Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

pKa

Organic Chemistry

3. Acids and Bases

pKa

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3:52 minutes

Problem 42a

Textbook Question

Hydrogen peroxide (HOOH) has a pK_a of 11.6, making it roughly 10,000 times as strong an acid as water (pK_a = 15.7). Explain why H₂O₂ is a stronger acid than H₂O.

Verified step by step guidance

Understand the concept of pKa: The pKa value is a measure of the strength of an acid. A lower pKa value indicates a stronger acid because it implies the acid more readily donates protons (H⁺ ions) in solution.

Compare the pKa values: Hydrogen peroxide (HOOH) has a pKa of 11.6, while water (H₂O) has a pKa of 15.7. Since 11.6 is lower than 15.7, HOOH is a stronger acid than H₂O.

Analyze the molecular structure: The O-O bond in hydrogen peroxide is weaker and more polar than the O-H bond in water. This makes it easier for HOOH to donate a proton, contributing to its stronger acidic nature.

Consider the stability of the conjugate base: When HOOH donates a proton, it forms the conjugate base HO₂⁻. The presence of an additional oxygen atom in HO₂⁻ compared to OH⁻ (the conjugate base of water) can stabilize the negative charge through resonance and inductive effects, making the conjugate base more stable.

Relate acidity to conjugate base stability: The greater stability of the conjugate base (HO₂⁻) compared to OH⁻ is a key reason why HOOH is a stronger acid than H₂O. A more stable conjugate base corresponds to a stronger acid.

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Key Concepts

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

Acid Strength and pKa

The strength of an acid is determined by its ability to donate protons (H+ ions) in solution. The pKa value is a logarithmic measure of this ability; lower pKa values indicate stronger acids. Hydrogen peroxide (H2O2) has a pKa of 11.6, which is lower than water's pKa of 15.7, indicating that H2O2 is a stronger acid than H2O.

Molecular Structure and Stability

The molecular structure of a compound affects its acidity. In hydrogen peroxide, the presence of the O-O bond and the additional oxygen atom can stabilize the negative charge on the conjugate base (HO2-) after losing a proton, more effectively than in water (OH-). This increased stability of the conjugate base contributes to H2O2's stronger acidity.

Resonance and Inductive Effects

Resonance and inductive effects can influence acid strength. In H2O2, the electronegative oxygen atoms can delocalize the negative charge through resonance, stabilizing the conjugate base. Additionally, the inductive effect from the extra oxygen atom in H2O2 can pull electron density away, making it easier for the molecule to release a proton, thus enhancing its acidity compared to water.

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Textbook Question

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(b)

Textbook Question

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(e)

Textbook Question

Hydrogen gas (H₂) has a relatively high pK_a value. Is it a stable or unstable acid? Do you expect it to participate in acid–base reactions?

Textbook Question

Arrange each group of compounds in order of increasing basicity.

c. sodium benzoate, sodium ethoxide, and sodium phenoxide

d. pyridine, sodium ethoxide, and sodium acetate

Textbook Question

Peroxyacetic acid (pK_a = 8.2) is a much weaker acid than acetic acid (pK_a = 4.74). Explain why peroxyacetic acid is a weaker acid than acetic acid.

Textbook Question

Give the pK_a value of the following compounds.

Textbook Question

Using the table of pK_a values given in [Appendix I], answer the following:

a. Which is the most acidic organic compound in the table?

b. Which is the least acidic organic compound in the table?

c. Which is the most acidic carboxylic acid in the table?

Textbook Question

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(c)

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Hydrogen peroxide (HOOH) has a pKa of 11.6, making it roughly 10,000 times as strong an acid as water (pKa = 15.7). Explain why H2O2 is a stronger acid than H2O.

Key Concepts

Acid Strength and pKa

Molecular Structure and Stability

Resonance and Inductive Effects

Watch next

Hydrogen peroxide (HOOH) has a pK_a of 11.6, making it roughly 10,000 times as strong an acid as water (pK_a = 15.7). Explain why H₂O₂ is a stronger acid than H₂O.