15.13: Acid Strength and Molecular Structure

Binary Acids and Bases

In the absence of any leveling effect, the acid strength of binary compounds of hydrogen with nonmetals (A) increases as the H-A bond strength decreases down a group in the periodic table. For group 17, the order of increasing acidity is HF < HCl < HBr < HI. Likewise, for group 16, the order of increasing acid strength is H₂O < H₂S < H₂Se < H₂Te. Across a row in the periodic table, the acid strength of binary hydrogen compounds increases with increasing electronegativity of the nonmetal atom because the polarity of the H-A bond increases. Thus, the order of increasing acidity (for removal of one proton) across the second row is CH₄ < NH₃ < H₂O < HF; across the third row, it is SiH₄ < PH₃ < H₂S < HCl.

Ternary Acids and Bases

Ternary compounds composed of hydrogen, oxygen, and some third element (“E”) may be structured as depicted in the image below. In these compounds, the central E atom is bonded to one or more O atoms, and at least one of the O atoms is also bonded to an H atom, corresponding to the general molecular formula O_mE(OH)_n. These compounds may be acidic, basic, or amphoteric depending on the properties of the central E atom. Examples of such compounds include sulfuric acid, O₂S(OH)₂, sulfurous acid, OS(OH)₂, nitric acid, O₂NOH, perchloric acid, O₃ClOH, aluminum hydroxide, Al(OH)₃, calcium hydroxide, Ca(OH)₂, and potassium hydroxide, KOH.

If the central atom, E, has a low electronegativity, its attraction for electrons is low. Little tendency exists for the central atom to form a strong covalent bond with the oxygen atom, and bond a between the element and oxygen is more readily broken than bond b between oxygen and hydrogen. Hence bond a is ionic, hydroxide ions are released to the solution, and the material behaves as a base—this is the case with Ca(OH)₂ and KOH. Lower electronegativity is characteristic of the more metallic elements; hence, the metallic elements form ionic hydroxides that are, by definition, basic compounds.

If, on the other hand, the atom E has a relatively high electronegativity, it strongly attracts the electrons it shares with the oxygen atom, making a relatively strong covalent bond. The oxygen-hydrogen bond, bond b, is thereby weakened because electrons are displaced toward E. Bond b is polar and readily releases hydrogen ions to the solution, so the material behaves as an acid. High electronegativities are characteristic of the more nonmetallic elements. Thus, nonmetallic elements form covalent compounds containing acidic −OH groups that are called oxyacids.

Increasing the oxidation number of the central atom E also increases the acidity of an oxyacid because this increases the attraction of E for the electrons it shares with oxygen and thereby weakens the O-H bond. Sulfuric acid, H₂SO₄, or O₂S(OH)₂ (with a sulfur oxidation number of +6), is more acidic than sulfurous acid, H₂SO₃, or OS(OH)₂ (with a sulfur oxidation number of +4). Likewise, nitric acid, HNO₃, or O₂NOH (N oxidation number = +5), is more acidic than nitrous acid, HNO₂, or ONOH (N oxidation number = +3). In each of these pairs, the oxidation number of the central atom is larger for the stronger acid.

Carboxylic Acids

Carboxylic acids contain a carboxyl group. Carboxylic acids are weak acids meaning they are not 100% ionized in water.

Carboxylic acid acts as a weak acid because, as in the case of oxyacids, the second oxygen attached to the carbon atom increases the polarity of the O-H bond and makes it weaker. Further, after the loss of the proton, the carboxyl group is converted to the carboxylate ion, which exhibits resonance. The different resonance structures stabilize the carboxylate ion as its negative charge is delocalized over several atoms.

This text is adapted from Openstax, Chemistry 2e, Section 14.3: Relative Strengths of Acids and Bases and Openstax, Chemistry 2e, Section 20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters.

Hydrochloric acid is a strong acid, whereas hydrofluoric acid is a weak acid. But what determines their strength?

The strength of binary acids, with only two elements, is determined by bond energy and bond polarity.

An acid with a higher bond energy has a stronger bond and, therefore, will be a weaker acid. Comparing acids in a group, the bond in a weak acid, like hydrofluoric acid, is harder to break, so the acid is less likely to donate protons.

In contrast, an acid with lower bond energy has a weaker bond and, therefore, will be a stronger acid. For example, the bond in a strong acid, like hydrochloric acid, breaks more easily and donates protons more readily than hydrofluoric acid.

A bond, like that in hydrochloric acid, is polar when one atom is more electronegative than the other atom.

A compound can act as an acid when the atom attached to hydrogen has a higher electronegativity than hydrogen. The atom will have a partial negative charge, allowing the hydrogen to have a partial positive charge so it can be released as a proton.

An acid with higher bond polarity has a weaker bond and, therefore, will be a stronger acid.

Comparing compounds across a period, hydrochloric acid is stronger than hydrogen sulfide as chlorine is more electronegative than sulfur and, therefore, releases protons more easily.

If hydrogen has an equal or greater electronegativity than the other atom, that molecule can not donate protons and, therefore, can not act as an acid.

Oxyacids are acids where an OH is attached to a third atom that is more electronegative than hydrogen. The strength of an oxyacid depends upon the electronegativity and the number of oxygens attached to the third atom.

The higher the electronegativity of the atom, the more it polarizes, weakening the bond between the oxygen and hydrogen.

If the central atom is attached to additional oxygen atoms, it further increases the polarity of the bond between the oxygen and the hydrogen.

For example, perchloric acid with three additional oxygen atoms is stronger than chloric acid with two additional oxygen atoms. Chloric acid, in turn, is stronger than chlorous acid, which has only one additional oxygen and hypochlorous acid, with no additional oxygen atoms.

Carboxylic acids are weak acids that contain a carboxyl group. The second oxygen atom makes the oxygen-hydrogen bond more polar, and thereby allows the molecule to donate a proton. Acetic acid and formic acid are examples of carboxylic acids.