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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4:20 minutes

Problem 5ab

Textbook Question

a. Which is a stronger acid: one with a pK_a of 5.2 or one with a pK_a of 5.8?
b. Which is a stronger acid: one with an acid dissociation constant of 3.4 × 10⁻³ or one with an acid dissociation constant of 2.1 × 10⁻⁴?

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Step 1: Understand the relationship between pKa and acid strength. The lower the pKa value, the stronger the acid. This is because pKa is inversely related to the acid dissociation constant (Ka), which measures the extent of ionization of the acid in solution.

Step 2: Compare the pKa values given in part (a). The acid with a pKa of 5.2 is lower than the acid with a pKa of 5.8, indicating that the acid with a pKa of 5.2 is stronger.

Step 3: Understand the relationship between Ka and acid strength for part (b). The higher the Ka value, the stronger the acid. This is because a larger Ka indicates greater ionization of the acid in solution.

Step 4: Compare the Ka values given in part (b). The acid with a Ka of 3.4×10⁻³ is larger than the acid with a Ka of 2.1×10⁻⁴, indicating that the acid with a Ka of 3.4×10⁻³ is stronger.

Step 5: Summarize the findings: For part (a), the acid with a pKa of 5.2 is stronger. For part (b), the acid with a Ka of 3.4×10⁻³ is stronger. This demonstrates the inverse relationship between pKa and Ka when determining acid strength.

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Key Concepts

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

pKa and Acid Strength

The pKa value is a logarithmic measure of the acidity of a substance, where a lower pKa indicates a stronger acid. This is because pKa is derived from the acid dissociation constant (Ka), which quantifies the extent to which an acid donates protons in solution. Therefore, comparing pKa values allows us to determine which acid is stronger based on their numerical values.

Acid Dissociation Constant (Ka)

The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution. It represents the equilibrium constant for the dissociation of an acid into its conjugate base and a proton. A larger Ka value indicates a stronger acid, as it signifies a greater tendency to lose protons, making it essential for comparing acid strengths directly.

Relationship Between pKa and Ka

The relationship between pKa and Ka is expressed by the equation pKa = -log(Ka). This means that as the Ka increases, the pKa decreases, reinforcing the idea that stronger acids have lower pKa values. Understanding this relationship is crucial for interpreting acid strength comparisons, as it allows for the conversion between the two measures and aids in determining which acid is stronger.

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Related Practice

Textbook Question

Rank the following species in order of increasing basicity. Explain your reasons for ordering them as you do.

NH₃ CH₃O^– H₂O CH₃COO^– NaOH NH₂^– HSO₄^–

Textbook Question

The following compounds can all react as bases.

c. Rank the original compounds in order, from strongest base to weakest base.

Textbook Question

Compare the relative acidity of 1-molar aqueous solutions of the following acids.

Textbook Question

Estimate the pKa values of the following compounds:

c. CH₃CH₂COOH

d. CH₃CH₂CH₂N⁺H₃

Textbook Question

Like nitrogen and carbon, oxygen also shows this same hybridization effect on acidity. Both of the following compounds can lose a proton from a positively charged oxygen with three bonds to give a conjugate base containing a neutral oxygen with two bonds. One of these structures has pK_a= −2.4, while the other has pK_a= −8.0.

a. Show the reaction of each compound with water.

b. Match each structure with its pK_a, and explain your choice.

Textbook Question

Acetic acid can also react as a very weak base (pK_b = 20). Two different sites on acetic acid might become protonated to give the conjugate acid. Draw both of these possible conjugate acids, and explain (resonance) why the correct one is more stable. Calculate the pK_a of this conjugate acid.

Textbook Question

Given the data in Problem 47:

b. What pH would you make the water layer to cause the carboxylic acid to dissolve in the water layer and the amine to dissolve in the ether layer?

Textbook Question

Butyric acid, the compound responsible for the unpleasant odor and taste of sour milk, has a pK_a value of 4.82. What is its Ka value? Is it a stronger acid or a weaker acid than vitamin C?

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a. Which is a stronger acid: one with a pKa of 5.2 or one with a pKa of 5.8? b. Which is a stronger acid: one with an acid dissociation constant of 3.4 × 10−3 or one with an acid dissociation constant of 2.1 × 10−4?

Key Concepts

pKa and Acid Strength

Acid Dissociation Constant (Ka)

Relationship Between pKa and Ka

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a. Which is a stronger acid: one with a pK_a of 5.2 or one with a pK_a of 5.8?
b. Which is a stronger acid: one with an acid dissociation constant of 3.4 × 10⁻³ or one with an acid dissociation constant of 2.1 × 10⁻⁴?