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Further explanation
Carbonic acid is a chemical compound with the chemical formula H2CO3 (equivalent: OC (OH)2). This is also the name sometimes given for carbon dioxide solutions in water (carbonated water), because they contain small amounts of H2CO3. In physiology, carbonic acid is described as volatile acid or respiratory acid because it is the only acid that is excreted as gas by the lungs.
When carbon dioxide dissolves in water, it is in chemical balance with carbonic acid:
CO2 + H2O <==> H2CO3
The hydration equilibrium constant at 25 ° C is called K h, which in the case of carbonic acid is [H2CO3] / [CO2] ≈ 1.7 × 10 −3 in pure water and ≈ 1.2 × 10-3 in sea water. [6] Therefore, the majority of carbon dioxide is not converted to carbonic acid, but remains as a CO 2 molecule. In the absence of a catalyst, the balance is reached quite slowly. The rate constant is 0.039 s-1 for the forward reaction (CO2 + H2O → H2CO3) and 23 s-1 for the reverse reaction (H2CO 3 → CO2 + H2O). The addition of two water molecules to CO2 will produce orthocarbonic acid, C (OH) 4, which only exists in small amounts in aqueous solution.
Addition of a base with excess carbonic acid produces bicarbonate (hydrogen carbonate). With an excess of bases, carbonic acid reacts to produce carbonate salts.
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Details
Class: high School
Subject: chemistry
Keywords : carbon dioxide, carbonic acid, hydrogen carbonate
The expression for in an aqueous solution is [tex]\boxed{{{\text{k}}_{{\text{a1}}}} = \frac{{\left[ {{\text{HCO}}_3^ - } \right]\left[ {{{\text{H}}_{\text{3}}}{{\text{O}}^ + }} \right]}}{{\left[ {{{\text{H}}_2}{\text{C}}{{\text{O}}_3}} \right]}}}[/tex] .
Further Explanation:
Acids:
These are the type of substances that are known for hydrogen ion donation in aqueous solutions. In other words, proton donors are called acids. Based on the number of protons donated by acids, these can be classified as monoprotic and polyprotic, the description of which is given below.
Monoprotic acids
“Mono” means “single” while “protic” refers to “protons”. So monoprotic acids are those acidshaving capacityto donate single proton in aqueous solutions. HCl, HI and HF can donate single proton in solutions, thereby acting as monoprotic acids.
Polyprotic acids
“Poly” means “many” while “protic” refers to “protons”. So polyprotic acids are those acids having capacity to donate more than oneproton in aqueous solutions. [tex]{{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}[/tex], [tex]{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}[/tex] and [tex]{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}[/tex] can donate more than one proton in solutions, thereby acting as monoprotic acids.
Dissociation constant
It is a parameter required to determine strength of acids and is represented by .
As is evident from chemical formula of carbonic acid [tex]\left( {{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}} \right)[/tex], it has two hydrogens available for donation in solutions. Therefore [tex]{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}[/tex] acts as diprotic acid.
Dissociation of [tex]{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}[/tex] occurs as follows:
[tex]\text{H}_{2}\text{CO}_{3}(aq)+\text{H}_{2}\text{O}(aq)_{\rightleftharpoons}^{{K}_{a1}}\text{HCO}^{-}_{3}(aq)+\text{H}_{3}\text{O}^{+}(aq)\\\text{HCO}^{-}_{3}}(aq)+\text{H}_{2}\text{O}(aq)_{\rightleftharpoons}^{{K}_{a2}}\text{CO}^{2-}_{3}(aq)+\text{H}_{3}\text{O}^{+}(aq)[/tex]
Where [tex]{{\text{k}}_{{\text{a1}}}}[/tex] and [tex]{{\text{k}}_{{\text{a2}}}}[/tex] are first and second dissociation constants of [tex]{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}[/tex] respectively.
Since water is always present in larger quantity, its concentration is neglected while writing the expressions for dissociation constant of acid.
The expression of [tex]{{\text{k}}_{{\text{a1}}}}[/tex] for [tex]{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}[/tex] is as follows:
[tex]{{\text{k}}_{{\text{a1}}}} = \dfrac{{\left[ {{\text{HCO}}_3^ - } \right]\left[ {{{\text{H}}_{\text{3}}}{{\text{O}}^ + }} \right]}}{{\left[ {{{\text{H}}_2}{\text{C}}{{\text{O}}_3}} \right]}}[/tex]
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Answer details:
Grade: High School
Subject: Chemistry
Chapter: Acids, bases and salts
Keywords: H2CO3, acid, dissociation constant, ka2, H2O, H3O+, H2CO3,HCO3-, CO32-, ka1, hydrogens, monoprotic acid, polyprotic acid, expression, strength of acids, ka.
