Which factors influence the strength of acids

What does the pKa value depend on?

So what does it depend on whether an acid is a strong, a medium or a weak acid?

Acid strength depending on the main group of the acid residue

If we consider the three compounds ammonia, water and hydrogen fluoride, we find the following sequence of acidity:

NH3 2O

Hydrogen fluoride is therefore the strongest acid of the three compounds, ammonia the weakest (in itself ammonia is a base!).

Nitrogen can be seen to the left of oxygen in the periodic table, fluorine to the right. Should there be a connection?

In fact, it can be stated that - at least with these three compounds - the acid strength increases with the electronegativity of the central atom. Fluorine has the greatest electronegativity, oxygen also has a very high electronegativity, while nitrogen has the lowest electronegativity when the three elements are compared.

A possible cause for this dependence could be the degree of polarity of the covalent bond between the H atom and the central atom. The HF bond is very polar because of the EN difference H / F, while the HN bond is the least polar of all three bonds.

Is this relationship valid for all acids? I can't check that here either, but at least the following applies to acids with central atoms of the third period:

PH3 2S

Here, too, the electronegativity of the elements P, S and Cl increases from left to right, i.e. within the period, as does the acidity of the three acids.

The strength of an acid of the type HX (with X = NH2, OH, F, PH2, HS, Cl and the like) increases with the electronegativity of the central atom of X.

Acid strength as a function of the period of the central atom

Let us now consider acids whose central atoms are in the same group of the periodic table. Does the atom size or the number of electron shells - i.e. the period in the periodic table - have an influence on the acid strength?

If you look at the sixth group with the elements oxygen, sulfur, selenium and tellurium, you can see the following sequence of acid strength for the hydrogen compounds:

H2O 2S 2Se 2Te

There is obviously a connection here! The same phenomenon can be observed with acids with a central atom from the seventh main group:

HF

Why does the acidity increase with the size of the atom? The electronegativity decreases from top to bottom within a group. Fluorine, for example, is the most electronegative element not only of the seventh main group, but of the entire periodic table. Shouldn't HF also be the strongest acid?

The strength of a covalent bond is significantly influenced by two factors: the EN difference and the bond length.

A large EN difference means that the two bonding electrons of the covalent HX bond are more likely to be found on the X atom than on the H atom. The heterolytic cleavage of the HX bond is therefore favored by a high EN difference.

In the case of a large atom, the force of attraction of the atomic nucleus is shielded by the many electron shells. The larger the atom, the less noticeable the positive charge on the nucleus is. Splitting an H atom from a large X is much easier than separating an H atom from a small X.. It is therefore much easier to split off a proton from hydrogen iodide than from hydrogen fluoride.

For experts:
With large central atoms X "the valence electron cloud is distributed over a larger space and the H+-Ion therefore less firmly integrated "(Mortimer, Müller, Chemie, Thieme-Verlag, 9th edition).

Which influence is greater?

The example of hydrogen fluoride shows us that the size of the central atom X has the greater influence on the acid strength. The EN difference is greatest with HF, but HF is a weaker acid than HCl or even HBr, precisely because the central atoms are much larger here.

And what about the strength of oxo acids?

Oxo acids are acids in which the H atom to be released is attached to an oxygen atom. Sulfuric acid, nitric acid, phosphoric acid and carbonic acid are among the better-known representatives of the oxo acids.

Since the H atom is bound to an O atom in all oxo acids, there should actually be no differences in acid strength, because all O atoms have the same electronegativity and the same radius.

However, the O atom is itself connected to other atoms. In sulfuric acid, for example, the O atom is linked to an S atom, in phosphoric acid with a P atom, in nitric acid with an N atom and in carbonic acid with a C atom. The comparison of the very weak carbonic acid with the strong nitric acid suggests that this "next but one" atom definitely has an influence on the acid strength.

However, another factor has a much greater influence, which we cannot (yet) explain here because we lack the knowledge. It is about resonance structures (mesomerism) of these connections, so that the electrons of the bonds can be better distributed throughout the molecule. The negative acid residue ions of some oxo acids become more stable than the acid residue ions of other oxo acids, and this has a very significant influence on the acid strength. If an acid residue is stable, it is also formed more easily, which means that the acid gives up its proton more easily and is therefore stronger.

Another example comes from the chemistry of aromatics. Cyclopentadiene has a pKa of 16, making it one of the most acidic hydrocarbons. This relatively high acid strength (for a hydrocarbon!) Is due to the fact that the cyclopentadiene anion, which is formed when a proton is given off, acquires an aromatic character. The cyclopentadiene molecule was not aromatic, but the anion formed is aromatic - in other words: energetically much cheaper than the molecule. For details, see "The Aromatic State: Example Cyclopentadiene".