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 42b(3,4,5)

Chapter 3, Problem 42b(3,4,5)

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.
3. pH = 7
4. pH = 10
5. pH = 13

Verified step by step guidance

Step 1: Recall the relationship between pKa, pH, and the ratio of charged to neutral species. The Henderson-Hasselbalch equation is used:

pH = pKa + \log (\frac{[A⁻]}{[HA]})

, where [A⁻] is the concentration of the deprotonated (charged) form and [HA] is the concentration of the neutral form.

Step 2: For each pH value, calculate the ratio of [A⁻] to [HA] using the equation:

\frac{[A⁻]}{[HA]} = 10^{pH - pKa}

. Substitute the given pKa value of 15 and the respective pH values (7, 10.5, and 13) into the equation.

Step 3: Analyze the ratio for each pH value. If the ratio

\frac{[A⁻]}{[HA]}

is greater than 1, the solution has more charged molecules ([A⁻]). If the ratio is less than 1, the solution has more neutral molecules ([HA]). If the ratio equals 1, the concentrations of charged and neutral molecules are equal.

Step 4: For pH = 7, calculate

= 10^{7 - 15}

. Since the pH is much lower than the pKa, expect more neutral molecules ([HA]).

Step 5: For pH = 10.5 and pH = 13, repeat the calculation using

= 10^{10.5 - 15}

and

= 10^{13 - 15}

. Compare the results to determine whether the solution has more charged or neutral molecules at these pH 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, with lower pKa values corresponding to stronger acids. In a solution, the pH reflects the concentration of hydrogen ions (H+). When the pH is lower than the pKa, the compound is more likely to be protonated (neutral), while at a higher pH, it tends to lose a proton and become deprotonated (charged).

Protonation and Deprotonation

Protonation refers to the addition of a proton (H+) to a molecule, which can change its charge state. For alcohols, when the pH is lower than the pKa, the alcohol remains mostly in its neutral form (ROH). Conversely, at higher pH levels, the alcohol can lose a proton, resulting in a negatively charged alkoxide ion (RO-), thus affecting the balance between charged and neutral species in solution.

Equilibrium in Acid-Base Chemistry

In acid-base chemistry, equilibrium describes the balance between protonated and deprotonated forms of a compound in solution. The Henderson-Hasselbalch equation can be used to predict the ratio of charged to neutral species based on pH and pKa. Understanding this equilibrium is crucial for determining the predominant form of the alcohol at different pH levels, which directly influences its behavior in chemical reactions.

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Determining Acid/Base Equilibrium

Related Practice

Textbook Question

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

d. Draw the structure of alanine in a solution at pH = 12.

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Textbook Question

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

e. Is there a pH at which alanine is uncharged (that is, neither group has a charge)?

f. At what pH does alanine have no net charge (that is, the amount of negative charge is the same as the amount of positive charge)?

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Textbook Question

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).

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

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

1. pH = 1

2. pH = 3

3. pH = 5

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Textbook Question

Indicate whether a protonated amine (RN⁺H₃) with a pKa value of 9 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

1. pH = 1

2. pH = 3

3. pH = 5

4. pH = 7

5. pH = 10

6. pH = 13

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Indicate whether an alcohol (ROH) with a pKa value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.3. pH = 74. pH = 105. pH = 13

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Key Concepts

pKa and pH Relationship

Protonation and Deprotonation

Equilibrium in Acid-Base Chemistry

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.
3. pH = 7
4. pH = 10
5. pH = 13