Question

Acid Strength and Molecular Structure Based on their molecular structure, pick the stronger acid from each pair of oxyacids. Explain your choice. $$\begin{array}{l}{\text { a. } \mathrm{H}_{2} \mathrm{SO}_{4} \text { or } \mathrm{H}_{2} \mathrm{SO}_{3}} \\ {\text { b. } \mathrm{HClO}_{2} \text { or } \mathrm{HClO}} \\ {\text { c. } \mathrm{HClO}_{2} \text { or } \mathrm{HBrO}} \\ {\text { d. } \mathrm{CCl}_{3} \mathrm{COOH} \text { or } \mathrm{CH}_{3} \mathrm{COOH}}\end{array}$$

Short answer

The stronger acids are: H2SO4, HClO2, HClO2, CCl3COOH.

Step-by-step solution

- Comparing Acid Strength Based on Number of Oxygen Atoms

When comparing acidity of two oxyacids with the same central atom, the more oxygens generally indicate a stronger acid because they stabilize the negative charge of the conjugate base through resonance. More oxygens allow for better delocalization of the negative charge after deprotonation.

- Analyzing Acid Strength for H2SO4 vs H2SO3

H2SO4 (sulfuric acid) has one more oxygen atom than H2SO3 (sulfurous acid). Therefore, the conjugate base of H2SO4 is better stabilized by resonance, making H2SO4 the stronger acid.

- Analyzing Acid Strength for HClO2 vs HClO

HClO2 (chlorous acid) has one more oxygen atom than HClO (hypochlorous acid). Thus, HClO2 is the stronger acid because its conjugate base is more stabilized by resonance.

- Comparing Acid Strength of Oxyacids with Different Halo Atoms

For oxyacids with the same number of oxygen atoms but different central halogen atoms, the electronegativity and size of the halogen influence acid strength. A more electronegative or larger halogen will better stabilize the negative charge.

- Analyzing Acid Strength for HClO2 vs HBrO

HClO2 and HBrO have the same number of oxygen atoms, but bromine is larger than chlorine. Normally, a larger atom would stabilize the charge better, but the larger size of bromine makes the orbital overlap less effective compared to chlorine's. Hence, HClO2 is still the stronger acid.

- Analyzing Acid Strength for CCl3COOH vs CH3COOH

CCl3COOH (trichloroacetic acid) has three chlorine atoms which are highly electronegative and withdraw electron density through the inductive effect. CH3COOH (acetic acid) has only methyl groups, which are electron-donating. Therefore, CCl3COOH, having a stronger inductive effect pulling electron density away from the carboxyl group, is the stronger acid.

Key concepts

Molecular Structure of Acids

Understanding the molecular structure of acids is crucial to predicting their strength. Acids are substances that can donate a proton (H⁺) to another substance in a chemical reaction. The ability of an acid to donate a proton is influenced by its molecular structure. Some key structures subtopics include the presence of oxygens, halogens, and the overall arrangement of atoms within the acid.

In oxyacids, such as sulfuric acid (H₂SO₄) or chlorous acid (HClO₂), the central atom bonded to oxygen can have a significant impact on acid strength. For example, the presence of more oxygen atoms usually means a stronger acid because additional oxygens increase the likelihood of electron delocalization, which leads to better charge stabilization in the conjugate base after deprotonation. Additionally, in haloacids, the type of halogen connected to the central atom can affect the strength due to differences in electronegativity and size.

Oxyacids

Oxyacids are a specific type of acids where a hydrogen atom is bonded to an oxygen atom, which in turn is bonded to another atom, typically a non-metal. The acidity of oxyacids is commonly influenced by the number of oxygen atoms present. As seen in oxyacids like H₂SO₄ compared to H₂SO₃, the additional oxygen atom in sulfuric acid makes it a stronger acid than sulfurous acid.

This is because those extra oxygen atoms have the effect of distributing (or 'delocalizing') the negative charge that results when the acid loses a hydrogen proton. More oxygen atoms mean a more stable structure for the resulting anion, which is a reduced version of the original acid known as the conjugate base. Hence, more oxygens contribute to a higher acid strength.

Resonance Stabilization

Resonance stabilization is a concept that applies to molecules where the electronic structure can be depicted in multiple ways, none of which can fully capture the reality of the electron distribution on their own. These structures are known as resonance forms and contribute to the stability of the molecule.

In the context of acids, resonance stabilization is important for understanding the strength of an acid. For instance, in the conjugate base of chlorous acid (HClO₂), the negative charge can be delocalized across several oxygen atoms. This delocalization allows the negative charge to be spread out over a larger area, minimizing repulsion and leading to a more stable anion. A more stable conjugate base implies a stronger parent acid since it is more likely to donate its proton. This concept is central to understanding why HClO₂ is stronger than HClO.

Inductive Effect

The inductive effect is another molecular phenomenon affecting acid strength. It refers to the shifting of electron density through sigma bonds caused by the electronegativity of atoms within the molecule. Electronegative atoms or groups, such as chlorine in trichloroacetic acid (CCl₃COOH), pull electron density towards themselves.

This effect has significant implications for acid strength. When the electronegative atoms are attached to the carbon that's connected to the carboxyl group (COOH), they make the release of the hydrogen proton more favorable. The inductive effect in trichloroacetic acid pulls electron density away from the acidic proton, making it more apt to disassociate, and thereby increasing the acid's strength compared to acetic acid (CH₃COOH), which lacks highly electronegative groups.