Strong acid
Encyclopedia
A strong acid is an acid
that ionizes
completely in an aqueous solution
by losing one proton
, according to the equation
For sulfuric acid
which is diprotic
, the "strong acid" designation refers only to dissociation of the first proton
More precisely, the acid must be stronger in aqueous solution than hydronium
ion, so strong acids are acids with a pKa
< −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. 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 e corrosive and can dissolve, among other things, glass and all metals except iridium
.
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.
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: [H+] = [A−] = [HA]total and pH = −log[H+].
calculations by observing the following characteristics:
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
that ionizes
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
completely in an aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
by losing one proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
, according to the equation
- HA(aq) → H+(aq) + A−(aq)
For sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
which is diprotic
Diprotic acid
A diprotic acid is an acid such as H2SO4 that contains within its molecular structure two hydrogen atoms per molecule capable of dissociating in water. The complete dissociation of diprotic acids is of the same form as sulfuric acid:The dissociation does not happen all at once due to the two...
, 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
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
ion, so strong acids are acids with a pKa
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
< −1.74. An example is HCl
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
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. While strong acids are generally assumed to be the most corrosive, this is not always true. The carborane
Carborane
A carborane is a cluster composed of boron and carbon atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc...
superacid
Superacid
According to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure...
H(CHB11Cl11), which is one million times stronger than sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, is entirely non-corrosive, whereas the weak acid
Weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...
hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
(HF) is e corrosive and can dissolve, among other things, glass and all metals except iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
.
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
Weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...
is any acid that does not dissociate completely. The difference separating the acid dissociation constant
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
s 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 total concentration (ionized and un-ionized) of the acid introduced to solution: [H+] = [A−] = [HA]total and pH = −log[H+].
Determining acid strength
The strength of an acid, in comparison to other acids, can be determined without the use of pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
calculations by observing the following characteristics:
- Electronegativity: The higher the electronegativityElectronegativityElectronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
of a conjugate base in the same period, the more acidic. In other words, the less electronegative A- is, more acidic (where HA -> H+ + A-). - 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. Therefore, HI is ionized (deprotonated) more readily.
- 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).
Common strong acids
This is a list of strong acids with pKa < -1.74, which is stronger than hydronium ion, from strongest to weakest.- Hydroiodic acid HI (pKaAcid dissociation constantAn acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
= −9.3) - Hydrobromic acidHydrobromic acidHydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at 124.3 °C and contains 47.6% HBr by weight, which is 8.89 mol/L. Hydrobromic acid has a pKa of −9, making it a...
HBr (pKa = −8.7) - Perchloric acidPerchloric acidPerchloric acid is the inorganic compound with the formula HClO4. Usually encountered as an aqueous solution, this colourless compound is a strong acid comparable in strength to sulfuric and nitric acids. It is a powerful oxidizer, but its aqueous solutions up to appr. 70% are remarkably inert,...
HClO4 (pKa ≈ −8) - Hydrochloric acidHydrochloric acidHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
HCl (pKa = −6.3) - Sulfuric acidSulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
H2SO4 (first dissociation only, pKa1 ≈ −3) - p-Toluenesulfonic acidP-Toluenesulfonic acidp-Toluenesulfonic acid or tosylic acid is an organic compound with the formula CH3C6H4SO3H. It is a white solid that is soluble in water, alcohols, and other polar organic solvents. The 4-CH3C6H4SO2- group is known as the Tosyl group and is often abbreviated as Ts or Tos...
(pKa = −2.8) Organic soluble strong acid
Almost strong acids
These do not meet the strict criterion of being more acidic than H3O+, although in very dilute solution they dissociate almost completely, so sometimes they are included as "strong acids"- Hydronium ion H3O+ (pKa = -1.74). For purposes of simplicity, H3O+ is often replaced in a chemical equation with H+. However, 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 acidNitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
HNO3 (pKa = -1.64) - Chloric acidChloric acidChloric acid, HClO3, is an oxoacid of chlorine, and the formal precursor of chlorate salts. It is a strong acid and oxidizing agent....
HClO3 (pKa = -1.0) - Some chemists include bromic acidBromic acidBromic acid, also known as hydrogen bromate, is an oxoacid that only exists in aqueous solution. It is a colorless solution that turns yellow at room temperature as it decomposes to bromine. Bromic acid and bromates are powerful oxidizing agents and are common ingredients in Belousov-Zhabotinsky...
(HBrO3), perbromic acidPerbromic acidThe compound perbromic acid is the inorganic compound with the formula HBrO4. It is an oxoacid of bromine. Perbromic acid is unstable and cannot be formed by displacement of chlorine from perchlorate, as periodates are prepared. It can only be made by protonation of the perbromate ion.Perbromic...
(HBrO4), iodic acidIodic acidIodic acid, HIO3, can be obtained as a white solid. It dissolves in water very well, but it also exists in the pure state, as opposed to chloric acid or bromic acid. Iodic acid contains iodine in the oxidation state +5 and it is one of the most stable oxo-acids of the halogens in its pure state....
(HIO3), and periodic acidPeriodic acidPeriodic acid, or iodic acid is an oxoacid of iodine having chemical formula HIO4 or H5IO6.In dilute aqueous solution, periodic acid exists as discrete hydronium and metaperiodate ions. When more concentrated, orthoperiodic acid, H5IO6, is formed; this dissociates into hydronium and...
(HIO4) as strong acids, although these are not universally accepted.
Extremely strong acids (as protonators)
(Strongest to weakest)- Fluoroantimonic acidFluoroantimonic acidFluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2....
H[SbF6] - Magic acidMagic acidMagic acid , is a superacid consisting of a mixture, most commonly in a 1:1 molar ratio, of fluorosulfonic acid and antimony pentafluoride...
FSO3HSbF5 - Carborane superacid H(CHB11Cl11)
- Fluorosulfuric acidFluorosulfuric acidFluorosulfuric acid is the inorganic compound with the formula HSO3F. It is one of the strongest acids commercially available and is a superacid. The formula HFSO3 emphasizes its relationship to sulfuric acid, H2SO4; HSO3F is a tetrahedral molecule.-Chemical properties:Fluorosulfuric acid is a...
FSO3H - Triflic acid CF3SO3H
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