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Ch.22 - The Main Group Elements
Chapter 22, Problem 138a

Account for each of the following observations.
(a) H2SO4 is a stronger acid than H2SO3.

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1
Understand the concept of acid strength: Acid strength is determined by the ability of an acid to donate protons (H+ ions) in solution. A stronger acid will dissociate more completely in water, releasing more H+ ions.
Compare the structures of H2SO4 and H2SO3: H2SO4 (sulfuric acid) has two hydroxyl groups (OH) and two double-bonded oxygens, while H2SO3 (sulfurous acid) has two hydroxyl groups and one double-bonded oxygen.
Consider the role of electronegativity and resonance: The presence of more electronegative oxygen atoms in H2SO4 allows for greater stabilization of the negative charge on the conjugate base (HSO4-) through resonance, compared to H2SO3.
Evaluate the stability of the conjugate bases: The conjugate base of H2SO4 (HSO4-) is more stable than the conjugate base of H2SO3 (HSO3-) due to the additional resonance stabilization provided by the extra oxygen atom in H2SO4.
Conclude based on the stability of conjugate bases: Since H2SO4 has a more stable conjugate base than H2SO3, it is a stronger acid, as the stability of the conjugate base is a key factor in determining acid strength.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid Strength

Acid strength refers to the ability of an acid to donate protons (H⁺ ions) in a solution. Stronger acids dissociate more completely in water, leading to a higher concentration of H⁺ ions. The strength of an acid is influenced by its molecular structure, including the stability of its conjugate base after proton donation.
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Conjugate Base Stability

The stability of a conjugate base is crucial in determining the strength of its corresponding acid. A more stable conjugate base results from factors such as resonance, electronegativity, and size of the atoms involved. In the case of H₂SO₄ and H₂SO₃, the conjugate base of H₂SO₄ (HSO₄⁻) is more stable than that of H₂SO₃ (HSO₃⁻), making H₂SO₄ a stronger acid.
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Oxidation State and Acidic Behavior

The oxidation state of the central atom in an acid can influence its acidic behavior. In H₂SO₄, sulfur is in a higher oxidation state (+6) compared to H₂SO₃ (+4). Higher oxidation states typically lead to stronger acids due to increased positive charge on the central atom, which stabilizes the negative charge on the conjugate base after proton donation.
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