Lecture11_Feb25_Ch6_7
Page 1: Introduction
Lecture Overview
CHEM 260 Spring 2025, Lecture 11 on February 25
Chapter 6: Volumetric Titrations
HW 4 due on March 3 at 11:59 PM
Quiz 3 on March 4
Course Syllabus includes Chapters 6 and 7
Page 2: Learning Outcomes
Goals for Chapter 6
Learn to interpret precipitation titration curves
Focus on Argentometric Titrations
Page 3: Precipitation Titration Curves
Titration Curves
Show how reactant concentration varies as titrant is added
Understanding chemistry during titration is crucial
Experimental control is influential on analytical titration quality
Sharpness of Endpoint Influences:
Concentration of analyte and titrant
Magnitude of solubility product (Ksp)
Page 4: Equivalence Point of Precipitation Titration
Example Titration
25.00 mL of 0.1000 M I− titrated with 0.05000 M Ag+
Large equilibrium constant indicates near complete reaction of I− with Ag+
Sudden increase in [Ag+] at equivalence point, as no I− remains
Equilibrium Constant
K_sp = (Cation)(Anion) = 1.2 × 10^(-16)
Page 5: Volume of Titrant at Equivalence Point
Titrant Volume Calculation
Stoichiometry of reaction is 1:1, leading to: Ve = 50.00 mL
Titration curve has three distinct zones:
Before Ve
At Ve
After Ve
Equivalence point is at the steepest section of the curve (maximum slope)
Page 6: Reaction at Equivalence Point
Equilibrium Details
Enough Ag+ added to react with all I−
Precipitation of AgI(s) occurs, some of which redissolves due to Ksp equilibrium:
[Ag+][I−] = K_sp
Concentrations yield x value solutions for Ag+ at equivalence point
Page 7: Impact of Ksp on Titration Curves
Ksp’s Role
Curves for 25.00 mL of 0.1000 M halide with 0.05000 M Ag+
Equivalence points are sharply marked; steeper at lower solubility precipitates
Page 8: Endpoint Detection - Volhard Titration
Volhard Titers
Measures [Cl−], adaptable for other anions
Involves forming a soluble, colored complex at endpoint
Steps:
Back titration with excess Ag+ to evaluate Cl− in solution
Use of KSCN for titration of excess Ag+
Formation of visible red complex with Fe3+ at endpoint
Titrations involving Ag+ are known as argentometric titrations
Page 9: Endpoint Detection - Fajans Titration
Fajans Method
Can be utilized across various systems
Achieves endpoint via colored indicator adsorption on precipitate
Changes in charge enable color change through adsorption of dye after equivalence point
Page 10: Example Review Problems from Chapter 6
Review Problems
6-3, 6-9, 6-13, 6-18, 6-20, 6-21, 6-22
Page 11: Chapter 7 - Gravimetric and Combustion Analysis
Learning Outcomes
Understanding principles of gravimetric analysis
Direct vs. Indirect methods
Page 12: Gravimetric vs. Combustion Analysis
Definitions
Gravimetric Analysis: Measures mass of a precipitate for quantitative analysis, one of the earliest methods, very accurate but often impractical now
Combustion Analysis: Involves burning sample in excess oxygen and measuring resultant products (CO2, H2O) for organic compounds
Page 13: Types of Gravimetry
Categories
Precipitation: mass of precipitate
Electrogravimetry: weighing deposited analyte on electrode
Volatilization: thermal/chemical energy to remove volatile species
Particulate Gravimetry: filtration/extraction methods to isolate analyte
Page 14: Workflow for Precipitation Gravimetry
Process Steps
Formation and post-treatment of precipitate
Importance of stoichiometry and removal of interfering substances for accurate results
Common Terms:
Precipitate: insoluble product
Precipitant: causes precipitation (e.g., OH−)
Mother Liquor: solution where precipitate forms
Page 15: Criteria for Ideal Precipitation in Gravimetry
Characteristics of Ideal Product
Should be pure, insoluble, filterable, and have known composition
Larger, low solubility crystals with higher molar mass are preferred for lower impurity retention
Page 16: Colloidal Suspensions
Considerations
Colloidal suspensions are undesirable since particles can pass through filters
Colloids are too small to precipitate but larger than molecules and remain suspended
Page 17: Mechanism of Precipitation
Stages of Precipitation
Nucleation: Initial small particle formation
Particle Growth: Molecules condense to form larger crystals
Nucleation and particle growth dynamics are crucial for successful precipitation
Page 18: Supersaturated Solutions
Tuning Nucleation and Growth
Importance of managing nucleation versus growth rates in supersaturated solutions
Techniques to optimize yield: slow precipitant addition, increased temperature, and larger solution volumes