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Factors that Determine Acid Strength:
Ultimately, a weak bond strength between the proton (H⁺) and the conjugate base dictate how strongly acids dissociate.
I. Polarity Between H⁺ and Conjugate Base
Electronegativity is the strength of which an atom pulls on its electrons. A difference in electronegativity within a molecule results in the compound being polar.
The more polar a substance is, the more readily it dissolves into an aqueous environment due to the intermolecular dipole forces.
Acids, unlike most other molecular compounds, can dissociate into its constituent ions due to the large difference in electronegativity between the proton and the conjugate base.
II. Distance Between the Nuclei of H⁺ and Conjugate Base
Atomic radius is defined by the distance between the nucleus and the outermost electron shell.
The weaker the chemical bond between the proton and the conjugate base is, the easier it is for that acid to dissociate. According to Coulomb's law, the distance between oppositely charged particles and their attraction strength are inversely related. Because heavier elements have more electron shells, their bond with hydrogen would be weaker than lighter elements as the nuclei of heavier elements are further away from the electron shell of hydrogen.
The trend of acid strength is generally the opposite of the trend of base strength. For example, NH₃ is a stronger base than H₂O. However, while CH₄ is a weaker acid than NH₃, it cannot act as a base (accept a proton) because it lacks lone pairs that are needed to form a bond with a proton.
III. Stable Conjugate Base
A more chemically unstable conjugate base would have a higher chance of spontaneously reacting with protons, therefore decreasing solubility.
A factor that determines the chemical stability of polyatomic conjugate bases would be that resonance promotes stability by distributing charges across the molecule.
Another factor is that the stability of the ion also derives from the charge. The more powerful a charge is (away from 0), the less stable the ion.
For Polyatomic Ions
Polyatomic conjugate bases that contain more electronegative atoms increase the polarity in the bond between the H⁺ and conjugate base. This is because the electronegative atoms together pull the electron density of the molecule away from the hydrogen and towards the conjugate base.
For example, HClO₄ (perchloric acid) is stronger than HClO (hypochlorous acid) due to the presence of more oxygen atoms within HClO₄ molecules.
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