A strong acid is one that is completely dissociated or ionized in an aqueous solution. It is a chemical species with a high capacity to lose a proton, H+. In water, a strong acid loses one proton, which is captured by water to form the hydronium ion:
HA(aq) + H2O → H3O+(aq) + A−(aq)
Diprotic and polyprotic acids may lose more than one proton, but the "strong acid" pKa value and reaction refer only to the loss of the first proton.
Strong acids have a small logarithmic constant (pKa) and a large acid dissociation constant (Ka).
Most strong acids are corrosive, but some of the superacids are not corrosive. In contrast, some of the weak acids (e.g., hydrofluoric acid) can be highly corrosive.
As acid concentration increases, the ability to dissociate diminishes. Under normal conditions in water, strong acids dissociate completely, but extremely concentrated solutions do not.
Examples of Strong Acids
While there are many weak acids, there are few strong acids. The common strong acids include:
- HCl (hydrochloric acid)
- H2SO4 (sulfuric acid)
- HNO3 (nitric acid)
- HBr (hydrobromic acid)
- HClO4 (perchloric acid)
- HI (hydroiodic acid)
- p-toluenesulfonic acid (an organic soluble strong acid)
- methanesulfonic acid (a liquid organic strong acid)
The following acids dissociate almost completely in water, so they are often considered to be strong acids, although they are not more acidic than the hydronium ion, H3O+:
- HNO3 (nitric acid)
- HClO3 (chloric acid)
Some chemists consider the hydronium ion, bromic acid, periodic acid, perbromic acid, and periodic acid to be strong acids.
If the ability to donate protons is used as the primary criterion for acid strength, then the strong acids (from strongest to weakest) would be:
- HSbF6 (fluoroantimonic acid)
- FSO3HSbF5 (magic acid)
- H(CHB11Cl11) (carborane superacid)
- FSO3H (fluorosulfuric acid)
- CF3SO3H (triflic acid)
These are the "superacids," which are defined as acids that are more acidic than 100% sulfuric acid. The superacids permanently protonate water.
Factors Determining Acid Strength
You may be wondering why the strong acids dissociate so well, or why certain weak acids do not completely ionize. A few factors come into play:
- Atomic radius: As the atomic radius increases, so does acidity. For example, HI is a stronger acid than HCl (iodine is a larger atom than chlorine).
- Electronegativity: The more electronegative a conjugate base in the same period of the periodic table is (A-), the more acidic it is.
- Electrical charge: The more positive the charge on an atom, the higher its acidity. In other words, it's easier to take a proton from a neutral species than from one with a negative charge.
- Equilibrium: When an acid dissociates, equilibrium is reached with its conjugate base. In the case of strong acids, the equilibrium strongly favors the product or is to the right of a chemical equation. The conjugate base of a strong acid is much weaker than water as a base.
- Solvent: In most applications, strong acids are discussed in relation to water as a solvent. However, acidity and basicity have meaning in nonaqueous solvent. For example, in liquid ammonia, acetic acid ionizes completely and may be considered a strong acid, even though it is a weak acid in water.