Strong acid: Difference between revisions

Content deleted Content added

Inline

Revision as of 00:57, 16 April 2010

A strong acid is an acid that dissociates completely in an aqueous solution by losing one proton, according to the equation

HA(aq) → H⁺(aq) + A⁻(aq)

For sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first proton

H<superacid Chemistry, 2nd ed., Wiley, p. 41.</ref>^[1] 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^{[citation needed]}.

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⁺].

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:

Electronegativity: The higher the EN of a conjugate base in the same period, the more acidic.
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 its attraction for the proton (H⁺) is not as strong as the same attraction in HCl.

Some common strong acids (as defined above)

(Strongest to the weakest)

Hydroiodic acid HI (pK_a = −9.3)^[2]
Hydrobromic acid HBr (pK_a = −8.7)^[2]
Perchloric acid HClO₄ (pK_a ≈ −8)^[3]
Hydrochloric acid HCl (pK_a = −6.3)^[2]
Sulfuric acid H₂SO₄ (first dissociation only, pK_a1 ≈ −3)^[3]
p-Toluenesulfonic acid (pK_a = −2.8) Organic soluble strong acid

Almost strong acids

These do not meet the strict criterion of being more acid than H₃O⁺, although in very dilute solution they dissociate almost completely, so sometimes they are included as "strong acids"

Hydronium ion H₃O⁺ (pK_a = -1.74). For purposes of simplicity, H₃O⁺ is often replaced in a chemical equation with H⁺. However, it should be noted that a bare proton does not exist free in water but instead is bound to one of the lone pairs of electrons on the H₂O molecule.
Nitric acid HNO₃ (pK_a = -1.64)^[3]
Chloric acid HClO₃ (pK_a = -1.0)^[3]
Some chemists include bromic acid (HBrO₃)^{[citation needed]}, perbromic acid (HBrO₄)^{[citation needed]}, iodic acid (HIO₃)^{[citation needed]}, and periodic acid (HIO₄)^{[citation needed]} as strong acids, although these are not universally accepted.

Extremely strong acids (as protonators)

(Strongest to weakest)

Fluoroantimonic acid H[SbF₆]
Magic acid FSO₃HSbF₅
Carborane superacid H(CHB₁₁Cl₁₁)
Fluorosulfuric acid FSO₃H
Triflic acid CF₃SO₃H

References

^ That is, the ability of the carborane superacid to protonate a given base (B) is one million times greater than a solution of sulfuric acid, so that the ratio [BH⁺] / [B] is one million times higher. The relative acidities of strong acids can be evaluated using the Hammett acidity function.
^ ^a ^b ^c William L. Jolly "Modern Inorganic Chemistry" (McGraw-Hill, 1984), p.177
^ ^a ^b ^c ^d Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. p. 171. ISBN 978-0-13-039913-7.

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

External links

http://www.cm.utexas.edu/academic/courses/Spring2002/CH301/McDevitt/strong.htm
http://jchemed.chem.wisc.edu/Journal/Issues/2000/Jul/abs849.html
Titration of acids - freeware for data analysis and simulation of potentiometric titration curves
Acids and Bases - definitions

[[pt:Áci

[1] That is, the ability of the carborane superacid to protonate a given base (B) is one million times greater than a solution of sulfuric acid, so that the ratio [BH⁺] / [B] is one million times higher. The relative acidities of strong acids can be evaluated using the Hammett acidity function.

[Jolly-2] William L. Jolly "Modern Inorganic Chemistry" (McGraw-Hill, 1984), p.177

[Housecroft-3] Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. p. 171. ISBN 978-0-13-039913-7.

[1]

[2]

[3]

@@ Line 3: / Line 3: @@
 :HA(aq) → H<sup>+</sup>(aq) + A<sup>−</sup>(aq)
 For [[sulfuric acid]] which is [[diprotic acid|diprotic]], the "strong acid" designation refers only to dissociation of the first proton
+:H<superacid Chemistry'', 2nd ed., Wiley, p.&nbsp;41.</ref><ref>That is, the ability of the carborane superacid to protonate a given base (B) is one million times greater than a solution of sulfuric acid, so that the ratio [BH<sup>+</sup>] / [B] is one million times higher. The relative acidities of strong acids can be evaluated using the [[Hammett acidity function]].</ref> 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]]{{Citation needed|date=January 2010}}.
-:H<sub>2</sub>SO<sub>4</sub>(aq) → H<sup>+</sup>(aq) + HSO<sub>4</sub><sup>−</sup>(aq)
-More precisely, the acid must be stronger in aqueous solution than [[hydronium]] ion, so strong acids are acids with a [[acid dissociation constant|p''K''<sub>a</sub>]] > −1.74 (eg. -5.3). 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.  While strong acids are generally assumed to be the most [[corrosive]], this is not always true<!-- (why not?)-->.  The [[carborane]] [[superacid]] H(CHB<sub>11</sub>Cl<sub>11</sub></sub>), which is one million times stronger than [[sulfuric acid]],<ref>George A. Olah, et. al. ''Superacid Chemistry'', 2nd ed., Wiley, p.&nbsp;41.</ref><ref>That is, the ability of the carborane superacid to protonate a given base (B) is one million times greater than a solution of sulfuric acid, so that the ratio [BH<sup>+</sup>] / [B] is one million times higher. The relative acidities of strong acids can be evaluated using the [[Hammett acidity function]].</ref> 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]]{{Citation needed|date=January 2010}}.
 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 constant]]s of strong acids from all other acids is so small that this is a reasonable demarcation.
@@ Line 68: / Line 66: @@
 [[nl:Sterk zuur]]
 [[ja:強酸]]
-[[pt:Ácido forte]]
+[[pt:Áci
 [[fi:Vahva happo]]
 [[sv:Stark syra]]