Chapter 8: Phenomena Phenomena: Buffers are sometimes defined as: a - PowerPoint PPT Presentation

Chapter 8: Phenomena Phenomena: Buffers are sometimes defined as: a solution that resists changes in pH when an acid or base is added to it. This definition can be a little misleading. Look at the four titration curves below. The circled regions are considered buffer regions while the boxed regions are not. What do all of the boxed regions have in common and why are they not considered buffers? Weak Acid Titrated with Strong Base Strong Acid Titrated with Strong Base 14 [OH - ]=1 M 14 [OH - ]=1 M [OH - ]=0.1 M [OH - ]=0.1 M 12 12 10 10 8 [H + ]=1×10 -8 M 8 [OH - ]=1×10 -7 M pH pH 6 6 [H + ]=1×10 -7 M [H + ]=0.1 M 4 4 [H + ]=1×10 -7 M 2 2 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 [H + ]=1 M Amount Strong Base Added Amount Strong Base Added Strong Base Titrated with Strong Acid Weak Base Titrated with Strong Acid 14 14 [OH - ]=0.1 M [OH - ]=1×10 -4 M 12 12 10 10 [OH - ]=1 M [OH - ]=1×10 -6 M [OH - ]=1×10 -7 M 8 8 pH pH [H + ]=1×10 -7 M 6 6 4 4 [H + ]=0.1 M 2 2 [H + ]=0.1 M 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 [H + ]=1 M [H + ]=1 M Amount Strong Acid Added Amount Strong Base Added Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium

Chapter 8 Applications of Aqueous Equilibrium • Acid/Base Review Big Idea: Buffer systems maintain • Buffer Solutions the pH value of a • Titration Curves solution even when • Solubility small amounts of acid or bases are added to the system. In order to have a buffer, a weak acid and its conjugate base or a weak base and its conjugate acid must be present. 2

Acid/Base Review Determine the major species in solution:  Ca(OH) 2  Major Species:  HC 2 H 3 O 2  Major Species:  HClO 4  Major Species:  NaCN  Major Species:  CH 3 NH 3 Cl  Major Species: Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 3

Buffer Solutions Buffer: A solution that resists any change in pH when small amounts of acid or base are added.  Buffers Consist of:  Weak Acid and its Conjugate Base  (HC 2 H 3 O 2 / NaC 2 H 3 O 2 )  Weak Base and its Conjugate Acid  (NH 3 / NH 4 Cl) Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 4

Buffer Solutions  Step 1: Identify major species in solution.  Step 2(a): Identify IF any reaction will go to completion (this happens if you have H + and a weak base or OH - and a weak acid in your major species). If no reaction goes to completion go to step 3.  Step 2(b): If a reaction goes to completion, make an “IF” table to determine the major species in solution after the reaction goes to completion (products and excess reactants.) IF tables are in moles not molarity.  Step 3: Examine major species to see if you have a buffer solution. Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 5

Buffer Solutions Student Question How many of the following can be mixed to form a buffer solution? KOH & HF RbOH & HBr NaC 2 H 3 O 2 & HCl H 3 PO 4 & HBr NH 3 & NH 4 Cl a) 1 b) 2 c) 3 d) 4 e) 5 Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 6

Buffer Solutions  Buffer Problems  Assume that [HA] is constant  Assume that [A - ] is constant HA(aq) ⇌ H + (aq) + A - (aq) 𝐿 𝑏 = 𝐼 + 𝐵 − 𝐼𝐵  Solve for pH=-log[H + ] 𝐼 + = 𝐵 − 𝐿 𝑏 𝐼𝐵 𝐵 − 𝐿 𝑏 𝑚𝑝𝑕 = 𝑚𝑝𝑕 𝐼 + 𝐼𝐵 𝐵 − 𝑚𝑝𝑕 𝐿 𝑏 − 𝑚𝑝𝑕 𝐼 + = 𝑚𝑝𝑕 𝐼𝐵 𝐵 − −𝑚𝑝𝑕 𝐼 + = −𝑚𝑝𝑕 𝐿 𝑏 + 𝑚𝑝𝑕 𝐼𝐵 𝐵 − 𝑞𝐼 = 𝑞𝐿 𝑏 + 𝑚𝑝𝑕 𝐼𝐵 Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 7

Buffer Solutions Identify Major Species Find species after reactions goes to completion ONLY Weak Acid and ONLY Weak its Conjugate Strong Acid Acid Base or or or Strong Base Weak Weak Base and or Base its Conjugate Strong Acid and a Acid Weak Acid or Strong Base and a Weak Base Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium

Buffer Solutions  How to determine pH Strong Acid Strong Base Weak Weak Weak Base Weak or or Acid Base and its Acid and Strong Base Strong Acid its Conjugate and a and a Acid Conjugate Weak Base Weak Acid Base The amount of H + /OH - from the weak acid/base is negligible in comparison to the strong acid/base therefore just use the strong 𝑩 − acid/base to calculate the pH. 𝒒𝑰 = 𝒒𝑳 𝒃 + 𝒎𝒑𝒉 𝑰𝑩 𝒒𝑰 = −𝒎𝒑𝒉 𝑰 + 𝒒𝑷𝑰 = −𝒎𝒑𝒉 𝑷𝑰 − 𝒒𝑰 = 𝟐𝟓 − 𝒒𝑷𝑰 Ice Table Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 9

Buffer Solutions Situation #1: Solution #1 (1L) Add Solution #2 (1L) Water 1 drop HCl 0.010 M HCl [H + ]= [H + ]= pH = pH = Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium

Buffer Solutions Situation #2: Solution #1 (1L) Add Solution #2 (1L) 0.50 M HC 2 H 3 O 2 1 drop HCl 0.010 M HCl 0.50 M NaC 2 H 3 O 2 0.50 M HC 2 H 3 O 2 0.50 M NaC 2 H 3 O 2 [H + ]=1.8×10 -5 M [H + ]= pH=4.74 pH = Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 11

Buffer Solutions Student Question A buffer solution contains 0.0200 M acetic acid and 0.0200 M sodium acetate. What is the pH after 2.0 mmol of NaOH are added to 1.00 L of this buffer? Helpful Information: pK a =4.75 for acetic acid a) 4.75 b) 4.70 c) 4.80 d) 4.84 e) None of the above Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 12

Buffer Solutions Buffer Capacity: An indication of the amount of acid or base that can be added before a buffer loses its ability to resist the change in pH  A buffer has the greatest buffer capacity when:  there are equal amounts of [HA] and [A - ]  there are large quantities of [HA] and [A - ] Not ote: When choosing a buffer, pick a buffer that has a pK a closest to what you want the pH to be. Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 13

Titration Curves  Equivalence Point: The stage of a titration when exactly the right volume of solution needed to complete the reaction has been added . Not ote: The equivalence point is sometimes called the stoichiometric point. Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 14

Titration Curves  Weak Base/ Strong Acid (NH 3 /HCl) Strong Acid  Calculate the pH at the following four points of a titration curve in which 0.50 M HCl is added to 50. mL of 1.0 M NH 3 (K b =1.8×10 -5 ). Weak Base  Case #1: No acid added (50. mL of 1.0 M NH 3 )  Case #2: 50. mL of HCl added (50. mL of 1.0 M NH 3 and 50. mL of 0.50 M HCl)  Case #3: 100. mL of HCl added (50. mL of 1.0 M NH 3 and 100. mL of 0.50 M HCl)  Case #4: 150. mL of HCl added (50. mL of 1.0 M NH3 and 150. mL of 0.50 M HCl) Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 15

Titration Curves  Mark these points on your titration curve The equivalence point a. The region with maximum buffering b. pH = pK a c. pH depends only on [A - ] (weak base only present) d. pH depends only on [HA] (weak acid only present) e. The pH only depends on the amount of f. strong acid or base added Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 16

Buffer Solutions Identify Major Species Find species after reactions goes to completion ONLY Weak Acid and ONLY Weak its Conjugate Strong Acid Acid Base or or or Strong Base Weak Weak Base and or Base its Conjugate Strong Acid and a Acid Weak Acid or Strong Base and a Weak Base Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium

Buffer Solutions  What method would you use to calculate the pH at each of the points? Strong Acid Strong Base Weak Weak Weak Base Weak or or Acid Base and its Acid and Strong Base Strong Acid its Conjugate and a and a Acid Conjugate Weak Base Weak Acid Base The amount of H + /OH - from the weak acid/base is negligible in comparison to the strong acid/base therefore just use the strong 𝑩 − acid/base to calculate the pH. 𝒒𝑰 = 𝒒𝑳 𝒃 + 𝒎𝒑𝒉 𝑰𝑩 𝒒𝑰 = −𝒎𝒑𝒉 𝑰 + 𝒒𝑷𝑰 = −𝒎𝒑𝒉 𝑷𝑰 − 𝒒𝑰 = 𝟐𝟓 − 𝒒𝑷𝑰 Ice Table Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 18

Titration Curves  Mark these points on your titration curve The equivalence point a. The region with maximum buffering b. pH = pK a c. pH depends only on [A - ] (weak base only present) d. pH depends only on [HA] (weak acid only present) e. The pH only depends on the amount of f. strong acid or base added Chapt pter er 8: Applicat cation ions s of Aque queous ous Equi quilib ibrium ium 19