Acids and Bases Dissociation of Strong Bases MOHs
Acids and Bases
Dissociation of Strong Bases MOH(s) M+(aq) + OH-(aq) Ø Strong bases are metallic hydroxides ØGroup I hydroxides (Na. OH, KOH) are very soluble ØGroup II hydroxides (Ca, Ba, Mg, Sr) are less soluble Ø p. H of strong bases is calculated directly from the concentration of the base in solution
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
Read 14. 6: Bases Page 706 questions #71, 72, 79, 80, 83, 84
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