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.