Acid Equilibrium and pH Søren Sørensen

Acid/Base Definitions  Arrhenius Model  Acids produce hydrogen ions in aqueous solutions  Bases produce hydroxide ions in aqueous solutions  Bronsted-Lowry Model  Acids are proton donors  Bases are proton acceptors  Lewis Acid Model  Acids are electron pair acceptors  Bases are electron pair donors

Acid Dissociation HA  H + + A - Acid Proton Conjugate base Alternately, H + may be written in its hydrated form, H 3 O + (hydronium ion)

Dissociation of Strong Acids Strong acids are assumed to dissociate completely in solution. Large K a or small K a ? Reactant favored or product favored?

Dissociation Constants: Strong Acids

Dissociation of Weak Acids Weak acids are assumed to dissociate only slightly (less than 5%) in solution. Large K a or small K a ? Reactant favored or product favored?

Dissociation Constants: Weak Acids

Self-Ionization of Water H 2 O + H 2 O  H 3 O + + OH - At 25 , [H 3 O + ] = [OH - ] = 1 x K w is a constant at 25  C: K w = [H 3 O + ][OH - ] K w = (1 x )(1 x ) = 1 x

Calculating pH, pOH pH = -log 10 (H 3 O + ) pOH = -log 10 (OH - ) Relationship between pH and pOH pH + pOH = 14 Finding [H 3 O + ], [OH - ] from pH, pOH [H 3 O + ] = 10 -pH [OH - ] = 10 -pOH

pH and pOH Calculations

The pH Scale Graphic: Wikimedia Commons user Slower

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #1: Write the dissociation equation HC 2 H 3 O 2  C 2 H 3 O H +

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #2: ICE it! HC 2 H 3 O 2  C 2 H 3 O H x +x x xx

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #3: Set up the law of mass action HC 2 H 3 O 2  C 2 H 3 O H xxx E

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #4: Solve for x, which is also [H + ] HC 2 H 3 O 2  C 2 H 3 O H xxx E [H + ] = 3.0 x M

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #5: Convert [H + ] to pH HC 2 H 3 O 2  C 2 H 3 O H xxx E

Text Pg. 671 Sample 16.1 and PracticeText Pg. 671 Sample 16.1 and Practice Sample 16.2 and PracticeSample 16.2 and Practice Pg. 673 probably will not appear on AP test.Pg. 673 probably will not appear on AP test. Equilibrium shifts from the stronger acid to the stronger base (table will not be available to id the strengths)Equilibrium shifts from the stronger acid to the stronger base (table will not be available to id the strengths) Pg. 674 and 675 Sample and practicePg. 674 and 675 Sample and practice Pg. 677 use Kw to calculate [H + ] or [OH - ]Pg. 677 use Kw to calculate [H + ] or [OH - ] Pg and 16.7 Sample and practicePg and 16.7 Sample and practice

Pg. 679 IndicatorsPg. 679 Indicators Most important phenolphthaleinMost important phenolphthalein Pink in the presence of a basePink in the presence of a base Pg and 16.9 Sample and PracticePg and 16.9 Sample and Practice Pg. 683 Sample and practicePg. 683 Sample and practice

Percent Ionization Percent ionization = concentration ionized X 100Percent ionization = concentration ionized X 100 original concentration original concentration Percent ionization = [H + ] equilibrium X 100Percent ionization = [H + ] equilibrium X 100 [HA] initial [HA] initial Pg. 684 Sample and practicePg. 684 Sample and practice Pg. 685 Sample and practicePg. 685 Sample and practice