Talk:Carbonic acid

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mol/L ?

How does the K_a have units? —Preceding unsigned comment added by 198.146.61.222 (talk) 20:41, August 30, 2007 (UTC)

The acid dissociation constant is defined as a ratio of concentrations, two in the numerator of the fraction and one in the denominator. It usually (but not necessarily) has the units of mol/L (or mol/dm³, which is the same but longer to type). Physchim62 (talk) 15:16, 2 September 2007 (UTC)[reply]

Pure Carbonic Acid

From the article: It is not possible to obtain pure carbonic acid. Why? -- The Anome 09:28, 29 Mar 2004 (UTC)

The previous sentance indicates what I believe is the basis for this statement: "exists in an equilibrium with water and carbon dioxide". If you were to somehow obtain a quantity of pure carbonic acid, some of it would immediately and spontaneously break into water and carbon dioxide and it wouldn't be pure any more. Bryan 09:34, 29 Mar 2004 (UTC)

If it were pure, then it wouldn't be carbonic acid; it would be hydrogen carbonate. Vacuum c 23:07, Feb 12, 2005 (UTC)

This web page indicates differently so I updated the article http://www.newton.dep.anl.gov/askasci/chem99/chem99661.htm Rjstott 09:37, 29 Mar 2004 (UTC)

Carbonic Acid / Water / Air Equilibrium Notes

Since one of the major components of earth's atmosphere is carbon dioxide, the equilibrium CO₂(g) <-> CO₂(aq) in combination with CO₂(aq) + H₂O <-> H₂CO₃ is important in many solutions that are in contact with air. It is worth noticing that the value of the equilibrium constant of the first dissociation reaction that is usually given (pK=6.36), is really the combined constant of the equilibria CO₂(aq) + H₂O <-> H₂CO₃ and H₂CO₃ <-> H⁺ + HCO₃^-. Under normal conditions only a small fraction of the dissolved carbon dioxide reacts with water to form carbonic acid (source: http://www.thuisexperimenteren.nl/science/carbonaatkinetiek/Carbondioxide%20in%20water%20equilibrium.doc, http://www.chem.usu.edu/~sbialkow/Classes/3650/Carbonate/Carbonic%20Acid.html). Therefore the true value of the first equilibrium constant of carbonic acid is approximately 2.6E-4 (pK=3.58). However, as long as the distinction between dissolved CO₂ and carbonic acid is irrelevant, the use of the greater pK-value is no problem, but the overall reaction concerned is CO₂(aq) + H₂O <-> H⁺ + HCO₃^-.

Another important quantity involved in the calculation of dilute aqueous solutions in equilibrium with air is the Henry coefficient of CO₂, which describes the first equilibrium CO₂(g) <-> CO₂(aq). At 293 K it is about 25.6 kg atm/mol (Edwards, Newman & Prausnitz, AIChE J., 1975, 21(2), 248-). This value corresponds very well with the observations that about 39 mmol of CO₂ dissolves in 1 L of pure water of 293 K (under pure CO₂ at 1 atm., final pH 3.9) and that the pH of water in equilibrium with air (0.03% CO₂ by volume) is 5.7.

--Watje22 13:46, 3 August 2005 (UTC)[reply]

Carbonic Acid Contradiction

The table says it exists only in solution, but the article links to a page which says that it DOES exist as an indipendent substance. Isn't that unconsistent?--Army1987 21:03, 15 August 2005 (UTC)[reply]

Both are wrong! The press release claims that carbonic acid has been isolated in the gas phase, but goes on to say that the preparation of gaseous H₂CO₃ is an objective for researchers into interstellar space... I am sceptical about gaseous H₂CO₃, the article cited describes only computer calculations, not actual preparation. However, carbonic acid can be prepared as solid, crystalline etherate (like a hydrate but with dimethyl ether), melting point -47 °C (Cotton, F. Albert; Wilkinson, Geoffrey (1980), Advanced Inorganic Chemistry (4th ed.), New York: Wiley, ISBN 0-471-02775-8). An etherate is not H₂CO₃, but it is certainly not a solution either. Physchim62 01:46, 16 August 2005 (UTC)[reply]

Carbonic Acid Density

The density listed for carbonic acid is shown as 1.0 g/cm³ in a dilute solution. However, the density listed for water is shown as 1,000 kg/m³, which is the same thing in different units. Shouldn't these entries use the same units? Also, I believe I read somewhere that carbonic acid is slightly less dense than water, so it tends to be a bit more common near the surface, but Googling only seems to turn up copies of the Wikipedia information. Can anyone verify density of carbonic acid? Thanks. --70.20.161.132 23:59, 10 October 2006 (UTC)[reply]

This is indeed the same figure in two different units: g/cm³ is the unit which is more commonly used. There is no significant difference in the density of carbonic acid and water, at least at normal pressures of carbon dioxide. Physchim62 (talk) 08:13, 12 October 2006 (UTC)[reply]

Acidity

The Ka value for this acid is diffrent in my chemistry text book. Ka1=4.4*10^-7 ka2=4.7*10^-11

Ka2 is (roughly) correct, Ka1 is false because of the equilibrium between carbonic acid and dissolved carbon dioxide, as discussed in the article. Physchim62 (talk) 17:39, 13 November 2006 (UTC)[reply]

Rate law for H₂CO₃ dissociation

What rate law is assumed for the rate constants given for carbonic acid dissociation / CO₂ hydration? I assume the rate law is first order in both directions - i.e. rate = k[CO₂] or rate = k[H₂CO₃]. However, this should be stated explicitly. Asteen 15:48, 3 October 2007 (UTC)[reply]