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Chapter 16: Problem 89

Explain the following observations: (a) \(\mathrm{HNO}_{3}\) is a stronger acid than \(\mathrm{HNO}_{2} ;\) (b) \(\mathrm{H}_{2} \mathrm{~S}\) is a stronger acid than \(\mathrm{H}_{2} \mathrm{O} ;(\mathrm{c})\) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a stronger acid than \(\mathrm{HSO}_{4}^{-} ;\) (d) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a stronger acid than \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) (e) \(\mathrm{CCl}_{3} \mathrm{COOH}\) is a stronger acid than \(\mathrm{CH}_{3} \mathrm{COOH}\)

Short Answer

Expert verified
(a) HNO₃ is a stronger acid than HNO₂ because its conjugate base (NO₃⁻) is more stable due to the delocalization of the negative charge. (b) H₂S is a stronger acid than H₂O because sulfur is less electronegative than oxygen, making the H-S bond weaker and easier to break. (c) H₂SO₄ is a stronger acid than HSO₄⁻ because the negative charge on HSO₄⁻ reduces its overall stability. (d) H₂SO₄ is a stronger acid than H₂SeO₄ due to sulfur being more electronegative than selenium, making the S-O bond more polar. (e) CCl₃COOH is a stronger acid than CH₃COOH since the CCl₃ group is an electron-withdrawing group, increasing the stability of the conjugate base (CCl₃COO⁻).

Step by step solution

01

Observation (a)

HNO₃ is a stronger acid than HNO₂: The acidity of an acid depends on the stability of the conjugate base that forms after losing a hydrogen ion. In this case, the conjugate bases are NO₃⁻ and NO₂⁻. The more stable the conjugate base, the stronger the acid. In case of HNO₃, the negative charge is delocalized on the nitrate ion (NO₃⁻), which provides greater stability to the conjugate base. In the case of HNO₂, the negative charge is localized on the nitrite ion (NO₂⁻), making it less stable. Therefore, HNO₃ is a stronger acid than HNO₂.
02

Observation (b)

H₂S is a stronger acid than H₂O: The acidic strength of a compound depends on the electronegativity of the central atom and the ease of breaking the H-X bond (X being the central atom). In both cases, the central atom is from group 16 (oxygen in H₂O and sulfur in H₂S). Sulfur is less electronegative than oxygen, making the H-S bond weaker and easier to break compared to the H-O bond in H₂O. As a result, H₂S is a stronger acid than H₂O.
03

Observation (c)

H₂SO₄ is a stronger acid than HSO₄⁻: In this case, we compare the acidic strength of a molecule and its conjugate base. When H₂SO₄ loses a proton, it forms HSO₄⁻. The acidic strength is directly related to the stability of the formed conjugate base after losing the proton. The negative charge on the HSO₄⁻ ion reduces the overall stability compared to H₂SO₄. Therefore, H₂SO₄ is a stronger acid than HSO₄⁻.
04

Observation (d)

H₂SO₄ is a stronger acid than H₂SeO₄: H₂SO₄ and H₂SeO₄ are both oxyacids, consisting of a central atom (sulfur in H₂SO₄, and selenium in H₂SeO₄) surrounded by oxygen atoms. The electronegativity difference between the central atom and oxygen determines the acidic strength. Sulfur is more electronegative than selenium, making the S-O bond more polar than the Se-O bond. Consequently, H₂SO₄ is a stronger acid than H₂SeO₄.
05

Observation (e)

CCl₃COOH is a stronger acid than CH₃COOH: The acidity of carboxylic acids depends on the inductive effect of the substituent atoms/groups. The inductive effect is the electron-withdrawing or electron-donating property of a substituent group that influences the acidity of the carboxylic acid. The CCl₃ group in CCl₃COOH is an electron-withdrawing group due to the high electronegativity of Cl atoms, releasing the negative charge on the conjugate base (CCl₃COO⁻) and increasing the stability. In CH₃COOH, the methyl group (CH₃) has a weaker electron-withdrawing effect, which leads to a less stable conjugate base (CH₃COO⁻). Therefore, CCl₃COOH is a stronger acid than CH₃COOH.

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Most popular questions from this chapter

Determine the \(\mathrm{pH}\) of each of the following solutions \(\left(K_{a}\right.\) and \(K_{b}\) values are given in Appendix \(D\) ): (a) \(0.095 M\) hypochlorous acid, (b) \(0.0085 \mathrm{M}\) hydrazine, (c) \(0.165 \mathrm{M}\) hydroxylamine.

Although \(\mathrm{HCl}\) and \(\mathrm{H}_{2} \mathrm{SO}_{4}\) have very different properties as pure substances, their aqueous solutions possess many common properties. List some general properties of these solutions, and explain their common behavior in terms of the species present.

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Calculate \(\left[\mathrm{OH}^{-}\right]\) for each of the following solutions, and indicate whether the solution is acidic, basic, or neutral: (a) \(\left[\mathrm{H}^{+}\right]=0.0505 \mathrm{M} ;\) (b) \(\left[\mathrm{H}^{+}\right]=2.5 \times 10^{-10} \mathrm{M} ;(\mathrm{c})\) a solution in which \(\left[\mathrm{H}^{+}\right]\) is 1000 times greater than \(\left[\mathrm{OH}^{-}\right] .\)

Although pure \(\mathrm{NaOH}\) and \(\mathrm{NH}_{3}\) have very different properties, their aqueous solutions possess many common properties. List some general properties of these solutions, and explain their common behavior in terms of the species present.
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