Understanding Weak Acids and Bases Equilibrium
Explore the concept of weak acids and bases equilibrium through a step-by-step example involving acetic acid (HC2H3O2) with a Ka value of 1.8 x 10-5. Learn how to calculate the pH of a 0.50 M solution of acetic acid by setting up the dissociation equation, using the law of mass action, solving for x (H+), and converting it to pH. Additionally, discover the reaction of weak bases with water and common weak bases and their conjugate acids. Enhance your knowledge of acid-base chemistry in a comprehensive manner.
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Presentation Transcript
Acids and Bases Weak
A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #1: Write the dissociation equation HC2H3O2 C2H3O2- + H+
A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #2: ICE it! HC2H3O2 C2H3O2- + H+ 0.50 - x I C E 0 0 +x +x x x 0.50 - x
A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #3: Set up the law of mass action HC2H3O2 C2H3O2- + H+ 0.50 - x E x x 2 ( )( ) x x x = 5 8 . 1 10 x . 0 ( 50 ) . 0 ( 50 ) x
A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #4: Solve for x, which is also [H+] HC2H3O2 C2H3O2- + H+ 0.50 - x E x x 2 x = 5 8 . 1 10 [H+] = 3.0 x 10-3 M x . 0 ( 50 )
A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #5: Convert [H+] to pH HC2H3O2 C2H3O2- + H+ 0.50 - x = E x x 3= log( 0 . 3 10 ) . 4 52 pH x
Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH3NH2 + H2O CH3NH3+ + OH- Kb = 4.38 x 10-4 + [ ][ ] CH NH OH = = 4 4.38 10 3 CH NH 3 K x b [ ] 3 2
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? Conjugate Acid NH4+ CH3NH3+ C2H5NH3+ (C2H5)2NH2+ (C2H5)3NH+ HONH3+ H2NNH3+ C6H5NH3+ C5H5NH+ Base Formula Kb Ammonia Methylamine Ethylamine Diethylamine Triethylamine Hydroxylamine Hydrazine Aniline Pyridine NH3 CH3NH2 C2H5NH2 (C2H5)2NH (C2H5)3N HONH2 H2NNH2 C6H5NH2 C5H5N 1.8 x 10-5 4.38 x 10-4 5.6 x 10-4 1.3 x 10-3 4.0 x 10-4 1.1 x 10-8 3.0 x 10-6 3.8 x 10-10 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 + H2O BH+ + OH- [ b K = + ][ B ] BH OH [ ] (Yes, all weak bases do this DO NOT endeavor to make this complicated!)
A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #1: Write the equation for the reaction NH3 + H2O NH4+ + OH-
A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #2: ICE it! NH3 + H2O NH4+ + OH- I C E 0.50 - x 0 0 +x +x x x 0.50 - x
A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #3: Set up the law of mass action NH3 + H2O NH4+ + OH- E 0.50 - x x x 2 ( )( ) x x x = 5 8 . 1 10 x . 0 ( 50 ) . 0 ( 50 ) x
A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #4: Solve for x, which is also [OH-] NH3 + H2O NH4+ + OH- E 0.50 - x x x 2 x = 5 8 . 1 10 [OH-] = 3.0 x 10-3 M x . 0 ( 50 )
A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #5: Convert [OH-] to pH NH3 + H2O NH4+ + OH- E 0.50 - x x x = = 3= = log( 0 . 3 10 ) . 4 52 pOH x 14 00 . . 9 48 pH pOH
