Question

Perchloric acid, \(\mathrm{HClO}_{4},\) is the strongest of the halogen oxoacids, and hypoiodous acid, \(\mathrm{HIO}\), is the weakest. What two factors govern this difference in acid strength?

Short answer

Electronegativity and oxidation state differences make \(\text{HClO}_4\) stronger than \(\text{HIO}\).

Step-by-step solution

- Halogen Electronegativities

Understand that the electronegativity of halogens (like chlorine in \(\text{HClO}_4\) and iodine in \(\text{HIO}\)) greatly impacts the acid strength. The higher the electronegativity, the more it stabilizes the negative charge on the conjugate base, making the acid stronger. Chlorine is more electronegative than iodine.

- Oxidation States

Assess the oxidation state of the halogen in each acid. Higher oxidation states of the central atom (halogen) increase the acid strength by drawing more electron density away from the oxygen-hydrogen bond, making it easier to donate \(\text{H}^+\). Perchloric acid (\(\text{HClO}_4\)) has a higher oxidation state for chlorine (+7) compared to the oxidation state of iodine in hypoiodous acid (\(\text{HIO}\), +1).

- Structure and Resonance

Consider the molecular structure and possible resonance stabilization. More resonance structures typically mean greater stabilization of the conjugate base. \(\text{HClO}_4\) has multiple resonance forms for its conjugate base \(\text{ClO}_4^-\) which helps delocalize the negative charge, whereas \(\text{HIO}\) has fewer possibilities for resonance stabilization.

Final Conclusion

Summarize: The two factors are the higher electronegativity of chlorine and the higher oxidation state in \(\text{HClO}_4\) compared to \(\text{HIO}\), which govern the stronger acid strength of \(\text{HClO}_4\).

Key concepts

Halogen Electronegativity

Halogen electronegativity plays a crucial role in determining the strength of halogen oxoacids, such as perchloric acid \(\mathrm{HClO}_{4}\) and hypoiodous acid \(\mathrm{HIO}\). Electronegativity is a measure of an atom's ability to attract and hold onto electrons. Chlorine is more electronegative than iodine. This means that chlorine can more effectively stabilize the negative charge on the conjugate base after the acid donates a proton (\(\text{H}^+\)).

In simpler terms, the stronger pull of electronic charge from the central halogen (chlorine in this case) attracts the electron density towards itself, which spreads out the negative charge more evenly. This stabilization makes \(\text{HClO}_{4}\) release its \(\text{H}^+\) more readily, thus making it a stronger acid compared to \(\text{HIO}\), where iodine is less electronegative and therefore less effective at stabilizing the conjugate base.

• High electronegativity = stronger acid
• Chlorine (Cl) is more electronegative than iodine (I)
• This leads to better stabilization of the conjugate base

Oxidation States

The oxidation state of the halogen plays a significant role in the acid strength of halogen oxoacids. The oxidation state is a number that reflects the total number of electrons an atom gains or loses when forming a compound. In the case of perchloric acid \(\text{HClO}_4\), chlorine exhibits an oxidation state of +7. In contrast, in hypoiodous acid \(\text{HIO}\), iodine has an oxidation state of +1.

Higher oxidation states mean that the central atom (halogen) can pull more electron density away from the surrounding atoms due to its increased positive charge. This electron withdrawal stabilizes the negative charge on the conjugate base better when the acid molecule loses a proton. Thus, higher oxidation states correlate with stronger acids.

• Higher oxidation state = stronger acid
• Chlorine in \(\text{HClO}_4\) is +7 vs. iodine in \(\text{HIO}\) which is +1
• Higher positive charge means better electron withdrawal

Resonance Stabilization

Resonance stabilization is another important factor in determining acid strength. Resonance refers to the delocalization of electrons within a molecule, which can be shown by drawing multiple structures (resonance structures) that represent the same molecule. These structures help distribute the negative charge more evenly, stabilizing the molecule.

For perchloric acid \(\text{HClO}_4\), the conjugate base \(\text{ClO}_4^-\) can form multiple resonance structures, which means the negative charge is spread out over several oxygen atoms. This extensive delocalization significantly stabilizes the conjugate base, making \(\text{HClO}_{4}\) a strong acid. In contrast, hypoiodous acid \(\text{HIO}\) has fewer resonance structures, limiting the delocalization and consequently offering less stabilization for the conjugate base.

• More resonance structures = greater stabilization
• \text{HClO}_4\text{ has} multiple resonance forms for \(\text{ClO}_4^-\)
• Hypoiodous acid \(\text{HIO}\) has fewer resonance forms