Predict the stronger acid in each pair: (a) HCl or HF; (b) H3 PO4 or H3 AsO4 ; (c) HBrO3 or HBrO

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Predict the stronger acid in each pair: (a) HCl or HF; (b)...

Predict the stronger acid in each pair: (a) $\mathrm{HCl}$ or HF; (b) $\mathrm{H}_{3} \mathrm{PO}_{4}$ or $\mathrm{H}_{3} \mathrm{AsO}_{4} ;$ (c) $\mathrm{HBrO}_{3}$ or $\mathrm{HBrO}_{2}$ (d) $\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}$ or $\mathrm{HC}_{2} \mathrm{O}_{4} \overline{;} ;(\mathbf{e})$ benzoic acid $\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\right)$ or phenol $\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\right) .$

Predict the stronger acid in each pair: (a) $\mathrm{HCl}$ or HF;
(b) $\mathrm{H}_{3} \mathrm{PO}_{4}$ or $\mathrm{H}_{3} \mathrm{AsO}_{4} ;$
(c) $\mathrm{HBrO}_{3}$ or $\mathrm{HBrO}_{2}$
(d) $\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}$ or
$\mathrm{HC}_{2} \mathrm{O}_{4} \overline{;} ;(\mathbf{e})$ benzoic acid $\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\right)$ or phenol $\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\right) .$

Key Concepts

Acid Strength and pKa

Acid strength is typically quantified by the acid dissociation constant (Ka) or its logarithmic form, pKa. A lower pKa value corresponds to a stronger acid, meaning it more readily donates a proton in solution. This concept is central in comparing acids, as it reflects the inherent tendency of a compound to lose a proton under equilibrium conditions.

Bond Strength and Bond Polarity

The ease with which an acid donates its proton often depends on the strength and polarity of the bond between the hydrogen and the attached atom. Weaker bonds or bonds that are highly polar can facilitate proton donation. In acids where this bond involves a halogen, for instance, a larger, more polarizable halogen can create a weaker bond with hydrogen, enhancing acid strength.

Electronegativity and Atomic Size Effects

Electronegativity, which is the tendency of an atom to attract electrons, and the size of the atom, play significant roles in acid behavior. High electronegativity can stabilize the negative charge in the conjugate base after deprotonation. However, larger atomic size often leads to weaker hydrogen bonds due to longer bond distances and increased polarizability, which can make proton removal easier.

Resonance Stabilization

Resonance stabilization is a key factor in determining acid strength because the ability to delocalize the negative charge in the conjugate base greatly stabilizes it. When the conjugate base can spread the charge over multiple atoms through resonance structures, the original acid is more likely to donate its proton, thus increasing its acidity.

Inductive Effects

Inductive effects refer to the transmission of charge through a chain of atoms in a molecule, which can influence the stability of the conjugate base. Electron-withdrawing groups positioned near the acidic proton can pull electron density away, stabilizing the negative charge that results from deprotonation. This stabilization enhances acid strength.

Oxoacid Structure and Oxygen Content

In oxoacids, the number and arrangement of oxygen atoms bonded to the central atom are critical in determining acidity. More oxygen atoms typically increase the electron-withdrawing effect and enhance resonance stabilization, both of which stabilize the conjugate base. Additionally, the oxidation state of the central atom can influence the overall electron density and thus the acid's proton-donating ability.

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