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One method of preparing sulfuric acid is by hydrating a molecule of sulfur trioxide (SO3) with a molecule of water (H2O). The reaction is SO3 + H2O → H2SO4. Clearly, sulfuric acid bears a close connection to water; in addition, it is completely soluble in water. Combining concentrated sulfuric acid with water is potentially dangerous as the reaction can be violent. This is because the two substances interact — not through only one or two mechanisms — but through a host of mechanisms, each adding incrementally to the total energy release.
Pure sulfuric acid is not just polar; it can actually ionize itself through proton transfer, a process known as "auto-protolysis." This ionization reaction is written 2 H2SO4 → [H3SO4]+[HSO4]-. This phenomenon makes it easier for sulfuric acid and water to form hydrogen bonds. In so doing, energy in the form of heat enters the surrounding system.
Wherever there are hydrogen, nitrogen, oxygen and fluorine atoms available to do so, weak bonds called "hydrogen bonds" may form. In the case of sulfuric acid, one or two hydrogen ions may leave the acid to become associated with nearby molecules of water. Such positively charged "hydronium" ions (H3O+) readily form because the oxygen atoms of water molecules offer electron-rich environments to which hydrogen ions are drawn. The geometry of a hydronium ion is more symmetrical than a water molecule. This enables uniform charge distribution, adding to the releasing of energy into the system when sulfuric acid and water are combined.
Yet another energy release aided by symmetry is the formation of the doubly charged sulfate anion (SO4-2). The two free electrons can take up residence on any of the four oxygen atoms. Like charges repel each other, 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 as the reverse — bringing like charges together — takes energy. The high dielectric constants of sulfuric acid and water enable a high degree of charge shielding when they are combined. Add to this further stabilization enabled by additional layers of water surrounding the innermost hydrogen-bonded layer.
For the above reasons, care must be exercised when combining concentrated sulfuric acid and water. The acid should be added gradually to water being stirred, rather than the other way around. This prevents excessive heat release, leading to sudden boiling with the violent ejection of acid either onto the skin or into the eyes. The combination of sulfuric acid and water is applied in the manufacture of fertilizers, the production of steel, the bleaching of Kraft pulp and in automotive batteries.