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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Ch. 4 - Acids and Bases: Electron Flow
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
Chapter 3, Problem 33
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')?
Verified step by step guidance1
Understand the relationship between pKₐ and acid strength: A high pKₐ value indicates a weak acid, meaning the acid does not readily donate protons (H⁺). This is because the equilibrium for proton dissociation lies far to the left, favoring the undissociated form of the acid.
Recall the conjugate acid-base relationship: A weak acid has a strong conjugate base. This is because the weak acid does not easily lose its proton, so its conjugate base is more likely to accept a proton, making it a strong base.
Analyze the stability of the base: A strong base is less stable because it has a high tendency to react with protons (H⁺) to form its conjugate acid. Stability is inversely related to reactivity; the more reactive a base, the less stable it is.
Connect the pKₐ value to the base's stability: Since a high pKₐ value corresponds to a weak acid and a strong conjugate base, the base in question is less stable. This is because it is highly reactive and eager to accept protons.
Summarize the reasoning: The stability of a base is determined by the strength of its conjugate acid. A high pKₐ value for the conjugate acid means the base is strong and therefore less stable due to its high reactivity.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Conjugate Acid-Base Pairs
In acid-base chemistry, a conjugate acid is formed when a base accepts a proton (H⁺). The strength of the acid is often measured by its pKₐ value, which is the negative logarithm of its acid dissociation constant (Kₐ). A high pKₐ indicates a weak acid, meaning it does not dissociate easily in solution, which is a key factor in determining the stability of the conjugate acid.
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Stability of Conjugate Acids
The stability of a conjugate acid is influenced by its ability to hold onto the proton it has gained. A conjugate acid with a high pKₐ value is typically more stable because it is less likely to donate the proton back to the solution. This stability can be attributed to factors such as electronegativity, resonance, and the overall structure of the molecule, which affect how well the positive charge is delocalized.
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Relationship Between pKₐ and Acid Strength
The pKₐ value is inversely related to the strength of an acid; lower pKₐ values correspond to stronger acids that dissociate more readily. Therefore, when considering a base with a conjugate acid that has a high pKₐ, it indicates that the conjugate acid is weak and stable. This relationship helps predict the behavior of acids and bases in chemical reactions and their relative stabilities.
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Related Practice
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 base or the conjugate base?
(ii) Which side of the reaction is favored?
(iii) Would you expect a Keq greater than or less than 1?
(a)
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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?
(a)
1
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Textbook Question
How do you know that a proton with a low pKa value is acidic (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.
(b)
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