CHEM1312H - Equilibrium Constants for Acids and Bases

equilibrium constants for acids and bases

-acids and bases can be very broadly classified in strong and weak electrolytes

-later on, it will be important to distinguish if the substance is a strong or weak electrolyte for pH calculations (pH calculations of strong acids and bases are different from weak acids and bases)

strong acids

-strong acids are strong electrolytes

-if the ionization equilibrium lies far to the right the acid is strong (can lose protons easily or product-favored

-we consider strong acid as completely ionized in this course (indicated by the unidirectional arrow or normal arrow)

strong acids to memorize

-hydrochloric acid (HCl)

-hydrobromic acid (HBr)

-hydroiodic acid (HI)

-chloric acid (HClO₃)

-nitric acid (HNO₃)

-perchloric acid (HClO₄)

-sulfuric acid (H₂SO₄) (diprotic)

strong acids additional information

-HCl and HNO₃ are monoprotic (contains one ionizable proton)

-sulfuric acid, H₂SO₄ is a diprotic acid (contains two ionizable protons)

strong bases

-most strong bases are ionic compounds (strong electrolytes) containing OH^-

-they are either group I or II metal hydroxides

-since strong bases are strong electrolytes, they completely dissociate in solution

strong bases to memorize

-lithium hydroxide (LiOH)

-sodium hydroxide (NaOH)

-potassium hydroxide (KOH)

-strontium hydroxide (Sr(OH)₂)

-calcium hydroxide (Ca(OH)₂)

-barium hydroxide (Ba(OH)₂)

weak acids

-weak acids are weak electrolytes

-if the equilibrium lies far to the left (partially ionizes) the acid is weak (does not lose protons easily)

-a weak acid yields a strong conjugate base (has a strong affinity for a proton)

-weak acids can also be monoprotic, diprotic, or triprotic (contains three ionizable protons)

-carboxylic acids such as methanoic acid and ethanoic acid are also weak acids

equilibrium constant for weak acids

-for weak acids, we will use the acid ionization constant K_a, the equilibrium constant for the ionization of weak acids to express its ionization in water

-the value of K_a reflects the extent to which a weak acid ionizes and in turn the relative strength of a weak acid

-the stronger the acid, the higher the value of K_a

-another way to report and compare the relative acid strength is pKa (pK_a = -logK_a)

-the stronger the acid, the higher the K_a, the lower the pK_a and vice versa

weak bases

-like weak acids, weak bases partially ionize in water (as indicated by the reversible arrows)

-most common weak bases produce OH^- by accepting a proton from water to form OH^-

-weak bases that are neutral substances are nitrogen containing compounds amines

weak bases equilibrium constant

-you can quantify the extent of ionization with the base ionization constant, K_b that is used to compare the relative strengths of weak bases

-the stronger the base, the higher the value of K_b

logarithmic scale of relative base strength, pK_b

-another way to report and compare the relative base strength is pK_b (pK_b = -logK_b)

-the stronger the base, the higher the K_b, the lower the pK_b and vice versa

relating the ionization constant for an acid and its conjugate base

-for a conjugate acid-base pair we can derive the relationship: K_w = K_a x K_b

-pK_a + pK_b = 14.00

polyprotic acids (honorable mention)

-like other acids, polyprotic acids have more than one ionizable hydrogen dissociating one at a time with a K_a value for each step (ex: H₃PO₄)

-similarly, the K_b value conjugates of polyprotic bases can be calculated from the K_a of the conjugate acid (look at slides)