- Structure of disulfuric acid
- Properties
- Sugar dehydration reaction
- Nomenclature
- Applications
- Stores sulfuric acid
- 100% sulfuric acid synthesis
- Sulfonation
- References
The disulfuric acid is one of many oxoacids of sulfur, with the chemical formula H 2 S 2 O 7. The formula explicitly indicates that there are two sulfur atoms, and as its name also indicates, there must be two H 2 SO 4 molecules in one; however, there are seven oxygens instead of eight, so a sulfur must have three.
This acid is considered one of the main components of oleum (or fuming sulfuric acid), which is a "dense" form of sulfuric acid. Such is its predominance, that some consider oleum as a synonym for disulfuric acid; which is also known by the traditional name of pyrosulfuric acid.
Oleum. Source: W. Oelen, via Wikimedia Commons
The image shows a container with oleum, with a concentration of sulfur trioxide, SO 3, at 20%. This substance is characterized by being oily and colorless; although it can turn yellowish or brown depending on the concentration of SO 3 and the presence of some impurities.
The foregoing relates the participation of SO 3 in H 2 S 2 O 7; that is, a molecule of H 2 SO 4 reacts with one of SO 3 to form disulfuric acid. This being the case, H 2 S 2 O 7 can also be written as H 2 SO 4 · SO 3.
Structure of disulfuric acid
H2S2O7 molecule. Source:. Jynto (more from this user), from Wikimedia Commons
Above is the structure of H 2 S 2 O 7 in a spheres and bars model. Note at first glance the symmetry of the molecule: the left side of the oxygen bridge is identical to the right. From that perspective, the structure can be described with the formula HO 3 SOSO 3 H.
At both ends are each hydroxyl group, whose hydrogen, by inductive effect of all these oxygen atoms, increases its positive partial charge; and consequently, disulfuric acid has an even stronger acidity than sulfuric acid.
The molecule can be written as H 2 SO 4 · SO 3. However, this refers more to a composition of oleum than to the structure of a molecule.
However, if a molecule of water is added to H 2 S 2 O 7, two molecules of H 2 SO 4 will be released:
H 2 S 2 O 7 + H 2 O <=> 2H 2 SO 4
The equilibrium can be shifted to the left: to the formation of H 2 S 2 O 7 from H 2 SO 4 if heat is supplied to it. It is for this reason that H 2 S 2 O 7 is also known as pyrosulfuric acid; since it can be generated after heating sulfuric acid.
Also, as mentioned at the beginning, H 2 S 2 O 7 is formed by the direct combination of H 2 SO 4 and SO 3:
SO 3 + H 2 SO 4 <=> H 2 S 2 O 7
Properties
The properties of disulfuric acid are not very well defined, because it is extremely difficult to isolate purely from oleum.
It should be remembered that in oleum there may be other compounds, with formulas H 2 SO 4 · xSO 3, where depending on the value of x, even polymeric structures may be present.
One way to visualize it is to imagine that the upper structure of H 2 S 2 O 7 becomes more elongated by more SO 3 units and oxygen bridges.
In an almost pure state it consists of an unstable crystalline solid (fuming) that melts at 36ºC. However, this melting point can vary depending on the true composition.
Disulfuric acid can form what are known as disulfate (or pyrosulfate) salts. For example, if it reacts with potassium hydroxide, it forms potassium pyrosulfate, K 2 S 2 O 7.
In addition, it can be said that it has a molecular weight of 178 g / mol, and two acid equivalents (2 H + ions that can be neutralized with a strong base).
Sugar dehydration reaction
With this compound in the form of oleum, a curious reaction can be carried out: the transformation of sugar cubes into carbon blocks.
H 2 S 2 O 7 reacts with sugars eliminating all their OH groups in the form of water, which favors new bonds between carbon atoms; unions that end up causing black carbon.
Nomenclature
Disulfuric acid is considered the anhydride of sulfuric acid; in other words, it loses a water molecule as a result of the condensation between two acid molecules. Given so many possible names for this compound, the IUPAC nomenclature simply recommends disulfuric acid.
The term 'pyro' refers exclusively to that formed as a product of the application of heat. The IUPAC does not advise this name for H 2 S 2 O 7; but it is still used today by many chemists.
The -ico ending remains unchanged since the sulfur atom continues to maintain a valence of +6. For example, it can be calculated using the following arithmetic operation:
2H + 2S + 7O = 0
2 (+1) + 2S + 7 (-2) = 0
S = 6
Applications
Stores sulfuric acid
Disulfuric acid, given its property of solidifying like oleum, is able to dissolve and store sulfuric acid more safely. This is because SO 3 escapes creating an unbreathable "mist", which is much more soluble in H 2 SO 4 than in water. To obtain H 2 SO 4, it would be enough to add water to oleum:
H 2 S 2 O 7 + H 2 O <=> 2H 2 SO 4
100% sulfuric acid synthesis
From the previous equation, the H 2 SO 4 is diluted in the added water. For example, if you have an aqueous solution of H 2 SO 4, where SO 3 tends to escape and represents a risk for workers, when oleum is added to the solution it reacts with the water, forming more sulfuric acid; that is, increasing your concentration.
If some water remains, more SO 3 is added, which reacts with the H 2 SO 4 to produce more disulfuric acid or oleum, and then this rehydrates by "drying" the H 2 SO 4. The process is repeated enough times to obtain sulfuric acid at a concentration of 100%.
A similar process is used to obtain 100% nitric acid. Likewise, it has been used to dehydrate other chemical substances used to make explosives.
Sulfonation
It is used to sulfonate structures, such as those of dyes; that is, it adds -SO 3 H groups, which by losing their acidic proton can anchor themselves to the polymer of the textile fiber.
On the other hand, the acidity of oleum is used to achieve the second nitration (add -NO 2 groups) to the aromatic rings.
References
- Shiver & Atkins. (2008). Inorganic chemistry. (fourth edition). Mc Graw Hill.
- Wikipedia. (2018). Disulfuric acid. Recovered from: en.wikipedia.org
- PubChem. (2019). Pyrosulfuric acid. Recovered from: pubchem.ncbi.nlm.nih.gov
- Seong Kyu Kim, Han Myoung Lee, & Kwang S. Kim. (2015). Disulfuric acid dissociated by two water molecules: ab initio and density functional theory calculations. Phys. Chem. Chem. Phys., 2015, 17, 28556
- Contra Costa Health Services. (sf). Oleum / sulfur trioxide: chemical fact sheet.. Recovered from: cchealth.org