15.11
Acid/Base Strengths and Dissociation Constants
The relative strength of an acid or base is the extent to which it ionizes when dissolved in water. If the ionization reaction is essentially complete…
Acids and bases can be categorized by whether they are a strong acid, a strong base, a weak acid, or a weak base. There are very few strong acids and bases so the majority of acids and bases are weak.
A strong acid, like hydrochloric acid, completely dissociates into hydrogen ions and chloride ions when dissolved in water. A strong base, like sodium hydroxide, dissociates completely into sodium ions and hydroxide ions.
Weak acids and bases partially dissociate and are present in both ionized and un-ionized forms. For example, both acetic acid and its weak conjugate base, acetate, are found in an aqueous solution.
The degree of dissociation of a weak acid or base can be measured using its equilibrium constant. The equilibrium constant for weak acids has a special name, the acid dissociation constant, or, Ka.
For a generic weak acid HA, Ka at a given temperature can be calculated by an equilibrium equation: dividing the concentration of products, A ion and hydronium ion, by the concentration of reactants, HA and water.
As water is liquid and its concentration remains nearly unchanged in the reaction, it is excluded from the equation.
The higher the Ka, the stronger the acid. Nitrous acid is stronger than acetic acid because the Ka of nitrous acid is larger than the Ka of acetic acid.
The equilibrium constant for weak bases, the base dissociation constant, or Kb, acts in a similar manner to Ka. For a generic weak base B, Kb at a given temperature can be determined by dividing the concentration of products, BH ion, and hydroxide ion, by the concentration of reactant, B.
Like acids, the strength of the bases is also directly proportional to the Kb. For example, ethylamine is relatively stronger than urea because the Kb of ethylamine is larger than the Kb of urea.
The strength of an acid can also be expressed in terms of percent ionization. The percent ionization of an acid can be calculated by dividing hydronium ion concentration at equilibrium by the initial acid concentration and multiplying it by a hundred.
Similarly, the percent ionization for bases can be calculated by dividing the hydroxide ion concentration at equilibrium by the initial concentration of base and multiplying it by a hundred. The higher the percent ionization, the stronger the acid or base.
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Q1: What is the difference between strong and weak acids?
Strong acids completely dissociate into ions when dissolved in water, like hydrochloric acid forming hydrogen and chloride ions. Weak acids only partially dissociate, existing in both ionized and un-ionized forms in solution. Since there are very few strong acids, most acids encountered are weak. The degree of dissociation determines acid strength and can be measured using the acid dissociation constant, Ka.
Q2: How does the acid dissociation constant measure acid strength?
The acid dissociation constant, Ka, is calculated by dividing the concentration of products by the concentration of reactants at equilibrium. The higher the Ka value, the stronger the acid because more ionization occurs. For example, nitrous acid is stronger than acetic acid because its Ka is larger. Ka values allow direct comparison of weak acid strengths in solutions and solutions and dissociation constant calculations.
Q3: What is the base dissociation constant and how does it work?
The base dissociation constant, Kb, measures the strength of weak bases similarly to how Ka measures acids. It is calculated by dividing the concentration of products by the concentration of the base at equilibrium. A larger Kb indicates a stronger base. For instance, ethylamine is relatively stronger than urea because ethylamine has a larger Kb value, reflecting greater ionization in aqueous solution.
Q4: Why is percent ionization useful for comparing acid and base strength?
Percent ionization expresses acid or base strength as a percentage by dividing the concentration of ionized species at equilibrium by the initial concentration and multiplying by 100. Higher percent ionization indicates stronger acids or bases. Unlike Ka or Kb values, percent ionization varies with initial concentration, typically decreasing as concentration increases, providing context-dependent strength assessment.
Q5: How do conjugate acid-base pairs relate to dissociation constants?
In weak acid solutions, both the acid and its conjugate base exist in equilibrium. For example, acetic acid and acetate coexist in aqueous solution. The relative strengths of conjugate acid base pairs are inversely related: if an acid is weak, its conjugate base is relatively strong. Understanding this relationship helps predict ionization behavior and solution pH in buffer systems.
Q6: What are the most common strong acids and bases?
Common strong acids include hydrochloric acid, nitric acid, and sulfuric acid, which completely ionize in water. Strong bases include sodium hydroxide, potassium hydroxide, and calcium hydroxide. These substances are important reference points because they completely dissociate, making their ionization predictable and their Ka or Kb values immeasurably large compared to weak acids and bases.
Q7: How does water concentration affect the acid dissociation constant equation?
Although water is a reactant in acid dissociation, it is excluded from the Ka equation because water is the solvent and its concentration remains nearly constant during the reaction. This simplification allows Ka to accurately reflect only the ionization of the acid itself. The same principle applies to base dissociation constants, where water concentration is similarly omitted from the Kb expression.