Sulfuric acid is made by hydrating sulfur dioxide with water, and is completely soluble in water. When combined, the two substances interact through various mechanisms, releasing energy. Sulfuric acid can ionize itself through self-protolysis, forming hydrogen bonds with water. Care must be taken when combining the two, as it can lead to violent reactions. The combination is used in various industries.
One method of preparing sulfuric acid is to hydrate a molecule of sulfur dioxide (SO3) with a molecule of water (H2O). The reaction is SO3 + H2O → H2SO4. Clearly, sulfuric acid has a close relationship with water; it is also completely soluble in water. Combining concentrated sulfuric acid with water is potentially dangerous as the reaction can be violent. That’s because the two substances interact — not just through one or two mechanisms — but through a number of mechanisms, each adding incrementally to the total energy release.
Pure sulfuric acid is not just polar; it can actually ionize itself through the transfer of protons, a process known as “self-protolysis.” This ionization reaction is written 2 H2SO4 → (H3SO4)+(HSO4)-. This phenomenon makes it easier for sulfuric acid and water to form hydrogen bonds. Thus, energy in the form of heat enters the surrounding system.
Wherever hydrogen, nitrogen, oxygen and fluorine atoms are available to do so, weak bonds called “hydrogen bonds” can form. In the case of sulfuric acid, one or two hydrogen ions can leave the acid to associate with nearby water molecules. Such positively charged “hydronium” (H3O+) ions are easily formed because the oxygen atoms of water molecules provide electron-rich environments to which hydrogen ions are attracted. The geometry of a hydronium ion is more symmetrical than a water molecule. This allows for an even distribution of charge, adding to the energy release in the system when sulfuric acid and water are combined.
Yet another symmetry aided release of energy is the formation of the doubly charged sulfate anion (SO4-2). The two free electrons can reside on any of the four oxygen atoms. Like charges repel and thus release energy to the system if they can escape to the outer regions of the ion; it is clear that this action releases energy while the converse, bringing together similar charges, requires energy. The high dielectric constants of sulfuric acid and water allow for a high degree of charge shielding when combined. Add to this the additional stabilization enabled by additional layers of water surrounding the innermost hydrogen-bonded layer.
For the above reasons, care must be taken when combining concentrated sulfuric acid and water. The acid should be added gradually to the mixing water, rather than the other way around. This prevents excessive heat release, which leads to sudden boiling with the violent expulsion of acid on the skin or into the eyes. The combination of sulfuric acid and water is applied in the production of fertilizers, steel production, bleaching of Kraft pulp and automotive batteries.
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