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The strength of an acid refers to its ability or tendency to lose a proton (H+). A strong acid is one that completely ionizes (dissociates) in a solution. In water, one mole of a strong acid HA dissolves yielding one mole of H+ (as hydronium ion H3O+) and one mole of the conjugate base, A−. Essentially, none of the non-ionized acid HA remains. Examples of strong acids are hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid (HBr), perchloric acid (HClO4), nitric acid (HNO3) and sulfuric acid (H2SO4). In aqueous solution, each of these essentially ionizes 100%.Examples of strong acids are hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid (HBr), perchloric acid (HClO4), nitric acid (HNO3) and sulfuric acid (H2SO4). In aqueous solution, each of these essentially ionizes 100%. In contrast, a weak acid only partially dissociates. In contrast, a weak acid only partially dissociates. Examples in water include carbonic acid (H2CO3) and acetic acid (CH3COOH). At equilibrium, both the acid and the conjugate base are present in solution. Stronger acids have a larger acid dissociation constant (''K''a) and a smaller logarithmic constant (p''K''a = - 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 determines the strength of the H—A bond. Acid strengths also depend on the stability of the conjugate base. While ''K''a measures the strength of an acidic molecule, the strength of an aqueous acid solution is measured by pH, which is a function of the concentration of hydronium ions in solution. The pH of a simple solution of an acid in water is determined by both ''K''a and the acid concentration. For weak acid solutions, it depends on the degree of dissociation, which may be determined by an equilibrium calculation. For concentrated solutions of strong acids with a pH less than about zero, the Hammett acidity function is a better measure of acidity than the pH. Sulfonic acids, which are organic oxyacids, are a class of strong acids. A common example is p-toluenesulfonic acid (tosylic acid). Unlike sulfuric acid itself, sulfonic acids can be solids. In fact, polystyrene functionalized into polystyrene sulfonate is a solid strongly acidic plastic that is filterable. Superacids are acid solutions that are more acidic than 100% sulfuric acid.〔Miessler G.L. and Tarr D.A. ''Inorganic Chemistry'' (2nd ed., Prentice-Hall 1998, p.170) ISBN 0-13-841891-8〕 Examples of superacids are fluoroantimonic acid, magic acid and perchloric acid. Superacids can permanently protonate water to give ionic, crystalline hydronium "salts". They can also quantitatively stabilize carbocations. ==Strong acids in water== In aqueous solution, a strong acid is an acid that ionizes completely by losing one proton, according to the equation :HA(aq) → H+(aq) + A−(aq) The proton lost is captured by a water molecule to form a hydronium ion, so that the equation is more correctly written as :HA(aq) + H2O → H3O+(aq) + A−(aq) For sulfuric acid, which is diprotic, the "strong acid" designation refers only to dissociation of the first proton :H2SO4(aq) → H+(aq) + HSO4−(aq) More precisely, the acid must be stronger in aqueous solution than hydronium ions, so strong acids are acids with a p''K''a < −1.74. An example is HCl for which pKa = −6.3.〔 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. Due to the complete dissociation of strong acids in aqueous solution, the concentration of hydronium ions in the water is equal to the total concentration (ionized and un-ionized) of the acid introduced to solution: () = () = ()total and pH = −log(). No acid species stronger than H3O+(aq) can exist in aqueous solution, as any stronger acid is converted to H3O+. The acidity of the stronger acid is said to be leveled to the acidity of hydronium ion.〔Porterfield, William W. ''Inorganic Chemistry'' (Addison-Wesley 1984) p.260 ISBN 0-201-05660-7〕 Strong acids are distinguished from ''weak acids'' (see below), in which dissociation is incomplete and is 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 great that this is a reasonable demarcation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「acid strength」の詳細全文を読む スポンサード リンク
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