Everything about Acid Dissociation Constant totally explained
An
acid dissociation constant, denoted by K
a, is an
equilibrium constant for the
dissociation of a
weak acid. According to the
Brønsted-Lowry theory of
acids and
bases, an acid is a
proton donor (HA, where H represents an acidic
hydrogen atom), and a base is a proton acceptor. In
aqueous solution, water can function as a base, as in the following general example.:HA + H
2O
A
- + H
3O
+
Acid dissociation constants are also known as the
acidity constant or the
acid-ionization constant. The term is also used for pK
a, which is equal to the negative decimal
logarithm of K
a (
cologarithm of K
a).
Definitions
Monoprotic acids
When an acid, HA, dissolves in water, some molecules of the acid 'dissociate' to form
hydronium ions and the
conjugate base, (A
-), of the acid.
» , pH = pK
a + 2
It follows that the range of pH within which there's partial dissociation of the acid is about pK
a 2.This is shown graphically at the right.
A
weak acid may be
defined as an acid with pK
a greater than about -2. An acid with pK
a = -2 would be 99% dissociated at pH 0, that is, in a 1M HCl solution. Any acid with a pK
a less than about -2 is said to be a
strong acid. Strong acids are said to be fully dissociated. There is no precise pK
a value that distinguishes between strong and weak acids because strong acids, such as
sulfuric acid, are associated in very concentrated solution.
On the pK
a scale of acid strength, a large value indicates a very weak acid, and a small value indicates a not so weak one.
The pH of a solution of a weak acid can be easily calculated when the
analytical concentration of the acid is known. See
ICE table for details.
Some polyprotic acids can be treated as a set of individual acids. This is possible when successive pK values differ by 4 or more. For example with phosphoric acid
» H
3PO
4 H
2PO
4- +H
+, pK
a1 = 2.15
H
2PO
4- HPO
42- +H
+, pK
a2 = 7.20
» HPO
42- PO
43- +H
+, pK
a3 = 12.37
Both the hydrogenphosphate and dihydrogenphosphate ions can be treated as acids in their own right. On the other hand, the two pKs for malonic acid are 2.51 and 5.05, so there are pH values at which both malonic acid and the hydrogenmalonate ion co-exist. More elaborate calculations are needed to calculate the composition of solutions of malonic acid.
Factors that determine the relative strengths of acids
Being an
equilibrium constant, the acid dissociation constant
Ka is determined
by the standard free energy difference ΔG
o between the reactants and products, specifically, between the protonated (HA) and deprotonated (A
−) forms of the substance.
Pauling's second rule It can be seen in the table above that DMSO is more basic than water, but its dielectric constant is less. DMSO is widely used as an alternative to water in evaluating acids and bases.
In solvents of low dielectric constant, ions tend to associate, which complicates the interpretation of pK
as. In particular, in aprotic solvents the process of homoconjugation occurs when the conjugate base forms a
hydrogen bond with the parent acid as in the following equilibrium
» HA + A
- HA
2-
Typically HA
2- would have the structure A---H---A. This process doesn't occur in water because H
2O molecules are strong hydrogen bond donors and acceptors.
In acetonitrile solution,
para-toluenesulfonic acid has a homoconjugation constant pK
f, of -2.9. This indicates that the toluenesulfonate anion has a strong tendency to form a hydrogen bond with the parent acid. Homoconjugation has the effect of enhancing the acidity of acids, lowering their effective pK
as, by stabilizing the conjugate base. Due to homoconjugation, the proton-donating power of toluenesulfonic acid in acetonitrile solution is enhanced by a factor of nearly 800.
pKa of some common substances
Measurements are at 25ºC in water for those with a p
Ka at or above -1.76:
- 25.00: Fluoroantimonic acid
- 15.00: Magic acid
- 10.00: Fluorosulfuric acid
- 10.00: Perchloric acid
- 10.00: Hydroiodic acid
- 9.00: Hydrobromic acid
- 8.00: Hydrochloric acid
- 3.00, 1.99: Sulfuric acid
- 2.00: Nitric acid
- 1.76: Hydronium ion
3.15: Hydrofluoric acid
3.75: Formic acid
4.04: Ascorbic acid (Vitamin C)
4.19: Succinic acid
4.20: Benzoic acid
4.63: Aniline*
4.76: Acetic acid
4.76: Dihydrogencitrate ion (Citrate)
5.21: Pyridine*
6.37: Carbonic acid
6.40: Monohydrogencitrate ion Citrate
6.99: Ethylenediamine*
7.00: Hydrogen sulfide, Imidazole* (as an acid)
7.50: Hypochlorous acid
9.25: Ammonia*
9.33: Benzylamine*
9.81: Trimethylamine*
9.99: Phenol
10.08: Ethylenediamine*
10.33: Bicarbonate
10.66: Methylamine*
10.73: Dimethylamine*
10.81: Ethylamine*
11.01: Triethylamine*
11.09: Diethylamine*
11.65: Hydrogen peroxide
12.50: Guanidine*
12.67: Monohydrogenphosphate ion (Phosphate)
14.58: Imidazole (as a base)
15.76: Water
- 19.00 (pKb) Sodium amide
37.00: Lithium diisopropylamide (LDA)
45.00: Propane
50.00: Ethane
* Listed values for ammonia and amines are the pKa values for the corresponding ammonium ions.Further Information
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