Acids and Bases: Part 1
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Questions and Answers

Which of the following correctly defines an acid based on the Brønsted-Lowry theory?

  • A substance that increases the concentration of OH- in a solution.
  • A substance that will not change in an acidic environment.
  • A compound that donates H+ ions in a chemical reaction. (correct)
  • A molecule that accepts protons from acids.

In the dissociation of HCl in water, what is the primary species formed besides Cl⁻?

  • Na+
  • H2O
  • H3O+ (correct)
  • OH-

Which of the following statements about the pH range in biological systems is correct?

  • All biological systems maintain a neutral pH of 7.
  • Organisms usually regulate their pH within a narrow range for optimal function. (correct)
  • An increase in pH always decreases biological activity.
  • Most enzymes function optimally at a highly acidic pH level.

What characterizes a strong acid in an aqueous solution?

<p>It readily dissociates to form its conjugate base. (A)</p> Signup and view all the answers

In the equilibrium reaction of a weak acid, what is true about its conjugate base?

<p>It is a weak base that does not accept protons. (A)</p> Signup and view all the answers

What is a common misconception about bases in aqueous solutions?

<p>They solely increase the concentration of H+ ions in water. (D)</p> Signup and view all the answers

How does the strength of a weak base compare to that of a strong base?

<p>The weak base does not readily accept H+ ions. (B)</p> Signup and view all the answers

What is the role of buffers in biological systems?

<p>To prevent drastic changes in pH levels. (A)</p> Signup and view all the answers

What is the acid dissociation constant (Kc) calculated from in an acid-base equilibrium?

<p>The concentration of the conjugate base and hydronium ions over the undissociated acid and water. (C)</p> Signup and view all the answers

In the reaction CN⁻ + H⁺ ⇌ HCN, what is the role of CN⁻?

<p>Weak base (B)</p> Signup and view all the answers

Which statement accurately describes water's role in acid-base reactions?

<p>Water can act as both an acid and a base depending on the reaction. (A)</p> Signup and view all the answers

What happens when a weak acid is placed in water?

<p>It establishes a chemical equilibrium with little dissociation. (B)</p> Signup and view all the answers

What occurs to a water molecule when it acts as a base in an acid-base reaction?

<p>It becomes a hydroxide ion (OH-). (D)</p> Signup and view all the answers

In the reaction NH3 + H2O ⇌ NH4+ + OH-, which component acts as a base?

<p>NH3 (B)</p> Signup and view all the answers

Which statement is correct regarding strong acids and their conjugate bases?

<p>Strong acids have weak conjugate bases. (B)</p> Signup and view all the answers

What role does water play in ammonia's reaction mixture?

<p>It acts as an acid in the reaction. (A)</p> Signup and view all the answers

What is the value of the ion product of water (Kw) at 25℃?

<p>1 x 10 -14 mol2 dm-6 (C)</p> Signup and view all the answers

If the concentration of hydronium ions [H3O+] is 1 x 10 -5 M, what is the concentration of hydroxide ions [OH-] in that solution?

<p>1 x 10 -9 M (A)</p> Signup and view all the answers

How are Ka, Kb, and Kw related in the context of acid-base chemistry?

<p>Kw = Ka x Kb (C)</p> Signup and view all the answers

What happens to the concentration of OH- ions if the concentration of H3O+ ions in a solution is increased?

<p>The concentration of OH- ions decreases. (C)</p> Signup and view all the answers

If a weak acid has a Ka of 1 x 10 -4, what can be inferred about the corresponding Kb of its conjugate base?

<p>Kb can be determined using Kw and Ka. (A)</p> Signup and view all the answers

What is the significance of the relationship $Ka \times Kb = Kw$?

<p>It allows calculations of the acid or base strength based on known values. (B)</p> Signup and view all the answers

Which statement correctly describes the relationship between $Ka$, $Kb$, and the strength of acids and bases?

<p>A strong conjugate base corresponds to a weak acid. (B)</p> Signup and view all the answers

What does the pKa value represent in terms of acid strength?

<p>It simplifies the comparison of acid strengths by converting $Ka$ values. (B)</p> Signup and view all the answers

When ammonia reacts with water, which species is formed that acts as a base?

<p>Hydroxide ion ($OH^-$) (D)</p> Signup and view all the answers

Which of the following statements about $Kw$ is true?

<p>$Kw$ is defined as the product of the hydronium and hydroxide ion concentrations. (B)</p> Signup and view all the answers

What does a large value of Ka indicate about an acid?

<p>The acid readily donates a proton. (C)</p> Signup and view all the answers

How is pKa related to Ka?

<p>pKa is the negative logarithm of Ka. (C)</p> Signup and view all the answers

What is denoted by a small value of Kb?

<p>The base is classified as weak. (B)</p> Signup and view all the answers

What does the pKa value of a strong acid generally look like?

<p>It is usually greater than 7. (D)</p> Signup and view all the answers

What does Kw represent in aqueous solutions?

<p>The ion product of water. (A)</p> Signup and view all the answers

What indicates that water is not a strong acid or base?

<p>The dissociation of water lies to the left at equilibrium. (D)</p> Signup and view all the answers

If an acid has a pKa of 5.5, what can be inferred about its Ka value?

<p>The Ka value is approximately 3.0 x 10-6. (C)</p> Signup and view all the answers

Which of the following statements is true regarding the ion product of water (Kw)?

<p>Kw changes with temperature and is defined at 25°C. (A)</p> Signup and view all the answers

How is the acid dissociation constant, Ka, derived from the equilibrium constant, Kc?

<p>Ka = Kc x [H2O] (D)</p> Signup and view all the answers

What does a small value of Kb suggest about a base?

<p>The base is weak and does not dissociate significantly. (D)</p> Signup and view all the answers

Which equilibrium reaction correctly represents the derivation of Kb?

<p>B + H2O ⇌ BH+ + OH- (C)</p> Signup and view all the answers

What does the acid dissociation constant Ka represent mathematically?

<p>[H3O+] * [A-] / [HA] (B)</p> Signup and view all the answers

In the reaction HA + H2O ⇌ A- + H3O+, what happens if Ka is decreased?

<p>The acid becomes weaker. (B)</p> Signup and view all the answers

What does the equilibrium position indicate when Ka is large?

<p>The reaction lies predominantly to the right. (D)</p> Signup and view all the answers

Which of the following statements is true regarding Kc and the dissociation constants?

<p>Ka and Kb provide insight into the strength of acids and bases respectively. (B)</p> Signup and view all the answers

Flashcards

Acid

A substance that donates a hydrogen ion (H+).

Base

A substance that accepts a hydrogen ion (H+).

Brønsted-Lowry definition

Describes acids and bases as H+ donors and acceptors, respectively.

Hydronium ion

H3O+. Formed when H+ ions combine with water molecules.

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Hydroxide ion

OH-. Formed when a base reacts with water and accepts a proton.

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Acid dissociation

A reversible process where an acid separates into ions.

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H+ ion

A proton.

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pH

A measure of the concentration of H+ ions in a solution.

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Conjugate pair

An acid and its corresponding base that differ only by a proton (H+). They are always present in a reaction involving acid-base interactions.

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Strong acid

An acid that completely dissociates in solution, releasing all its hydrogen ions (H+) and forming hydronium ions (H3O+).

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Weak acid

An acid that only partially dissociates in solution, releasing some but not all of its hydrogen ions (H+).

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Conjugate base

The species that remains after an acid donates a proton (H+). It is the base part of the conjugate pair.

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Strong base

A base that readily accepts a proton (H+) from solution, forming hydroxide ions (OH-).

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Weak base

A base that only partially accepts protons (H+) from solution, forming only a limited amount of hydroxide ions (OH-).

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Acid dissociation constant (Ka)

A measure of the strength of an acid. A higher Ka value indicates a stronger acid, meaning it dissociates more readily.

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Equilibrium constant (Kc)

A value that represents the ratio of products to reactants at equilibrium in a reversible reaction. For acid dissociation, it's denoted as Ka.

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What is Ka?

Ka is the acid dissociation constant, representing the equilibrium constant for an acid's dissociation in water. It reflects how much an acid dissociates into ions.

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Large Ka

A large Ka value indicates that the acid strongly dissociates in solution, resulting in a high concentration of H3O+ ions. This makes the acid a strong acid.

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Small Ka

A small Ka value suggests that the acid only partially dissociates in solution, leading to a low concentration of H3O+ ions. This makes the acid a weak acid.

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What is Kb?

Kb is the base dissociation constant, measuring the equilibrium constant for a base's reaction with water to produce OH- ions.

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Large Kb

A large Kb value means the base readily reacts with water, creating a high concentration of OH- ions. This makes the base a strong base.

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Small Kb

A small Kb value indicates that the base reacts weakly with water, producing a low concentration of OH- ions. This makes the base a weak base.

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Ka and Kb Relationship

Ka and Kb are inversely related. If one is large, the other tends to be small. This reflects the complementary nature of acids and bases.

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Dissociation Equilibrium

Acid and base dissociation reactions are equilibrium reactions, meaning they proceed in both directions simultaneously. The balance between reactants and products determines the strength (Ka or Kb).

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Large Ka value

Indicates a strong acid that readily donates a proton, resulting in an equilibrium that favors the product side.

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Small Ka value

Indicates a weak acid that donates protons less readily, resulting in an equilibrium that favors the reactant side.

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Large Kb value

Indicates a strong base that readily accepts a proton, resulting in an equilibrium that favors the product side.

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Small Kb value

Indicates a weak base that accepts protons less readily, resulting in an equilibrium that favors the reactant side.

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Water as an acid and a base

Water can act as both an acid and a base, donating or accepting protons in different reactions. This is known as amphoterism.

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Ion product of water

The product of the concentrations of hydronium ions (H3O+) and hydroxide ions (OH-) in any aqueous solution, represented by Kw = [H3O+][OH-]. At 25°C, Kw has a value of 1 x 10^-14 mol² dm^-6.

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How does Kw relate to pH and pOH?

Kw is a crucial factor for calculating pH and pOH. The relationship is: Kw = [H3O+][OH-] = 1 x 10^-14 mol² dm^-6. This means that, if you know the concentration of either H3O+ or OH-, you can calculate the other using Kw.

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What is the meaning of Kw = 1 x 10^-14?

This equation signifies that in pure water, the concentrations of H3O+ and OH- are equal and are both 1 x 10^-7 mol dm^-3. This means that pure water is neutral, with neither acidic nor basic properties.

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Kw relationship with Ka and Kb

The ion product of water (Kw) is related to the acid dissociation constant (Ka) and the base dissociation constant (Kb) by the equation: Kw = Ka x Kb. This means that you can calculate the value of Ka or Kb if you know the value of the other.

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How can we use Kw to find the concentration of OH-?

If we know the concentration of hydronium ions (H3O+) in a solution, we can use Kw to calculate the concentration of hydroxide ions (OH-). This is done by rearranging the equation: [OH-] = Kw / [H3O+].

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Kb vs. Ka

Kb is the base dissociation constant, measuring a base's strength. Ka is the acid dissociation constant, measuring an acid's strength. They're linked for conjugate acid-base pairs.

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Kw: The Water Constant

Kw represents the equilibrium constant for water's self-ionization. It's a fixed value, meaning the product of hydronium ion [H3O+] and hydroxide ion [OH-] concentrations is always constant at a given temperature.

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How are Ka and Kb related to Kw?

The product of Ka and Kb for a conjugate acid-base pair is always equal to Kw, the ion product of water.

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pKa: What is it?

pKa is the negative logarithm of Ka. It's a convenient way to express the strength of an acid. A lower pKa value indicates a stronger acid.

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Strong Acid/Weak Base

A strong acid readily releases H+ ions, while its conjugate base is weak, accepting fewer H+ ions.

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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

  • Bases are H⁺ acceptors (compounds that combine with an H⁺ ion).

  • In water, H⁺ ions don't exist independently; they combine with water to form hydronium ions (H₃O⁺).

  • Example: HCl dissociates in water as HCl + H₂O ⇌ H₃O⁺ + Cl⁻.

  • Brønsted-Lowry definition:

    • Acid = H⁺ donor
    • Base = H⁺ acceptor

  • 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⁻
  • 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⁺]).
  • 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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Acids and Bases Part 1 PDF

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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.

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