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 39e,f

Chapter 3, Problem 39e,f

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)

Verified step by step guidance

Step 1: Understand the relationship between pH and pKa. The pKa value of a compound indicates the pH at which the compound is 50% ionized. If the pH of the solution is lower than the pKa, the acidic form of the compound predominates. If the pH is higher than the pKa, the basic form predominates.

Step 2: Compare the pH of the solution (5.5) to the pKa of +NH4 (9.4). Since the pH is significantly lower than the pKa, the acidic form (+NH4) will predominate in the solution.

Step 3: Compare the pH of the solution (5.5) to the pKa of HC≡N (9.1). Similarly, since the pH is lower than the pKa, the acidic form (HC≡N) will predominate in the solution.

Step 4: Recall that the acidic form of +NH4 is the ammonium ion itself (+NH4), and the acidic form of HC≡N is hydrogen cyanide (HC≡N). These are the forms that will predominate at pH 5.5.

Step 5: Summarize the findings: At pH 5.5, the predominant forms of the compounds are +NH4 for ammonium and HC≡N for hydrogen cyanide, as the pH is lower than their respective pKa values.

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Key Concepts

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

pKa and pH Relationship

The pKa value of a compound indicates the acidity of that compound, specifically the pH at which half of the species are deprotonated. A lower pKa means a stronger acid. In a solution with a specific pH, if the pH is lower than the pKa, the protonated form predominates; if higher, the deprotonated form is favored.

Acid-Base Equilibrium

Acid-base equilibrium describes the balance between protonated and deprotonated forms of a compound in solution. The equilibrium can shift based on the pH of the solution, which affects the concentration of hydrogen ions. Understanding this equilibrium is crucial for predicting which form of a compound will be predominant at a given pH.

Ammonium Ion and Cyanide Ion

The ammonium ion (NH4+) and cyanide ion (HC≡N) are examples of weak acids. Their pKa values (9.4 and 9.1, respectively) indicate that they are weak acids, meaning they do not fully dissociate in solution. At a pH of 5.5, which is significantly lower than their pKa values, both compounds will predominantly exist in their protonated forms.

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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:

i. HON⁺H₃ (pK_a = 6.0)

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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:

c. H₃O⁺ (pK_a = −1.7)

d. HBr (pK_a = −9)

Textbook Question

As long as the pH is not less than ___________, at least 50% of a protonated amine with a pK_a value of 10.4 will be in its neutral, nonprotonated form.

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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:

a. CH₃COOH(pK_a= 4.76)

b. CH₃CH₂N⁺H₃ (pK_a= 11.0)

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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:

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:e. +NH4 (pKa = 9.4)f. HC≡N (pKa = 9.1)

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Key Concepts

pKa and pH Relationship

Acid-Base Equilibrium

Ammonium Ion and Cyanide Ion

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)