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-…
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.
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Q1: Why is hydrochloric acid stronger than hydrofluoric acid?
Hydrochloric acid is stronger because it has lower bond energy than hydrofluoric acid. A weaker H-Cl bond breaks more easily, allowing the acid to donate protons readily. In contrast, hydrofluoric acid has a stronger, harder-to-break H-F bond, making it less likely to release protons and therefore a weaker acid.
Q2: How does electronegativity affect acid strength in binary acids?
Higher electronegativity of the atom bonded to hydrogen increases bond polarity, making the hydrogen more likely to be released as a proton. The electronegative atom attracts electrons, giving hydrogen a partial positive charge. Comparing across a period, hydrochloric acid is stronger than hydrogen sulfide because chlorine is more electronegative than sulfur.
Q3: What makes oxyacids stronger when additional oxygen atoms are added?
Additional oxygen atoms bonded to the central atom increase the polarity of the oxygen-hydrogen bond, weakening it and making proton release easier. Perchloric acid with three additional oxygens is stronger than chloric acid with two, which is stronger than chlorous acid with one. This pattern demonstrates how oxygen atoms progressively enhance acid strength.
Q4: Why are carboxylic acids considered weak acids?
Carboxylic acids are weak acids because the second oxygen atom attached to the carbon increases the polarity of the oxygen-hydrogen bond, weakening it only partially. Additionally, after losing a proton, the carboxylate ion exhibits resonance, which stabilizes the negative charge by delocalizing it over several atoms, preventing complete ionization in water.
Q5: How does bond energy determine whether a binary acid is strong or weak?
An acid with higher bond energy has a stronger bond that resists breaking, making it a weaker acid because protons are less readily donated. Conversely, an acid with lower bond energy has a weaker bond that breaks easily, allowing protons to be released more readily and making it a stronger acid. Bond strength is therefore inversely related to acid strength.
Q6: What determines whether a hydrogen-containing molecule can act as an acid?
A molecule can act as an acid only when the atom bonded to hydrogen has higher electronegativity than hydrogen itself. This creates a polar bond where the atom has a partial negative charge and hydrogen has a partial positive charge, allowing hydrogen to be released as a proton. If hydrogen has equal or greater electronegativity, the molecule cannot donate protons.
Q7: How does the oxidation number of the central atom affect oxyacid strength?
Increasing the oxidation number of the central atom increases the acid's strength because it enhances the atom's attraction for electrons shared with oxygen, thereby weakening the oxygen-hydrogen bond. Sulfuric acid with sulfur oxidation number +6 is more acidic than sulfurous acid with +4. Similarly, nitric acid with nitrogen oxidation number +5 is stronger than nitrous acid with +3.