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Ch. 4 - Acids and Bases: Electron Flow
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 4 - Acids and Bases: Electron FlowProblem 32
Chapter 3, Problem 32

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

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1
Understand the concept of pKₐ: The pKₐ value is a measure of the strength of an acid. It is the negative logarithm of the acid dissociation constant (Kₐ), which quantifies how easily a proton (H⁺) is released from an acid molecule. Mathematically, pKₐ = -log(Kₐ). Lower pKₐ values correspond to stronger acids because they dissociate more readily in solution.
Relate pKₐ to proton acidity: A proton associated with a molecule is considered acidic if it can be easily donated to a base. Molecules with low pKₐ values have protons that are more likely to dissociate, making them more acidic. This is because a low pKₐ indicates a high Kₐ, meaning the equilibrium strongly favors the dissociated form (H⁺ and the conjugate base).
Examine molecular structure and stability: Acidity is influenced by the stability of the conjugate base formed after the proton is donated. If the conjugate base is stabilized (e.g., through resonance, inductive effects, or electronegativity), the proton is more likely to dissociate, resulting in a lower pKₐ value. For example, carboxylic acids have low pKₐ values because their conjugate bases are resonance-stabilized.
Consider solvent effects: The solvent can also affect the acidity of a proton. In polar solvents, acidic protons are more likely to dissociate because the solvent stabilizes the ions formed during dissociation. This contributes to the observed low pKₐ values for strong acids in aqueous solutions.
Apply this knowledge to predict acidity: When analyzing a molecule, look for factors such as electronegativity of the atom bonded to the proton, resonance stabilization of the conjugate base, and inductive effects from nearby electronegative groups. These factors collectively explain why a proton with a low pKₐ value is acidic, beyond simply memorizing the value.

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Key Concepts

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

pKₐ and Acidity

The pKₐ value is a quantitative measure of the strength of an acid in solution. It is the negative logarithm of the acid dissociation constant (Kₐ), which indicates how readily an acid donates protons (H⁺) to the solution. A lower pKₐ value signifies a stronger acid, meaning it dissociates more completely in water, thus increasing the concentration of protons and making the solution more acidic.
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Acid-Base Equilibrium

Acid-base equilibrium refers to the balance between the concentrations of acids and their conjugate bases in a solution. When an acid donates a proton, it forms its conjugate base. The position of this equilibrium is influenced by the strength of the acid, which is reflected in its pKₐ value. Understanding this equilibrium helps predict how changes in concentration or pH affect the acidity of a solution.
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Factors Affecting Acidity

Several factors influence the acidity of a compound, including electronegativity, atomic size, and resonance stabilization. For instance, if the conjugate base of an acid is stabilized by resonance, the acid will be stronger and have a lower pKₐ. Additionally, the presence of electronegative atoms can pull electron density away from the bond to the proton, making it easier for the acid to donate the proton and thus increasing its acidity.
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Related Practice
Textbook Question

Hydrogen gas (H2) has a relatively high pKa value. Is it a stable or unstable acid? Do you expect it to participate in acid–base reactions?

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Textbook Question

Using qualitative reasoning for the acid–base reactions shown,

(i) which is stronger, the acid or the conjugate acid?

(ii) Which side of the reaction is favored?

(iii) Would you expect a Keq greater than, equal to, or less than 1?

(b)

Textbook Question

Using qualitative reasoning for the acid–base reactions shown,

(i) which is stronger, the acid or the conjugate acid?

(ii) Which side of the reaction is favored?

(iii) Would you expect a Keq greater than, equal to, or less than 1?

(a)

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Textbook Question

If a base has a conjugate acid with a high pKa value, is it stable or unstable? How do you know this is true (besides 'I just remember')?

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Textbook Question

Given that the indicated pKa values correspond to the acid dissociation reactions shown, calculate the ratio of acid to conjugate base for the reactions shown.

(a)

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Textbook Question

Given that the indicated pKa values correspond to the acid dissociation reactions shown, calculate the ratio of acid to conjugate base for the reactions shown.

(b)

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