The disulfuric acid is one of the many sulfur oxoacids, whose chemical formula is HtwoStwoOR7. The formula explicitly states that there are two sulfur atoms, and as its name also indicates, there must be two molecules of HtwoSW4 only one; however, there are seven oxygens instead of eight, so a sulfur must have three.
This acid is 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.
The image shows a container with oleum, with a concentration of sulfur trioxide, SO3, at 20%. This substance is characterized by being oily and colorless; although it may turn yellowish or brown depending on the SO concentration3 and the presence of some impurities.
The above relates to the participation of the SO3 in the HtwoStwoOR7; that is, a molecule of HtwoSW4 react with a SO3 to form disulfuric acid. This being the case then, the HtwoStwoOR7 can also be written as HtwoSW4·SW3.
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Above is the structure of HtwoStwoOR7 in a model of spheres and bars. 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 HO3SOSO3H.
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 HtwoSW4·SW3. However, this refers more to a composition of oleum than to the structure of a molecule..
However, if a water molecule is added to HtwoStwoOR7, two molecules of H will be releasedtwoSW4:
HtwoStwoOR7 + HtwoOR <=> 2HtwoSW4
The equilibrium can shift to the left: to the formation of HtwoStwoOR7 from HtwoSW4 if heat is supplied to it. It is for this reason that at HtwoStwoOR7 it is also known as pyrosulfuric acid; as it can be generated after heating sulfuric acid.
Also, as mentioned at the beginning, the HtwoStwoOR7 is formed by the direct combination of HtwoSW4 And so3:
SW3 + HtwoSW4 <=> HtwoStwoOR7
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 HtwoSW4XSO3, where depending on the value of x you can even have polymeric structures.
One way to visualize it is to imagine that the upper structure of HtwoStwoOR7 becomes more elongated by more units of SO3 and oxygen bridges.
In an almost pure state it consists of an unstable (fuming) crystalline solid 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, KtwoStwoOR7.
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).
With this compound in the form of oleum, a curious reaction can be carried out: the transformation of sugar cubes into carbon blocks.
The HtwoStwoOR7 reacts with sugars eliminating all their OH groups in the form of water, which favors new bonds between carbon atoms; joints that end up causing black carbon.
Disulfuric acid is considered as the anhydride of sulfuric acid; that is, 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 the HtwoStwoOR7; 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
Disulfuric acid, given its property of solidifying like oleum, is able to dissolve and store sulfuric acid more safely. This is because the OS3 escapes creating an unbreathable "mist", which is much more soluble in HtwoSW4 than in water. To get the HtwoSW4, It would be enough to add to the oleum water:
HtwoStwoOR7 + HtwoOR <=> 2HtwoSW4
From the above equation, the HtwoSW4 it is diluted in the added water. For example, if you have an aqueous solution of HtwoSW4, where the SO3 It 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 is added3, which reacts with HtwoSW4 to produce more disulfuric acid or oleum, and then this is rehydrated by "drying" the HtwoSW4. 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.
It is used to sulfonate structures, such as those of dyes; that is, add -SO groups3H, which by losing its acidic proton can anchor itself to the polymer of the textile fiber.
On the other hand, the acidity of oleum is used to achieve the second nitration (add -NO groupstwo) to aromatic rings.
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