Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry

Problem 39a,b

Chapter 3, Problem 39a,b

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:
a. CH₃COOH(pK_a= 4.76)
b. CH₃CH₂N⁺H₃ (pK_a= 11.0)

Verified step by step guidance

Step 1: Understand the problem. The question asks for the predominant form of each compound at a pH of 5.5. This involves determining whether the compound exists in its protonated (acidic) or deprotonated (basic) form based on the pKa values provided.

Step 2: Recall the relationship between pH, pKa, and the acid-base equilibrium. The Henderson-Hasselbalch equation is used to determine the ratio of protonated to deprotonated forms:

pH = {pK}_{a} + \log (\frac{[A -]}{[HA]})

, where [A⁻] is the deprotonated form and [HA] is the protonated form.

Step 3: Analyze compound (a) CH3COOH (acetic acid). The pKa of acetic acid is 4.76. Compare the pH (5.5) to the pKa. Since pH > pKa, the solution is more basic than the acid's pKa, meaning the deprotonated form (CH3COO⁻) will predominate.

Step 4: Analyze compound (b) CH3CH2N+H3 (ethylammonium ion). The pKa of the conjugate acid is 11.0. Compare the pH (5.5) to the pKa. Since pH < pKa, the solution is more acidic than the base's pKa, meaning the protonated form (CH3CH2N+H3) will predominate.

Step 5: Summarize the results. For compound (a), the predominant form at pH 5.5 is CH3COO⁻. For compound (b), the predominant form at pH 5.5 is CH3CH2N+H3. These conclusions are based on the relative pH and pKa values.

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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 protonated (acidic) and deprotonated (basic) forms of a compound in solution. The pKa value indicates the strength of an acid; lower pKa values correspond to stronger acids. At a given pH, the predominant form of an acid or base can be determined using the Henderson-Hasselbalch equation, which relates pH, pKa, and the ratio of protonated to deprotonated species.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a mathematical formula used to calculate the pH of a buffer solution. It is expressed as pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the deprotonated form and [HA] is the concentration of the protonated form. This equation helps predict the predominant form of a compound at a specific pH, which is essential for understanding the behavior of weak acids and bases in solution.

pH and its Relation to pKa

pH is a measure of the acidity or basicity of a solution, with lower values indicating higher acidity. The relationship between pH and pKa is crucial for determining the predominant species of an acid or base in solution. If the pH is lower than the pKa, the protonated form predominates; if the pH is higher, the deprotonated form is favored. This concept is vital for predicting the behavior of compounds like acetic acid and ammonium ion in the given question.

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Identifying pKa values

Related Practice

Textbook Question

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:

c. H₃O⁺ (pK_a = −1.7)

d. HBr (pK_a = −9)

Textbook Question

Fosamax (shown on the previous page) has six acidic groups. The active form of the drug, which has lost two of its acidic protons, is shown in the box.

(Notice that the phosphorus atom in Fosamax and the sulfur atom in [Problem 36] can be surrounded by more than eight electrons because P and S are below the second row of the periodic table.)

a. Why are the OH groups bonded to phosphorus the strongest acids of the six groups?

b. Which of the remaining four groups is the weakest acid?

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Textbook Question

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

d. Which is more electronegative: an sp³ oxygen or an sp² oxygen?

e. Which compounds demonstrate that the relative electronegativities of a hybridized nitrogen atom are sp > sp² > sp³?

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Textbook Question

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:

e. ⁺NH4 (pK_a= 9.4)

f. HC≡N (pK_a= 9.1)

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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?

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Textbook Question

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:

g. HNO₂ (pK_a= 3.4)

h. HNO₃ (pK_a= −1.3)

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For each of the following compounds (here shown in their acidic forms), write the form that ­predominates in a solution with a pH = 5.5:a. CH3COOH(pKa = 4.76)b. CH3CH2N+H3 (pKa = 11.0)

Verified video answer for a similar problem:

Key Concepts

Acid-Base Equilibrium

Henderson-Hasselbalch Equation

pH and its Relation to pKa

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:
a. CH₃COOH(pK_a= 4.76)
b. CH₃CH₂N⁺H₃ (pK_a= 11.0)