What is pKa?
pKa is a value that indicates how weak or powerful an acid is in simple terms. The pKa of a strong acid is less than zero. pKa is the negative log base ten of the Ka value, to be precise (acid dissociation constant). It determines an acid’s strength, or how tightly a proton is held by a Bronsted acid. The lower the pKa value, the more powerful the acid and its capacity to transfer protons.
The concentration of hydrogen ions in an aqueous solution is measured by pH. The higher the hydrogen ion concentration in a solution, the lower the pH value; thus, the stronger the acid. pKa and pH are linked because pKa aids in predicting what a molecule will do at a particular pH. In a nutshell, pKa indicates what pH a chemical species must have in order to donate or absorb a proton.
Table of Contents
pka and Acidity
The molecular structures of acids and bases have a direct impact on pH and pKa values. Chemists can also use pKa to determine acidity that isn’t dependent on the amount of acid present. A substance’s pKa value indicates how many acidic ions it will produce in a solution.
Assume you have two solutions, one with the same number of sulfuric acid molecules and the other with citric acid molecules. Because sulfuric acid loses more protons to form more hydronium (H3O+) ions, the solution will be more acidic. The lower the pKa, similar to pH, the stronger the acid. As a result, the pKa of sulfuric acid is lower than that of citric acid.
Acids and their conjugate bases are in a state of equilibrium. The stability of the base determines the acid’s strength (pKa). When a proton departs an acid, it takes its electrons with it. Those are extremely negative, and the conjugate base has a large negative charge.
pKa Table (pKa Chart)
| Compound | Formula | Ka value | pKa value |
|---|---|---|---|
| Acetic acid | CH3COOH | 1.7 x 10 -5 | 4.75 |
| Benzoic acid | C6H5COOH | 6.3 x 10-5 | 4.20 |
| Boric acid | H3BO3 | 5.9 x 10-10 | 9.15 |
| Carbonic acid | H2CO3
HCO3– |
4.3 x 10-7
4.8 x 10-11 |
6.35
10.33 |
| Cyanic acid | HOCN | 3.5 x 10-4 | 3.46 |
| Formic acid | HCOOH | 1.7 x 10-4 | 3.75 |
| Hydrocyanic acid | HCN | 4.9 x 10-10 | 9.3 |
| Hydrofluoric acid | HF | 6.8 x 10-4 | 3.20 |
| Sulphuric acid | H2SO4
HSO4– |
strong
1.1 x 10-2 |
-3
1.99 |
| Hydrogen sulphide | H2S
HS– |
8.9 x 10-8
1.2 x 10-19 |
7.05
19 |
| Nitrous acid | HNO2 | 4.5 x 10-4 | 3.25 |
| Oxalic acid | H2C2O4
HC2O4– |
5.6 x 10-2
5.1 x 10-5 |
1.2
4.2 |
| Water | H2O | 10-14 | 14 |
| Alkyne | CH☰CH | 10-25 | 25 |
| Amine | NH3 | 10-35 | 35 |
| Alkane | CH4 | 10-50 | 50 |
| Phenol | C6H5OH | 10-10 | 10 |
| Phosphoric acid | H3PO4
H2PO4– HPO42- |
6.9 x 10-3
6.2 x 10-8 4.8 x 10-13 |
2.16
7.21 12.32 |
| Sulfurous acid | H2SO3
HSO3– |
1.3 x 10-2
6.3 x 10-8 |
1.85
7.2 |
Frequently Asked Questions on pka Table
How pKa value is related to acidity?
The pKa value is one method used to indicate the strength of an acid. pKa is the negative log of the acid dissociation constant or Ka value. A lower pKa value indicates a stronger acid.
Does pKa change with pH?
pKa is an equilibrium constant. pH is an indication of hydrogen ion content in a solution. Any changes to pH will therefore affect one of the factors in the pKa equation. The amount and direction of change in the pKa value will depend on whether the H+ ions are part of the reactant or product side of the equation.
Why are larger atoms more acidic?
As the oxidation state of the central atom becomes larger, the acidity of the molecule increases. An atom becomes more electronegative as the oxidation number increases.
What is the pKa of an acid?
The pKa measures how tightly a proton is held by a Bronsted acid. A pKa may be a small, negative number, such as -3 or -5. It may be a larger, positive number, such as 30 or 50. The lower the pKa of a Bronsted acid, the more easily it gives up its proton.
Is HCl a strong acid?
HCl is a strong acid because it dissociates almost completely. By contrast, a weak acid like acetic acid (CH3COOH) does not dissociate well in water; many H+ ions remain bound-up within the molecule. The stronger the acid the more free H+ ions are released into solution.
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