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Questions and Answers
Which of the following correctly defines an acid based on the Brønsted-Lowry theory?
Which of the following correctly defines an acid based on the Brønsted-Lowry theory?
In the dissociation of HCl in water, what is the primary species formed besides Cl⁻?
In the dissociation of HCl in water, what is the primary species formed besides Cl⁻?
Which of the following statements about the pH range in biological systems is correct?
Which of the following statements about the pH range in biological systems is correct?
What characterizes a strong acid in an aqueous solution?
What characterizes a strong acid in an aqueous solution?
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In the equilibrium reaction of a weak acid, what is true about its conjugate base?
In the equilibrium reaction of a weak acid, what is true about its conjugate base?
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What is a common misconception about bases in aqueous solutions?
What is a common misconception about bases in aqueous solutions?
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How does the strength of a weak base compare to that of a strong base?
How does the strength of a weak base compare to that of a strong base?
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What is the role of buffers in biological systems?
What is the role of buffers in biological systems?
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What is the acid dissociation constant (Kc) calculated from in an acid-base equilibrium?
What is the acid dissociation constant (Kc) calculated from in an acid-base equilibrium?
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In the reaction CN⁻ + H⁺ ⇌ HCN, what is the role of CN⁻?
In the reaction CN⁻ + H⁺ ⇌ HCN, what is the role of CN⁻?
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Which statement accurately describes water's role in acid-base reactions?
Which statement accurately describes water's role in acid-base reactions?
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What happens when a weak acid is placed in water?
What happens when a weak acid is placed in water?
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What occurs to a water molecule when it acts as a base in an acid-base reaction?
What occurs to a water molecule when it acts as a base in an acid-base reaction?
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In the reaction NH3 + H2O ⇌ NH4+ + OH-, which component acts as a base?
In the reaction NH3 + H2O ⇌ NH4+ + OH-, which component acts as a base?
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Which statement is correct regarding strong acids and their conjugate bases?
Which statement is correct regarding strong acids and their conjugate bases?
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What role does water play in ammonia's reaction mixture?
What role does water play in ammonia's reaction mixture?
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What is the value of the ion product of water (Kw) at 25℃?
What is the value of the ion product of water (Kw) at 25℃?
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If the concentration of hydronium ions [H3O+] is 1 x 10 -5 M, what is the concentration of hydroxide ions [OH-] in that solution?
If the concentration of hydronium ions [H3O+] is 1 x 10 -5 M, what is the concentration of hydroxide ions [OH-] in that solution?
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How are Ka, Kb, and Kw related in the context of acid-base chemistry?
How are Ka, Kb, and Kw related in the context of acid-base chemistry?
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What happens to the concentration of OH- ions if the concentration of H3O+ ions in a solution is increased?
What happens to the concentration of OH- ions if the concentration of H3O+ ions in a solution is increased?
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If a weak acid has a Ka of 1 x 10 -4, what can be inferred about the corresponding Kb of its conjugate base?
If a weak acid has a Ka of 1 x 10 -4, what can be inferred about the corresponding Kb of its conjugate base?
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What is the significance of the relationship $Ka \times Kb = Kw$?
What is the significance of the relationship $Ka \times Kb = Kw$?
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Which statement correctly describes the relationship between $Ka$, $Kb$, and the strength of acids and bases?
Which statement correctly describes the relationship between $Ka$, $Kb$, and the strength of acids and bases?
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What does the pKa value represent in terms of acid strength?
What does the pKa value represent in terms of acid strength?
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When ammonia reacts with water, which species is formed that acts as a base?
When ammonia reacts with water, which species is formed that acts as a base?
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Which of the following statements about $Kw$ is true?
Which of the following statements about $Kw$ is true?
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What does a large value of Ka indicate about an acid?
What does a large value of Ka indicate about an acid?
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How is pKa related to Ka?
How is pKa related to Ka?
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What is denoted by a small value of Kb?
What is denoted by a small value of Kb?
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What does the pKa value of a strong acid generally look like?
What does the pKa value of a strong acid generally look like?
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What does Kw represent in aqueous solutions?
What does Kw represent in aqueous solutions?
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What indicates that water is not a strong acid or base?
What indicates that water is not a strong acid or base?
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If an acid has a pKa of 5.5, what can be inferred about its Ka value?
If an acid has a pKa of 5.5, what can be inferred about its Ka value?
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Which of the following statements is true regarding the ion product of water (Kw)?
Which of the following statements is true regarding the ion product of water (Kw)?
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How is the acid dissociation constant, Ka, derived from the equilibrium constant, Kc?
How is the acid dissociation constant, Ka, derived from the equilibrium constant, Kc?
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What does a small value of Kb suggest about a base?
What does a small value of Kb suggest about a base?
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Which equilibrium reaction correctly represents the derivation of Kb?
Which equilibrium reaction correctly represents the derivation of Kb?
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What does the acid dissociation constant Ka represent mathematically?
What does the acid dissociation constant Ka represent mathematically?
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In the reaction HA + H2O ⇌ A- + H3O+, what happens if Ka is decreased?
In the reaction HA + H2O ⇌ A- + H3O+, what happens if Ka is decreased?
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What does the equilibrium position indicate when Ka is large?
What does the equilibrium position indicate when Ka is large?
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Which of the following statements is true regarding Kc and the dissociation constants?
Which of the following statements is true regarding Kc and the dissociation constants?
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Study Notes
Acids and Bases: Part 1
- Biologists study acids and bases because the building blocks of proteins and DNA/RNA are amino acids and nucleic acids, respectively. Phospholipids have both acidic and basic headgroups.
- Enzymatic function often depends on pH (acid/base equilibrium).
- Protein function, like oxygen binding in hemoglobin, can be affected by pH.
- Organisms maintain a narrow pH range in tissues and cells.
- Understanding acids, bases, pH, and buffers is crucial to understanding life processes.
Acid and Base Definitions
- Acids are H⁺ donors (compounds that undergo chemical change to produce an H⁺ ion).
- H⁺ is equivalent to a proton.
- The dissociation of an acid into ions is an equilibrium process.
- Example: Ethanoic acid (CH₃COOH) dissociates into CH₃COO⁻ and H⁺ (CH₃COOH ⇌ CH₃COO⁻ + H⁺).
Acids and Bases: Definitions in Water
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Bases are H⁺ acceptors (compounds that combine with an H⁺ ion).
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In water, H⁺ ions don't exist independently; they combine with water to form hydronium ions (H₃O⁺).
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Example: HCl dissociates in water as HCl + H₂O ⇌ H₃O⁺ + Cl⁻.
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Brønsted-Lowry definition:
- Acid = H⁺ donor
- Base = H⁺ acceptor
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Example: NH₃ + H⁺ ⇌ NH₄⁺ (Ammonia accepts a proton to become ammonium).
Acids and Bases: Definitions. Self Test
- When a base dissolves in water, it accepts an H⁺ ion from a water molecule, forming a hydroxide ion (OH⁻).
- Example: B + H₂O ⇌ BH⁺ + OH⁻
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Self test questions: Determine if the molecule on the left-hand side of the equilibrium is an acid or a base in the following reactions:
- HBr ⇌ H⁺ + Br⁻
- CN⁻ + H⁺ ⇌ HCN
Pairing Up Acids and Bases: Conjugate Pairs
- Acids and bases react in pairs.
- An acid must have a base to which it donates an H⁺.
- Water can act as both an acid and a base.
- Example: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (Ammonia acts as a base, accepting H⁺ from water).
- Conjugate acid-base pairs are formed before and after the transfer of an H⁺ ion.
Extent of a Dissociation Reaction
- The extent of acid dissociation (the position of the equilibrium) is called its strength.
- The readiness with which an acid donates H⁺ to form H₃O⁺ is a measure of its strength.
- The readiness with which a base accepts H⁺ from water to form OH⁻ is a measure of its strength.
- Strong acids dissociate extensively; weak acids do not dissociate much.
- Strong bases readily accept H⁺; weak bases do not.
The Acid Dissociation Constant (Ka)
- Ka is the equilibrium constant for the dissociation of an acid in water.
- Ka = [H₃O⁺][A⁻]/[HA]
- In dilute solutions, the concentration of water (55.5 M) is constant and can be removed from the equation. The acid dissociation constant, Ka is then defined as Ka = [H₃O⁺][A⁻]/[HA].
- Large Ka values indicate strong acids (mostly dissociated); small Ka values indicate weak acids.
Interpretation of the Acid Dissociation Constant (Ka)
- The magnitude of Ka reflects the position of the acid dissociation reaction at equilibrium.
- A large Ka means the reaction lies to the right at equilibrium, indicating a strong acid.
- A small Ka means the reaction lies to the left at equilibrium, indicating a weak acid.
Definition and Interpretation of the Base Dissociation Constant (Kb)
- Kb is the equilibrium constant for the dissociation of a base in water.
- Kb = [BH⁺][OH⁻]/[B]
- A large Kb means the base readily accepts H⁺, indicating a strong base.
- A small Kb means the base weakly accepts H⁺, indicating a weak base.
Summary of Acid and Base Dissociation Reactions
- Acid and base dissociation reactions are equilibrium reactions.
- Large Ka values indicate strong acids.
- Large Kb values indicate strong bases.
- Strong acids have weak conjugate bases; strong bases have weak conjugate acids..
Introducing pKa and pKb
- The values of Ka and Kb can vary widely, so pKa and pKb are used to express their values in a more manageable and easier-to-compare form.
- pKa = -log₁₀(Ka)
- pKb = -log₁₀(Kb)
Ka, pKa, and the Strength of Acids
- Table of weak acids, their formulas, Ka, and pKa values.
The Ion Product of Water (Kw)
- Water acts as both an acid and a base.
- The equilibrium reaction of water is: H₂O + H₂O ⇌ H₃O⁺ + OH⁻.
- Kw = [H₃O⁺][OH⁻] = 1 x 10⁻¹⁴ mol² dm⁻⁶
Using Kw: A Worked Example
- Calculating the concentration of hydroxide ions ([OH⁻]) given the concentration of hydronium ions ([H₃O⁺]).
More Generally Ka, Kb, and Kw are Related
- Kw = Ka x Kb
- This relationship is useful for finding Ka if Kb is known (and vice versa) for conjugate acid/base pairs.
Summary of the Relationship Between Ka, Kb, and Kw
- pKa and pKb are used to express the strength of an acid or base.
- Strong acids have small pKa values, weak acids have large pKa values.
- Strong bases have small pKb values, weak bases have large pKb values.
- Kw = Ka x Kb
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Description
Explore the foundational concepts of acids and bases in biological contexts. This quiz covers important definitions, pH effects on enzymatic and protein functions, and the role of these substances in life processes. Test your understanding of how these chemical properties influence biology.
