- Structure
- Molecule
- Molecular interactions
- Pure carbonic acid
- Properties
- Synthesis
- Dissolution
- Liquid-vapor equilibrium
- Pure solid
- Applications
- Risks
- References
The carbonic acid is an inorganic compound, although some debate actually is organic, the chemical formula H 2 CO 3. It is therefore a diprotic acid, capable of donating two H + ions to the aqueous medium to generate two molecular cations H 3 O +. From it arise the well-known bicarbonate (HCO 3 -) and carbonate (CO 3 2-) ions.
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 undissolved CO 2 is observed whenever there is a bubbling in the water, rising towards the surface.
Glass with carbonated water, one of the most common beverages that contain carbonic acid. Source: Pxhere.
This phenomenon is seen very regularly in carbonated drinks and carbonated water.
In the case of carbonated or aerated water (upper image), such a quantity of CO 2 has dissolved that its vapor pressure is more than double that of atmospheric pressure. When uncapping it, the pressure difference inside the bottle and the outside decreases the solubility of CO 2, which is why bubbles appear that end up escaping from the liquid.
To a lesser degree, the same thing happens in any body of fresh or saline water: when heated they will release their dissolved CO 2 content.
However, CO 2 is not only dissolved, but undergoes transformations in its molecule that convert it into H 2 CO 3; 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.
Structure
Molecule
Carbonic acid molecule represented by a spheres and bars model. Source: Jynto and Ben Mills via Wikipedia.
Above we have the H 2 CO 3 molecule, represented by 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) 2, 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 released into the environment come from.
Molecular structure of carbonic acid.
Also note that the formula CO (OH) 2 can be written as OHCOOH; that is to say, 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.
Molecular interactions
Of the molecule H 2 CO 3 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, H 2 CO 3 has a strong tendency to interact with protic or oxygenated (and nitrogenous) solvents.
And coincidentally, water fulfills these two characteristics, and the affinity of H 2 CO 3 for it is such that almost immediately it gives up an H + and a hydrolysis equilibrium begins to be established that involves the HCO 3 - and H 3 O species. +.
That is why the mere presence of water breaks down carbonic acid and makes it too difficult to isolate it as a pure compound.
Pure carbonic acid
Returning to the H 2 CO 3 molecule, 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's 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 H 2 CO 3 consists of a crystalline structure composed of layers or sheets of molecules interacting with lateral hydrogen bonds. This is to be expected, the H 2 CO 3 molecule being flat and triangular. When it sublimates, cyclic dimers (H 2 CO 3) 2 appear, which are joined by two hydrogen bonds C = O-OH.
The symmetry of the H 2 CO 3 crystals has not been defined for the moment. It was considered to crystallize as two polymorphs: α-H 2 CO 3 and β-H 2 CO 3. However, α-H 2 CO 3, synthesized from a mixture of CH 3 COOH-CO 2, was shown to be actually CH 3 OCOOH: a monomethyl ester of carbonic acid.
Properties
It was mentioned that H 2 CO 3 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:
H 2 CO 3 (aq) + H 2 O (l) <=> HCO 3 - (aq) + H 3 O + (aq) (Ka 1 = 2.5 × 10 −4)
HCO 3 - (aq) + H 2 O (l) <=> CO 3 2- (aq) + H 3 O + (aq) (Ka 2 = 4.69 × 10 −11)
HCO 3 - is the bicarbonate or hydrogen carbonate anion, and CO 3 2- the carbonate anion. Their respective equilibrium constants, Ka 1 and Ka 2, are also indicated. Since Ka 2 is five million times smaller than Ka 1, the formation and concentration of CO 3 2- are negligible.
Thus, even though it is a diprotic acid, the second H + can barely release it appreciably. However, the presence of dissolved CO 2 in large quantities is enough to acidify the medium; in this case, water, lowering its pH values (below 7).
To speak of carbonic acid is to refer practically to an aqueous solution where the species HCO 3 - and H 3 O + predominate; it cannot be isolated by conventional methods, as the slightest attempt would shift the CO 2 solubility balance to the formation of bubbles that would escape from the water.
Synthesis
Dissolution
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 CO 2, it will bubble into the water, dissolving a small fraction of it.
When we do this the following reaction occurs:
CO 2 (g) + H 2 O (l) <=> H 2 CO 3 (aq)
But in turn, the solubility of CO 2 in water must be considered:
CO 2 (g) <=> CO 2 (aq)
Both CO 2 and H 2 O are inorganic molecules, so H 2 CO 3 is inorganic from this point of view.
Liquid-vapor equilibrium
As a result, we have an equilibrium system that is highly dependent on the partial pressures of CO 2, as well as the temperature of the liquid.
For example, if the pressure of CO 2 increases (in the case that we blow the air with more force through the straw), more H 2 CO 3 will be formed and the pH will become more acidic; since, the first equilibrium shifts to the right.
On the other hand, if we heat the H 2 CO 3 solution, the solubility of CO 2 in water will decrease because it is a gas, and the equilibrium will then shift to the left (there will be less H 2 CO 3). It will be similar if we try to apply a vacuum: the CO 2 will escape as well as the water molecules, which would shift the balance to the left again.
Pure solid
The above allows us to reach a conclusion: from a H 2 CO 3 solution 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 CO 2 and H 2 O.
This solid mixture of 50% CO 2 -H 2 O is bombarded with protons (a type of cosmic radiation), so that neither of the two components will escape and the formation of H 2 CO 3 occurs. For this purpose, a CH 3 OH-CO 2 mixture has also been used (remember α-H 2 CO 3).
Another method is to do the same but using dry ice directly, nothing more.
From 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.
Applications
Carbonic acid by itself is a useless compound. From their solutions, however, buffer solutions based on the pairs HCO 3 - / CO 3 2- or H 2 CO 3 / HCO 3 - can be prepared.
Thanks to these solutions and the action of the carbonic anhydrase enzyme, present in red blood cells, the CO 2 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 CO 2 is used to give soft drinks the pleasant and characteristic sensation that they leave in the throat when drinking them.
Likewise, the presence of H 2 CO 3 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 (NaHCO 3, for example).
Risks
Carbonic acid has such a minimal 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, that 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 CO 2 dissolved in them is much higher than in normal water. However, and again, there are no studies that have shown that drinking carbonated water poses a fatal 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.
References
- Day, R., & Underwood, A. (1989). Quantitative Analytical Chemistry (fifth ed.). PEARSON Prentice Hall.
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
- Wikipedia. (2019). Carbonic acid. Recovered from: en.wikipedia.org
- Danielle Reid. (2019). Carbonic Acid: Formation, Structure & Chemical Equation Video. Study. Recovered from: study.com
- Götz Bucher & Wolfram Sander. (2014). Clarifying the structure of carbonic acid. Vol. 346, Issue 6209, pp. 544-545. DOI: 10.1126 / science.1260117
- Lynn Yarris. (October 22, 2014). New Insights on Carbonic Acid in Water. Berkeley Lab. Recovered from: newscenter.lbl.gov
- Claudia Hammond. (2015, September 14). Is sparkling water really bad for you? Recovered from: bbc.com
- Jurgen Bernard. (2014). Solid and gaseous carbonic acid. Institute of Physical Chemistry. University of Innsbruck.