Acids and Bases Weak A Weak Acid Equilibrium

Acids and Bases …Weak…

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #1: Write the dissociation equation HC 2 H 3 O 2 C 2 H 3 O 2 - + H+

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #2: ICE it! I C E HC 2 H 3 O 2 C 2 H 3 O 2 - + H+ 0. 50 - x 0 0 +x +x x x

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #3: Set up the law of mass action E HC 2 H 3 O 2 C 2 H 3 O 2 - + H+ 0. 50 - x x x

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #4: Solve for x, which is also [H+] E HC 2 H 3 O 2 C 2 H 3 O 2 - + H+ 0. 50 - x x x [H+] = 3. 0 x 10 -3 M

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #5: Convert [H+] to p. H E HC 2 H 3 O 2 C 2 H 3 O 2 - + H+ 0. 50 - x x x

Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH 3 NH 2 + H 2 O CH 3 NH 3+ + OH- Kb = 4. 38 x 10 -4

Kb for Some Common Weak Bases Many students struggle with identifying weak bases and their conjugate acids. What patterns do you see that may help you? Formula Conjugate Acid Kb NH 3 NH 4+ 1. 8 x 10 -5 Methylamine CH 3 NH 2 CH 3 NH 3+ 4. 38 x 10 -4 Ethylamine C 2 H 5 NH 2 C 2 H 5 NH 3+ 5. 6 x 10 -4 Diethylamine (C 2 H 5)2 NH 2+ 1. 3 x 10 -3 Triethylamine (C 2 H 5)3 NH+ 4. 0 x 10 -4 Hydroxylamine HONH 2 HONH 3+ 1. 1 x 10 -8 Base Ammonia Hydrazine H 2 NNH 2 H 2 NNH 3+ 3. 0 x 10 -6 Aniline C 6 H 5 NH 2 C 6 H 5 NH 3+ 3. 8 x 10 -10 Pyridine C 5 H 5 NH+ 1. 7 x 10 -9

Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H 2 O BH+ + OH- (Yes, all weak bases do this – DO NOT endeavor to make this complicated!)

A Weak Base Equilibrium Problem What is the p. H of a 0. 50 M solution of ammonia, NH 3, Kb = 1. 8 x 10 -5 ? Step #1: Write the equation for the reaction NH 3 + H 2 O NH 4+ + OH-

A Weak Base Equilibrium Problem What is the p. H of a 0. 50 M solution of ammonia, NH 3, Kb = 1. 8 x 10 -5 ? Step #2: ICE it! I C NH 3 + H 2 O NH 4+ + OH- 0. 50 - x E 0. 50 - x 0 0 +x +x x x

A Weak Base Equilibrium Problem What is the p. H of a 0. 50 M solution of ammonia, NH 3, Kb = 1. 8 x 10 -5 ? Step #3: Set up the law of mass action E NH 3 + H 2 O NH 4+ + OH 0. 50 - x x x

A Weak Base Equilibrium Problem What is the p. H of a 0. 50 M solution of ammonia, NH 3, Kb = 1. 8 x 10 -5 ? Step #4: Solve for x, which is also [OH-] E NH 3 + H 2 O NH 4+ + OH 0. 50 - x x x [OH-] = 3. 0 x 10 -3 M

A Weak Base Equilibrium Problem What is the p. H of a 0. 50 M solution of ammonia, NH 3, Kb = 1. 8 x 10 -5 ? Step #5: Convert [OH-] to p. H NH 3 + H 2 O NH 4+ + OH- E 0. 50 - x x x