Acid strength refers to the tendency of an acid, symbolised by the chemical formula , to dissociate into a proton, and an anion .The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions.
Examples of strong acids are hydrochloric acid (HCl), perchloric acid (HClO4), nitric acid (HNO3) and sulfuric acid (H2SO4).
A weak acid is only partially dissociated, with both the undissociated acid and its dissociation products being present, in solution, in equilibrium with each other.
Acetic acid (CH3COOH) is an example of a weak acid. The strength of a weak acid is quantified by its acid dissociation constant, pKa value.
The strength of a weak organic acid may depend on substituent effects. The strength of an inorganic acid is dependent on the oxidation state for the atom to which the proton may be attached. Acid strength is solvent-dependent. For example, hydrogen chloride is a strong acid in aqueous solution, but is a weak acid when dissolved in glacial acetic acid
The usual measure of the strength of an acid is its acid dissociation constant (Ka), which can be determined experimentally by titration methods. Stronger acids have a larger Ka and a smaller logarithmic constant (pKa = −log Ka) than weaker acids. The stronger an acid is, the more easily it loses a proton, H+. Two key factors that contribute to the ease of deprotonation are the polarity of the H—A bond and the size of atom A, which determine the strength of the H—A bond. Acid strengths also depend on the stability of the conjugate base.
While the pKa value measures the tendency of an acidic solute to transfer a proton to a standard solvent (most commonly water or DMSO), the tendency of an acidic solvent to transfer a proton to a reference solute (most commonly a weak aniline base) is measured by its Hammett acidity function, the H0 value. Although these two concepts of acid strength often amount to the same general tendency of a substance to donate a proton, the pKa and H0 values are measures of distinct properties and may occasionally diverge. For instance, hydrogen fluoride, whether dissolved in water (pKa = 3.2) or DMSO (pKa = 15), has pKa values indicating that it undergoes incomplete dissociation in these solvents, making it a weak acid. However, as the rigorously dried, neat acidic medium, hydrogen fluoride has an H0 value of –15, making it a more strongly protonating medium than 100% sulfuric acid and thus, by definition, a superacid. (To prevent ambiguity, in the rest of this article, "strong acid" will, unless otherwise stated, refer to an acid that is strong as measured by its pKa value (pKa < –1.74). This usage is consistent with the common parlance of most practicing chemists.)
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