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Introduction to Chem II. Instructors Course Objectives Course Topics Laboratory Exercises Course Website Today’s Agenda Syllabus. Course Objectives. Review some familiar topics Investigate some of these topics at a more in-depth level Model sound pedagogy

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Introduction to Chem II

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Introduction to chem ii l.jpg

Introduction to Chem II

  • Instructors

  • Course Objectives

  • Course Topics

  • Laboratory Exercises

  • Course Website

  • Today’s Agenda

  • Syllabus


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Course Objectives

  • Review some familiar topics

  • Investigate some of these topics at a more in-depth level

  • Model sound pedagogy

  • Obtain hand-on practice with Venier Data Collection

  • Show some effective demonstrations


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Course Topics

  • Stoichiometry

  • Calorimetry

  • Equilibrium

  • Solubility

  • Acid-base chemistry

  • Redox chemistry

  • Thermochemistry


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5 Lab Exercises

  • A calorimetry experiment using a temperature probe

  • Solubility using a Ca ion selective electrode

  • Equilibrium constant using a Colorimeter

  • Acid-base titration

  • Ag Ion Indicator electrode


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Course website

http://alpha.chem.umb.edu/chemistry/bpschemII/

Syllabus

Lab experiments

Course notes

Homework solutions


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Today’s Agenda

  • Take a 2 hr exam

  • Paperwork, surveys

  • Lunch

  • Lecture; g/mol, Classification of reactions, Stoichiometry, LR, Energetics of Reactions

  • Lab Lecture; Calorimetry

  • Lab Experiment 1

  • Early start on HW


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Investigating Stoichiometry using Calorimetry

Experiment 1


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Heat of Reaction - DH

  • At constant pressure – most lab experiments

  • aA + bB → productsDH/mol A

  • . DH = q (heat produced or absorbed)


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Calorimetry

  • Method of measuring the heat of reaction

  • Calorimeter-coffee cup

  • q = cmDT

    • c is the specific heat [J/(g ºC)] of solution

    • m = mass of solution

    • .DT is change in temperature

  • .DT is directly proportional to the heat of reaction


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The experiment

  • Mix reactants in different molar ratios

  • Predict the stoichiometry of the reaction from the ratio that gives the maximum temperature increase


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Example of the Experiment

  • 1 to 1,A + B → products

  • Mixing molar ratios

  • Constant total volume - cmDT


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1:1 Stoichiometry (mol ratio A/B)


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Example 2

  • 2 to 1,2A + B → products


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2:1 Stoichiometry (mol ratio A/B)


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Example 3

  • 3 to 1,3A + B → products


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3:1 Stoichiometry (mol ratio A/B)


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Determining the DHm

  • .DH = cmDT = (4.4 J/gC)*(50 g)*(36)

    = 7920 J

  • mol A reacted = 18.8 mmol A

  • .DHm = DH/(mol A reacted)

    = (7920)/(.0188 mol) = 421276 J/mol

    = 421 kJ/mol


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Products

  • Thiosulfate is a classic reducing agent

  • 2S2O32-↔ S4O62- + 2e-

  • Cl- is the product of the reduction of OCl-

  • Write a balanced redox equation

    • Step 1: determine half reactions.

    • Step 2 Make the reduction half reaction and oxidation half reaction have the same number of electrons by multiply reactions by common denominator

    • Step 3: Add reactions


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  • OCl- + H2O + 2 e- ↔Cl- + 2OH-

  • 2S2O32-↔ S4O62-+ 2e-

  • ________________________

  • OCl- + H2O + 2S2O32-→ Cl- + 2OH- + S4O62-


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Solubility of CaSO4

Experiment 2


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Goals

  • Determine the solubility of CaSO4 in three different solution

    • Saturated CaSO4 in H2O

    • Saturated CaSO4 in 0.10 M KNO3

    • Saturated CaSO4 in 0.10 M Na2SO4

  • Compare and rationalize the results


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Major concepts

  • Solubility Product Constants and saturated solution

  • LeChatlier’s principle and the common ion effect

  • Effect of ionic strength and ion activities on Ksp

  • Ion Selective Electrodes


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Ksp of CaSO4

  • CaSO4(s)↔ Ca2+ + SO42-

  • Ksp(CaSO4) = [Ca2+][SO42-] = 2.4∙10-5


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Saturated solution in water

  • Add several grams of CaSO4 to 1 L of water

  • Shake and mix for weeks

  • Allow CaSO4 that did not dissociate to settle to bottom

  • Ksp(CaSO4) = [Ca2+][SO42-] = 2.4∙10-5

    = x2

    [Ca2+] = 5.0∙10-3 M


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Saturated solution in 0.10 M Na2SO4

  • Add several grams of CaSO4 to 1 L of 0.10 M Na2SO4

  • Common Ion effect

  • Ksp(CaSO4) = [Ca2+][SO42-] = 2.4∙10-5

    = x(x+0.10)

    Assume x <<< 0.10x = 2.4∙10-4 M

    [Ca2+] = 2.4∙10-4 M


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Saturated solution in 0.10 M KNO3

  • Activities

  • Ksp(CaSO4) = ACa2+ASO42- = [Ca2+]gCa2+[SO42-]gSO42- = 2.4∙10-5

  • Activity coefficient (g) is dependent on the ionic strength of the solution, and the size and charge of the ion. It is a number between 0 and 1. At very low ionic strength, g approaches 1


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Ionic strength

  • A measure of the concentration of ions in solution

    m = ½ ∑ cizi2

    Sat. solution in 0.10 M KNO3

    m = ½ ([K+](+1)2 + [NO3-](-1)2 + [Ca2+](2+)2 + [SO42-](-2)2) = 0.12 M

    [email protected]=0.12 =


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Take home message

  • The common ion effect decreases the solubility by over an order of magnitude

  • At high ionic strengths, solubility increases slightly ( by a factor of 1.5 -5).


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Ion Selective Electrode

  • A probe that consists of two reference electrodes connected electrically through a specific type of salt bridge through the solution being measured.

  • The salt bridge is a membrane that specifically binds the ion of interest

  • A junction potential develops at this membrane that is proportional to the concentration of the ion of interest


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voltmeter

Cathode

Reference electrode

Ag/AgCl, sat. KCl

Anode

Reference electrode

Ag/AgCl, sat. KCl

solution

Ion selective

membrane

Engineer this whole set-up in one probe

pH meter

Ca2+ selective electrode


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Response of Ca2+ Selective Electrode

  • Ecell = constant + 29.58 logACa2+


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