Introduction to Chem II
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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
• Obtain hand-on practice with Venier Data
Collection
• Show some effective demonstrations
Course Topics
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Stoichiometry
Calorimetry
Equilibrium
Solubility
Acid-base chemistry
Redox chemistry
Thermochemistry
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
Course website
http://alpha.chem.umb.edu/chemistry/bpschemII/
Syllabus
Lab experiments
Course notes
Homework solutions
Today’s Agenda
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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
Investigating Stoichiometry
using Calorimetry
Experiment 1
Heat of Reaction - DH
• At constant pressure – most lab
experiments
• aA + bB → products
DH/mol A
• . DH = q (heat produced or absorbed)
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
The experiment
• Mix reactants in different molar ratios
• Predict the stoichiometry of the reaction
from the ratio that gives the maximum
temperature increase
Example of the Experiment
• 1 to 1,
A + B → products
• Mixing molar ratios
• Constant total volume - cmDT
mmol A
mmol B
LR
A cons fraction ratio
DT
5.0
20.0 A
5.0
0.25 1 to 4
10
7.5
17.5 A
7.5
0.43 3 to 7
15
10.0
15.0 A
10.0
0.67 2 to 3
20
12.5
12.5 A
12.5
1.00 1 to 1
25
15.0
10.0 B
10.0
1.50 3 to 2
20
17.5
7.5 B
7.5
2.33 7 to 3
15
20.0
5.0 B
5.0
4.00 4 to 1
10
Temp Change (C)
1:1 Stoichiometry (mol ratio A/B)
30
25
20
15
10
5
0
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Mole Ratio
Example 2
• 2 to 1,
mmol A
2A + B → products
mmol B
LR
A cons
fraction
ratio
DT
5.0
20.0 A
5.0
0.25 1 to 4
10
7.5
17.5 A
7.5
0.43 3 to 7
15
10.0
15.0 A
10.0
0.67 2 to 3
20
12.5
12.5 A
12.5
1.00 1 to 1
25
15.0
10.0 B
15.0
1.50 3 to 2
30
17.5
7.5 B
15.0
2.33 7 to 3
30
20.0
5.0 B
10.0
4.00 4 to 1
20
16.7
8.4 B
16.7
1.99 2 to 1
32
Temp Change (C)
2:1 Stoichiometry (mol ratio A/B)
40
30
20
10
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0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Mole Ratio
Example 3
• 3 to 1,
3A + B → products
mmol A
mmol B
LR
A cons
fraction
ratio
DT
5.0
20.0 A
5.0
0.25 1 to 4
10
7.5
17.5 A
7.5
0.43 3 to 7
15
10.0
15.0 A
10.0
0.67 2 to 3
20
12.5
12.5 A
12.5
1.00 1 to 1
25
15.0
10.0 A
15.0
1.50 3 to 2
30
17.5
7.5 A
17.5
2.33 7 to 3
35
20.0
5.0 B
15.0
4.00 4 to 1
30
18.8
6.3 B
18.8
3.00 3 to 1
36
Temp Change (C)
3:1 Stoichiometry (mol ratio A/B)
40
30
20
10
0
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Mole Ratio
Determining the DHm
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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
Products
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Thiosulfate is a classic reducing agent
2S2O32- ↔ S4O62- + 2eCl- is the product of the reduction of OClWrite 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- + 2OH2S2O32- ↔ S4O62- + 2e________________________
OCl- + H2O + 2S2O32- → Cl- + 2OH- +
S4O62-
Solubility of CaSO4
Experiment 2
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
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
Ksp of CaSO4
• CaSO4(s) ↔ Ca2+ + SO42-
• Ksp(CaSO4) = [Ca2+][SO42-] = 2.4∙10-5
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
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 <<< 0.10
x = 2.4∙10-4 M
[Ca2+] = 2.4∙10-4 M
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
Ionic strength
• A measure of the concentration of ions in
solution
m = ½ ∑ c izi2
Sat. solution in 0.10 M KNO3
m = ½ ([K+](+1)2 + [NO3-](-1)2 + [Ca2+](2+)2
+ [SO42-](-2)2) = 0.12 M
gCa2+@m=0.12
=
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).
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
voltmeter
Cathode
Anode
Reference
electrode
Reference
electrode
Ag/AgCl,
sat. KCl
Ag/AgCl,
sat. KCl
solution
Ion selective
membrane
Engineer this whole set-up in one probe
pH meter
Ca2+ selective electrode
Response of
Ca2+ Selective Electrode
• Ecell = constant + 29.58 logACa2+
200
Ecell (mV)
150
100
50
0
-6
-5
-4
-3
log A(Ca2+)
-2
-1
0