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Acid Dissociation Constant: Unlocking Chemical Equilibria
Dive into the world of acid-base chemistry with our comprehensive guide on acid dissociation constant (Ka) and base dissociation constant (Kb). Master equilibrium expressions and ion calculations for real-world applications.

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Now Playing:Acid dissociation constant – Example 0a
Intros
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  1. What is the acid/base dissociation constant?
  2. What is the acid/base dissociation constant?
    From Kw to Ka and Kb.
  3. What is the acid/base dissociation constant?
    The Ka and Kb expressions.
Examples
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  1. Use the relationship between the Ka and Kw expressions to find an unknown Kb value.
    1. Ethanoic acid, or acetic acid CH3COOH is a weak acid with Ka = 1.4*10-5.1
      1. Write the formula of its conjugate base.
      2. Find the Kb for its conjugate base using the relationship between Ka/Kb and Kw.

    2. Explain using the Ka/Kb expression why Ka and Kb values are not normally used when studying strong acids and bases.

      1 Source for Ka acid dissociation constants: ATKINS, P. W., & DE PAULA, J. (2006). Atkins' Physical chemistry. Oxford, Oxford University Press.

Acid dissociation constant
Jump to:NotesConceptFAQsPrerequisites
Notes

In this lesson, we will learn:

  • To recall the equilibrium ionization expressions for weak acids and bases.
  • How to relate Ka and Kb for conjugate acid/base pairs.
  • How to calculate concentration of aqueous ions in weak acid/base solutions.

Notes:

  • In Autoionization of water, we looked at the equilibrium:

    2 H2O(l)  ⇌ \, \rightleftharpoons \, H3O+ (aq) + OH- (aq)

    The equilibrium constant was expressed as:

    Kw = [H3O+] [OH-] = 1 * 10 -14 at 25oC

    We saw the effect of adding strong acids/bases to equilibrium concentrations of water and dissolved ions. That was straightforward because strong acids and bases experience 100% dissociation in water.
    We also saw in
  • In strong and weak acids and bases that weak acids and bases do not experience 100% dissociation. This makes expressions for their dissociation in water more complicated.

  • Every weak acid has an acid dissociation constant, Ka and a weak base a base dissociation constant, Kb. These are equilibrium constants showing how much the acid/base dissociates when dissolved in water (aqueous solution).
    For a weak acid HX dissolved in water:

    Ka = [H3O+][X−][HX]\frac{[H_3O^{+}] [X^{-}]}{[HX]}

    For a weak base B dissolved in water:

    Kb = [HB+][OH−][B]\frac{[HB^{+}] [OH^{-}]}{[B]}

    • Just like in the ionization of water and other equilibria; this is an equilibrium constant expression. This means that the higher the Ka/Kb value, the greater the degree of dissociation (because the concentrations of the dissociated ions, in the numerator, are larger values) and therefore the stronger the acid or base.
    • Remember that for strong acids and bases Ka/Kb values are not normally used. This is because in the Ka expression, their [HX] or [B] is equal to or almost zero due to complete dissociation, so the values are incredibly large.
    • For strong acids, pKa is used instead of Ka. pKa is the negative logarithm of the Ka value and is more appropriate to use for strong acids, instead of the extremely large Ka values they have.

  • In Conjugate acids and bases, we learned in a conjugate pair that a stronger conjugate acid will have a weaker the conjugate base. This relationship affects the concentration of aqueous ions and therefore affects the Ka and Kb values for a conjugate acid/base pair!
    Consider the equations for the conjugate pair acid-base pair HX and X-:

    Conjugate acid: \qquad HX + H2O  ⇌ \, \rightleftharpoons \, X- + H3O+ \qquad

    Ka = [H3O+][X−][HX]\frac{[H_3O^{+}] [X^{-}]}{[HX]}

    Conjugate base: \qquad X- + H2O  ⇌ \, \rightleftharpoons \, HX + OH- \qquad

    Kb = [HX][OH−][X−]\frac{[HX] [OH^{-}]}{[X^{-}]}

    • Both equations depend on [X-] and [HX] so Ka and Kb themselves can be related, and terms cancelled out:

      buffer solutions weak acid or base ph = pka

      As you can see, the result is the product of [H3O+] and [OH-] which is the expression for Kw. Therefore for a conjugate pair:

      Ka * Kb = Kw
Concept

Introduction to Acid Dissociation Constant

The acid dissociation constant (Ka) and base dissociation constant (Kb) are fundamental concepts in chemistry, crucial for understanding the behavior of acids and bases in solution. Our introductory video provides a comprehensive overview of these constants, serving as an essential foundation for further study. This lesson aims to equip you with the knowledge to recall equilibrium ionization expressions for weak acids and bases, a skill vital for analyzing chemical reactions. You'll learn to relate Ka and Kb for conjugate acid-base pairs, understanding their inverse relationship. Additionally, we'll explore methods for calculating concentrations of aqueous ions in weak acids and bases solutions, a practical application of these concepts. By mastering these objectives, you'll gain a deeper understanding of acid-base chemistry and its real-world implications, setting the stage for more advanced topics in chemical equilibria.

FAQs

  1. What is the base dissociation constant KB?

    The base dissociation constant (Kb) is a measure of how completely a base dissociates in an aqueous solution. It quantifies the strength of a base by indicating its ability to produce hydroxide ions (OH-) when dissolved in water. A larger Kb value indicates a stronger base.

  2. What is the formula for the dissociation constant of a base?

    For a base B that dissociates in water according to the equation B + H2O BH+ + OH-, the formula for Kb is:

    Kb = (BH+)(OH-) / (B)

    Where (BH+), (OH-), and (B) represent the equilibrium concentrations of the conjugate acid, hydroxide ion, and undissociated base, respectively.

  3. Does a higher KB mean stronger base?

    Yes, a higher Kb value indicates a stronger base. Bases with larger Kb values dissociate more completely in water, producing more hydroxide ions. For example, a base with Kb = 1 × 10^-3 is stronger than a base with Kb = 1 × 10^-5.

  4. What is the base dissociation equation?

    The general base dissociation equation is:

    B + H2O BH+ + OH-

    Where B is the base, BH+ is the conjugate acid, and OH- is the hydroxide ion. This equation represents the equilibrium established when a base dissolves in water.

  5. How are Ka and Kb related for conjugate acid-base pairs?

    For a conjugate acid-base pair, Ka and Kb are inversely related. Their product is equal to the ion product of water (Kw):

    Ka × Kb = Kw = 1.0 × 10^-14 (at 25°C)

    This relationship means that if an acid is strong (high Ka), its conjugate base will be weak (low Kb), and vice versa.

Prerequisites

Understanding the acid dissociation constant (Ka) is crucial in chemistry, but to fully grasp this concept, it's essential to have a solid foundation in certain prerequisite topics. Two key areas that significantly contribute to comprehending acid dissociation constants are strong and weak acids and bases and the relationship between two variables.

Firstly, a thorough understanding of weak acids and bases is fundamental to grasping the concept of acid dissociation constants. The Ka value is directly related to the strength of an acid, indicating how readily it dissociates in water. Strong acids have large Ka values, while weak acids have smaller ones. By familiarizing yourself with the characteristics and behaviors of strong and weak acids, you'll be better equipped to interpret and apply Ka values in various chemical scenarios.

Moreover, the acid dissociation constant is intrinsically linked to equilibrium concepts in chemistry. Understanding how weak acids partially dissociate in solution and reach a state of dynamic equilibrium is crucial for comprehending Ka. This knowledge allows you to predict the extent of dissociation and calculate important parameters such as pH and concentration of species in solution.

Secondly, the relationship between two variables plays a significant role in understanding acid dissociation constants. In chemistry, this concept is particularly important when considering the relationship between Ka, Kb, and Kw. These constants are interconnected, and understanding how they relate to each other is crucial for solving problems involving acid-base equilibria.

For instance, the product of Ka and Kb for a conjugate acid-base pair is always equal to Kw, the ion product constant of water. This relationship allows chemists to calculate one constant if the other is known. Additionally, understanding how these variables relate to each other helps in predicting the behavior of acids and bases in different solutions and at varying concentrations.

By mastering these prerequisite topics, you'll develop a strong foundation for understanding acid dissociation constants. This knowledge will enable you to solve complex problems involving acid-base equilibria, predict chemical behaviors, and apply these concepts in real-world scenarios. Whether you're studying buffer solutions, titrations, or more advanced topics in chemistry, a solid grasp of these fundamental concepts will prove invaluable.

In conclusion, taking the time to thoroughly understand strong and weak acids and bases and the relationship between variables will significantly enhance your ability to work with acid dissociation constants. These prerequisite topics provide the necessary context and mathematical framework to fully appreciate the role of Ka in chemistry and its applications in various chemical processes.