⁻) and the second gives carbonate (CO₃²⁻), which is much less favorable. Its carbonyl carbon with two oxygen substituents makes it the parent of carbonates, carbamates and urea. In water it exists at low concentration and readily reverts to CO₂ and H₂O. The enzyme carbonic anhydrase speeds the interconversion. Extra CO₂ pushes the equilibrium right and releases H⁺.
Equation: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, and pH = pKa + log([HCO₃⁻]/[H₂CO₃]). Here pH and pKa are unitless, and the brackets are concentrations in mol/L. For the CO₂/bicarbonate system in blood, pKa is about 6.1.
On the MCAT: Passages describe blood buffering. You predict the direction of the shift when CO₂ rises (hypoventilation, lower pH) or falls (hyperventilation, higher pH), or when strong acid or base is added. Apply Le Châtelier's principle: HCO₃⁻ consumes added H⁺, and H₂CO₃ consumes added OH⁻. You may also be asked to use Henderson–Hasselbalch to find a pH or a ratio.
Common confusion: Students call CO₂ neutral or basic because it has no H. Dissolved CO₂ acts as an acid because it forms carbonic acid. Also, carbonic acid has no R group on its carbonyl carbon, so it is not a typical carboxylic acid (RCOOH).
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Chemistry: The Central Science (Brown, LeMay, Bursten)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.