Relative Strengths of Acids and Bases

Introduction

• Brønsted-Lowry Acids are H+ donors.
• The strength of an acid is measured by the extent to which it forms H3O+ in water.
  • HA(aq)    +    H2O(l)     ⇌ H3O+(aq)    +    A– (aq)
• Strong acids yield nearly 100% H3O+.
• Weak acids yield much less than 100% H3O+.

Acid Ionization Constant, Ka

• The relative strengths of weak acids can be determined by measuring their equilibrium constants
• Acid ionization constant (Ka): the equilibrium constant for an acid
• Ka values are related to the strength of the acids
  • The larger the Ka, the stronger the acid
  • The smaller the Ka, the weaker the acid
• Ka = ([H+][A-])/[HA] in a strong acid
  • As the molarity of HA approaches zero, the value of Ka becomes very, very large

Relative Strength of Bases

• Brønsted-Lowry Bases are H+ acceptors.
• The strength of a base is measured by the extent to which it forms hydroxide ions in water. 
  • B(aq)    +    H2O(l)     ⇌ HB+(aq)    +    OH–(aq)
• Strong bases yield nearly 100% OH–.
• Weak bases yield much less than 100% OH–.

Base Ionization Constant, Kb

• The relative strengths of weak bases can be determined by measuring their equilibrium constants
• Base ionization constant (Kb): the equilibrium constant for a base
• Kb = ([HB+][OH-])/[B]
• Kb values are related to the strength of the bases
  • The larger the Kb, the stronger the base
  • The smaller the Kb, the weaker the base

Ka and Kb

• Ka and Kb are inversely related.
  • The larger Ka is of an acid, the smaller the Kb is of its conjugate base.
  • The larger Kb is of a base, the smaller the Ka is of its conjugate acid.
• Therefore, the stronger the acid, the WEAKER its conjugate base.
• The extent to which an acid, HA, donates a proton to a water molecule depends upon the strength of that acid compared to the strength of  H3O+  
• Bronsted-Lowry acid base reactions go predominantly in the direction that forms the WEAKER acids and bases. 
• Any acid that is stronger than H3O+ is called a “strong acid,” and will be completely or nearly completely ionized in water.
• But,  if HA is a weaker acid than H3O+ then the ionization reaction goes only slightly ;  the reverse is favored over the forward.  Weak acids are only slightly ionized.
• The two-way yield sign indicates that HA is NOT a strong acid.  
• HA must be a weaker acid than H3O+ and,  A- must be a STRONGER base than water. The conjugate base of any weak acid must be more basic than water.