Acid strength, denoted by the formula HA, is the propensity of an acid to dissociate into a proton (H+) and an anion (A). A strong acid’s dissociation in solution is practically complete except for its most concentrated solutions.
HA → H+ + A−
Strong acid examples are hydrochloric acid (HCl), perchloric acid (HClO4), nitric acid (HNO3), and sulfuric acid (H2SO4).
A weak acid is partially dissociated, with both the dissociated acid and its undissociated product being present, in equilibrium.
HA ⇌ H+ + A−
The best example of a weak acid is Acetic acid (CH3COOH).
The strength of a weak acid is quantified by its acid equilibrium constant, the pKa value.
Acid Strength
Acid strength is a measure of the ability of an acid to donate protons (H+ ions) in an aqueous solution. The strength of an acid depends on its chemical structure and ability to release hydrogen ions into solution. Acids are classified as strong, weak, or intermediate based on the degree to which they ionize in water.
Are acids always strong?
Acids are not always strong. Acids can be classified as strong, weak, or intermediate based on the degree to which they ionize in water. Strong acids ionize completely in water, producing many H+ ions. On the other hand, weak acids only partially ionize in water, producing a small amount of H+ ions. Intermediate acids have properties that fall between those of strong and weak acids.
The strength of an acid depends on its chemical structure and ability to release hydrogen ions into solution. The stability of the resulting conjugate base and the strength of the bond between the hydrogen ion and the acid molecule are two factors determining acid strength.
Other factors that can affect acid strength include:
- The presence of resonance.
- The size of the molecule.
- The presence of other functional groups.
Some examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3), while examples of weak acids include acetic acid (CH3COOH), citric acid (C6H8O7), and carbonic acid (H2CO3). It is important to note that the strength of an acid is a relative measure, and an acid that is considered strong in one context may be considered weak in another.
- Strong acids:
These ionize completely in water, producing a large amount of H+ ions. Examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3). Strong acids have a pH of less than 3.
HA + S ⇌ SH+ + A−
To the extent that the concentration of the undissociated species HA is too low to be detected, S represents the solvents molecule, such as the water or DMSO molecule.
For practical purposes, a strong acid can be completely separated. Acid is an illustration of a powerful acid.
HCl → H+ + Cl− (in aqueous solution)
A robust acid is any acid with a pKa value less than or equal to about -2. This is explained by the leveling effect and the extremely high buffer capacity of solutions with a pH value of 1 or below.
- Weak acids:
Weak acids only partially ionize in water, producing several H+ ions. Examples of weak acids include acetic acid (CH3COOH), citric acid (C6H8O7), and carbonic acid (H2CO3). Weak acids have a pH between 4 and 6.
HA ⇌ H+ +A+
When the acid dissociation procedure successfully leaves the solvent’s concentration unaltered, such as in the case of water, the solvent is left out of the statement. A weak acid’s potency is sometimes expressed as an equilibrium constant, Ka, defined as follows, where X denotes the concentration of a chemical moiety, X.
The strength of an acid can be measured using pH, which measures the acidity or basicity of a solution. The lower the pH, the more acidic the solution is. The strength of an acid can also be expressed using the dissociation constant (Ka), which measures the extent to which an acid ionizes in water.
Factors Determining Acid Strength
Several factors, including the stability of the resulting conjugate base and the strength of the bond between the hydrogen ion and the acid molecule, determine acid strength.
1. Stability of the conjugate base
When an acid donates a proton (H+ ion), it forms a conjugate base. The stability of the resulting conjugate base determines the strength of the acid. Stronger acids form more stable conjugate bases, making them more likely to donate protons in solution. For example, hydrochloric acid (HCl) is a strong acid because its conjugate base (Cl-) is highly stable due to the electronegativity of chlorine.
2. Strength of the bond between the hydrogen ion and the acid molecule
The strength of the bond between the hydrogen ion and the acid molecule also affects acid strength. The stronger the bond, the less likely the acid is to donate a proton. For example, sulfuric acid (H2SO4) has two hydrogen ions. Still, only the first one is easily donated due to the strength of the bond between the second hydrogen ion and the acid molecule.
3. Electronegativity
Electronegativity is the ability of an atom to attract electrons towards itself. An atom with high electronegativity will hold onto its electrons more tightly, making it more difficult for the acid to donate a proton. For example, carboxylic acids (such as acetic acid) have a negatively charged oxygen atom that is highly electronegative, making it more difficult for the acid to donate a proton.
4. Resonance
Resonance occurs when electrons are shared between multiple atoms through a pi bond. In some molecules, resonance can stabilize the conjugate base, making the acid more likely to donate a proton.
For example, the carboxylic acid group (COOH) has resonance, making it a stronger acid than a similar molecule without resonance, such as an alcohol (ROH).
List of Strong Acids
Since there are only seven common strong acids, students can easily learn their names. Weak acids comprise all other acids that aren’t on the list of strong acids.
Here is a list of the chemical formulas for each of the seven-strong acids:
- Chloric acid: HClO3
- Hydrobromic acid: HBr
- Hydrochloric acid: HCl
- Hydroiodic acid: HI
- Nitric acid: HNO3
- Perchloric acid: HClO4
- Sulfuric acid: H2SO4
Also read: What is Peptide Synthesis? 6 Applications of Peptide Synthesis
