The carbonic acid It is an inorganic compound, although there are those who debate that it is actually organic, whose chemical formula is HtwoCO3. It is therefore a diprotic acid, capable of donating two H ions+ to the aqueous medium to generate two molecular cations H3OR+. From it arise the well-known bicarbonate ions (HCO3-) and carbonate (CO3two-).
This peculiar acid, simple, but at the same time involved in systems where numerous species participate in a liquid-vapor equilibrium, is formed from two fundamental inorganic molecules: water and carbon dioxide. The presence of COtwo undissolved is observed whenever there is a bubbling in the water, rising towards the surface.
This phenomenon is seen very regularly in carbonated drinks and carbonated water..
In the case of carbonated or carbonated water (image above), such a quantity of CO has been dissolvedtwo that its vapor pressure is more than double that of atmospheric pressure. When uncapped, the difference in pressure inside the bottle and outside decreases the solubility of COtwo, so the bubbles appear that end up escaping from the liquid.
To a lesser degree, the same occurs in any body of fresh or saline water: when heated they will release their dissolved CO content.two.
However, the COtwo It is not only dissolved, but undergoes transformations in its molecule that make it HtwoCO3; an acid that has too little a life time, but enough to mark a measurable change in the pH of its aqueous solvent medium, and also generate a unique carbonate buffer system.
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Above we have the H moleculetwoCO3, represented with spheres and bars. The red spheres correspond to the oxygen atoms, the black to the carbon atom, and the white to the hydrogen atoms.
Note that starting from the image you can write another valid formula for this acid: CO (OH)two, where CO becomes the carbonyl group, C = O, linked to two hydroxyl groups, OH. As there are two OH groups, capable of donating their hydrogen atoms, it is now understood where the H ions come from+ released to the middle.
Also note that the formula CO (OH)two can be written as OHCOOH; that is, of the RCOOH type, where R is in this case an OH group.
It is for this reason, in addition to the fact that the molecule is made up of oxygen, hydrogen and carbon atoms, all too common in organic chemistry, that carbonic acid is considered by some to be an organic compound. However, in the section on its synthesis, it will be explained why others consider it to be inorganic and non-organic in nature..
Of the H moleculetwoCO3 It can be commented that its geometry is trigonal plane, with the carbon located in the center of the triangle. In two of its vertices it has OH groups, which are hydrogen bond donors; and in the other remaining, an oxygen atom of the group C = O, acceptor of hydrogen bonds.
Thus, the HtwoCO3 has a strong tendency to interact with protic or oxygenated (and nitrogenous) solvents.
And coincidentally, water meets these two characteristics, and such is the affinity of HtwoCO3 for her that almost immediately gives her an H+ and a hydrolysis equilibrium begins to be established that involves the HCO species3- and H3OR+.
That is why the mere presence of water decomposes carbonic acid and makes its isolation as a pure compound too complicated..
Going back to the H moleculetwoCO3, It is not only flat, capable of establishing hydrogen bonds, but it can also present cis-trans isomerism; This is, in the image we have the cis isomer, with the two H pointing in the same direction, while in the trans isomer they would point in opposite directions.
The cis isomer is the more stable of the two, and that is why it is the only one that is usually represented.
A pure solid of HtwoCO3 It consists of a crystalline structure composed of layers or sheets of molecules interacting with lateral hydrogen bonds. This is to be expected, with the molecule HtwoCO3 flat and triangular. When sublimated, cyclic dimers (HtwoCO3)two, which are joined by two hydrogen bonds C = O - OH.
The symmetry of the H crystalstwoCO3 it has not been able to define itself at the moment. It was considered to crystallize as two polymorphs: α-HtwoCO3 and β- HtwoCO3. However, α-HtwoCO3, synthesized starting from a mixture of CH3COOH-COtwo, it was shown to be actually CH3OCOOH: a monomethyl ester of carbonic acid.
Mention was made that the HtwoCO3 it is a diprotic acid, so it can donate two H ions+ to a medium that accepts them. When this medium is water, the equations of its dissociation or hydrolysis are:
HtwoCO3(ac) + HtwoO (l) <=> HCO3-(ac) + H3OR+(ac) (Ka1 = 2.5 × 10−4)
HCO3-(ac) + HtwoO (l) <=> CO3two-(ac) + H3OR+(ac) (Katwo = 4.69 × 10−11)
The HCO3- is the bicarbonate or hydrogen carbonate anion, and the CO3two- the carbonate anion. Also indicated are their respective equilibrium constants, Ka1 and Katwo. Being Katwo five million times smaller than Ka1, the formation and concentration of CO3two- they are despicable.
Thus, even though it is a diprotic acid, the second H+ it can barely release it appreciably. However, the presence of COtwo dissolved in large quantities it is enough to acidify the medium; in this case, the water, lowering its pH values (below 7).
To speak of carbonic acid is to refer practically to an aqueous solution where the HCO species predominate3- and H3OR+; it cannot be isolated by conventional methods, as the slightest attempt would displace the CO solubility equilibriumtwo to the formation of bubbles that would escape from the water.
Carbonic acid is one of the easiest compounds to synthesize. How? The simplest method is to bubble, with the help of a straw or straw, the air we exhale into a volume of water. Because we essentially exhale COtwo, This will bubble in the water, dissolving a small fraction of it.
When we do this the following reaction occurs:
COtwo(g) + HtwoO (l) <=> HtwoCO3(ac)
But in turn, the solubility of CO must be considered.two in water:
COtwo(g) <=> COtwo(ac)
Both the COtwo like the HtwoO are inorganic molecules, so the HtwoCO3 it is inorganic from this point of view.
As a result we have a system in equilibrium that is highly dependent on the partial pressures of COtwo, as well as the temperature of the liquid.
For example, if the pressure of COtwo increases (in the case that we blow the air with more force through the straw), more H will be formedtwoCO3 and the pH will become more acidic; since the first equilibrium shifts to the right.
On the other hand, if we heat the solution of HtwoCO3, the solubility of CO will decreasetwo in water because it is a gas, and the equilibrium will then shift to the left (there will be less HtwoCO3). It will be similar if we try to apply a vacuum: the COtwo it will escape as well as the water molecules, which would shift the equilibrium to the left again.
The above allows us to reach a conclusion: from a solution of HtwoCO3 there is no way to synthesize this acid as a pure solid by a conventional method. However, it has been done, since the 90s of the last century, starting from solid mixtures of COtwo and HtwoOR.
To this solid mixture COtwo-HtwoOr at 50% it is bombarded with protons (a type of cosmic radiation), so that neither of the two components will escape and the formation of H occurs.twoCO3. A mixture of CH has also been used for this purpose.3OH-COtwo (remember α-HtwoCO3).
Another method is to do the same but using dry ice directly, nothing more..
Of the three methods, NASA scientists were able to reach one conclusion: pure carbonic acid, solid or gaseous, can exist in the icy satellites of Jupiter, in Martian glaciers, and in comets, where such solid mixtures are constantly irradiated. by cosmic rays.
Carbonic acid by itself is a useless compound. From its solutions, however, buffers based on the HCO pairs can be prepared.3-/CO3two- or HtwoCO3/ HCO3-.
Thanks to these solutions and the action of the enzyme carbonic anhydrase, present in red blood cells, COtwo produced in respiration can be transported in the blood to the lungs, where it is finally released to be exhaled outside our body.
The bubbling of COtwo it is used to give soft drinks the pleasant and characteristic sensation that they leave in the throat when drinking them.
Likewise, the presence of HtwoCO3 has geological importance in the formation of limestone stalactites, as it slowly dissolves them until they originate their pointed finishes.
And on the other hand, its solutions can be used to prepare some metallic bicarbonates; although for this it is more profitable and easier to directly use a bicarbonate salt (NaHCO3, for example).
Carbonic acid has such a short life span under normal conditions (they estimate around 300 nanoseconds) that it is practically harmless to the environment and living beings. However, as said before, this does not imply that it cannot generate a worrying change in the pH of ocean water, affecting marine fauna..
On the other hand, the real “risk” is found in the intake of carbonated water, since the amount of COtwo dissolved in them is much higher than in normal water. However, and again, there are no studies that have shown that drinking carbonated water represents a life threatening risk; if they even recommend it to fast and fight indigestion.
The only negative effect observed in those who drink this water is the feeling of fullness, as their stomachs fill with gases. Outside of this (not to mention sodas, since they are made up of much more than just carbonic acid), it can be said that this compound is not toxic at all.
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