Final answer:
Superacids are acids stronger than 100% sulfuric acid, and their common property is to stabilize carbocations. Acid strength is generally measured by the extent of ionization, with strong acids being completely ionized in solutions. The leveling effect in water necessitates using less basic solvents to compare strengths of strong acids, and pKa values help compare polyprotic acids to superacids.
Step-by-step explanation:
Superacids are indeed characterized as acids stronger than pure sulfuric acid. A common property of superacids is their ability to stabilize carbocations, which is indicative of the acid's strength. However, the strength of an acid is not solely determined by its ability to stabilize carbocations, but it is also often equated to the extent to which it can donate a proton (H+), and this forms the basis for comparing different acids.
The ionization in water of an acid is what categorizes it as strong or weak. A strong acid is completely ionized in solution, while a weak acid is only partially ionized. When it comes to comparing the strength of acids, factors such as the solvent used can affect the ionization and therefore the measured strength, due to the leveling effect.
Essentially, in water, all strong acids are completely ionized and exist primarily as hydronium ions (H3O+), thus a less basic solvent than water is necessary to observe the differences in their acid strength.
For polyprotic acids like H₂SO₄ (sulfuric acid), they lose protons in a stepwise manner, and the fully protonated species is the strongest acid. As protons are removed, the acid strength decreases, and the pKa value increases, so comparing the strength of such acids to superacids can be done through their pKa values and the extent of their ionization in the right solvent.