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

Equilibrium Constant

Organic Chemistry

3. Acids and Bases

Equilibrium Constant

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Problem 4

Textbook Question

Calculate K_eq for the acid–base reaction shown. Which side is favored and why?

Verified step by step guidance

Step 1: Identify the acid and base on both sides of the reaction. On the left side, the acid is the molecule with the hydroxyl group (HO-CH2-CO-CH3), and the base is the ethoxide ion (CH3CH2O-). On the right side, the conjugate acid is ethanol (CH3CH2OH), and the conjugate base is the deprotonated form of the acid (CH2-CO-CH3).

Step 2: Determine the pKa values of the acid and conjugate acid. The pKa of the acid (HO-CH2-CO-CH3) is approximately 4.8, and the pKa of ethanol (CH3CH2OH) is approximately 16. This information is crucial for calculating the equilibrium constant (Keq).

Step 3: Use the relationship between pKa values and Keq. The equilibrium constant for an acid-base reaction can be calculated using the formula: \( K_{eq} = 10^{(pKa_{acid} - pKa_{conjugate acid})} \). Substitute the pKa values into this formula.

Step 4: Analyze which side of the reaction is favored. The side with the weaker acid (higher pKa) is favored because the reaction tends to favor the formation of the weaker acid and base. Compare the pKa values to determine this.

Step 5: Conclude why the favored side is determined. Since ethanol (pKa ~16) is a much weaker acid than HO-CH2-CO-CH3 (pKa ~4.8), the equilibrium will favor the side with ethanol and the conjugate base (CH2-CO-CH3).

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Key Concepts

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

Acid-Base Equilibrium

Acid-base equilibrium refers to the balance between the reactants and products in an acid-base reaction. It is characterized by the transfer of protons (H+) from the acid to the base. The equilibrium constant, K_eq, quantifies this balance, indicating the extent to which reactants are converted to products. A larger K_eq value suggests that the products are favored, while a smaller value indicates that the reactants are favored.

Strength of Acids and Bases

The strength of acids and bases is determined by their ability to donate or accept protons. Strong acids completely dissociate in solution, while weak acids only partially dissociate. Similarly, strong bases readily accept protons, whereas weak bases do so less effectively. Understanding the relative strengths of the acids and bases involved in the reaction is crucial for predicting the direction of the equilibrium and determining which side is favored.

Le Chatelier's Principle

Le Chatelier's Principle states that if a system at equilibrium is disturbed, the system will adjust to counteract the disturbance and restore a new equilibrium. In the context of acid-base reactions, if the concentration of either reactants or products changes, the equilibrium will shift towards the side that reduces the effect of that change. This principle helps in predicting how changes in concentration, pressure, or temperature will affect the position of equilibrium in acid-base reactions.

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Related Practice

Textbook Question

We usually calculate K_eq for acid–base reactions using pK_a values.

(a) Derive an equation to calculate K_eq using pK_b values, then

(b) use it to calculate the equilibrium constant for the following reaction.

Textbook Question

Given the pK_b, calculate the pK_a of the conjugate acid.

(b)

Textbook Question

Calculate K_eq for these acid–base reactions.

(d)

Textbook Question

Using pK_a values for the conjugate acids of the bases on each side of the reaction arrow, identify which side of the equilibrium would be favored in the following hypothetical reactions.

(c)

Textbook Question

Write the K_eq expression for the following acid–base reactions. [You don't need to calculate K_eq here.]

(c)

Textbook Question

Parts (a)–(d) of this assessment assist in the development of what will become a common theme in organic reactions and should be worked in order. [Think carefully about how each question relates to the others.]

(c) Without worrying about the mechanism of the reaction, estimate an equilibrium constant for the following carbonyl addition reaction based on the relative stability of the Lewis bases.

Textbook Question

Calculate the equilibrium constant for the reaction of ethylamine and propanoic acid. Which side is favored?

Textbook Question

How do you know that a proton with a low pK_a value is acidic (besides 'I just remember')?

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