Strong acid

A strong acid is an acid that dissociates completely in an aqueous solution (not in the case of sulfuric acid as it is diprotic), or in other terms, with a pK_a < −1.74. This generally means that in aqueous solution at standard temperature and pressure, the concentration of hydronium ions is equal to the concentration of strong acid introduced to the solution. While strong acids are generally assumed to be the most corrosive, this is not always true. The carborane superacid (H(CHB₁₁Cl₁₁), which is one million times stronger than sulfuric acid, is entirely non-corrosive, whereas the weak acid hydrofluoric acid (HF) is extremely corrosive and can dissolve, among other things, glass and all metals except iridium. The equation for complete dissociation of an acid in aqueous solution is as follows:

HA(aq) → H⁺(aq) + A⁻(aq)

In all other acid-water reactions, dissociation is not complete, so will be represented as an equilibrium, not a completed reaction. The typical definition of a weak acid is any acid that does not dissociate completely. The difference separating the acid dissociation constants of strong acids from all other acids is so small that this is a reasonable demarcation.

Due to the complete dissociation of strong acids in aqueous solution, the concentration of hydronium ions in the water is equal to the re-duplication of the acid introduced to solution: [HA] = [H⁺] = [A⁻]; pH = −log[H+].

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Determining Acid Strength

The strength of an acid, in comparison to other acids, can be determined without the use of pH calculations by observing the following characteristics:

1. Electronegativity: The higher the EN of a conjugate base in the same period, the less acidic.

2. Atomic Radius: With increasing atomic radius, acidity also increases. For example, HCl and HI, both strong acids, ionize 100% in water to become their respective ionic constituents. However, HI is stronger than HCl. This is because the atomic radius of an atom of iodine is much larger than that of a chlorine atom. As a result, the negative charge over the I^- anion is dispersed over a larger electron cloud and it's attraction for the proton (H⁺) is not as strong as the same attraction in HCl. Therefore, HI is ionized (deprotonated) more readily.

3. Charge: The more positively charged a species is, the more acidic (neutral molecules can be stripped of protons more easily than anions, and cations are more acidic than comparable molecules).

Some Common Strong Acids (As Ionizers)

(Strongest to the weakest)

• Perchloric acid HClO₄
• Hydroiodic acid HI
• Hydrobromic acid HBr
• Hydrochloric acid HCl
• Sulfuric acid H₂SO₄ (K_a1/first dissociation only)
• Nitric acid HNO₃
• Hydronium ion H₃O⁺ or H⁺. For purposes of simplicity, H₃O⁺ is often replaced in a chemical equation with H⁺. However, it should be noted that a bare proton simply does not exist in water but instead is bound to one of the lone pairs of electrons on the H₂O molecule. This creates the hydronium ion and gives its single O atom a formal charge of +1.

• Some chemists include chloric acid (HClO₃), bromic acid (HBrO₃), perbromic acid (HBrO₄), iodic acid (HIO₃), and periodic acid (HIO₄) as strong acids, although these are not universally accepted.

Extremely Strong Acids (As Ionizers)

(Strongest to weakest)

• Fluoroantimonic acid HFSbF₅
• Magic acid FSO₃HSbF₅
• Carborane superacid H(CHB₁₁Cl₁₁)
• Fluorosulphuric acid FSO₃H
• Triflic acid CF₃SO₃H

See also

• Strong base
• Weak acid

References

• Hill, John W., et al. "General Chemistry." 4th ed. New Jersey: Prentice Hall, 2005.