Patrick Riley , Robert Hanson, Paul Fischer and Jeff Schwinefus
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Transcript of Patrick Riley , Robert Hanson, Paul Fischer and Jeff Schwinefus
Demos for Free (Energy, that is..)Chemical Demonstrations Explained
with Graphs of Free Energy vs. Temperature
Patrick Riley, Robert Hanson, Paul Fischer and Jeff Schwinefus
Department of Chemistry, St. Olaf College
Department of Chemistry, Macalaster College
July 19, 2004
“Demo Day”
A skit is performed to demonstrate recrystallization. Can we recover our “contaminated” goods?
Overall Concept Map For Molecular Thermodynamics
internal energy, U
heat, q
temperature, Tentropy, S enthalpy, H
work, w
free energy, G
equilibrium constant, Kreaction quotient, Q
Our Focus Today: Free Energy and Temperature
internal energy, U
heat, q
temperature, Tentropy, S enthalpy, H
work, w
free energy, G
equilibrium constant, Kreaction quotient, Q
Free Energy vs. Reaction Coordinate Graphs
Reactants
Products
Equilibrium mixture
Course of reaction
Free
ene
rgy
Equilibrium is the state where free energy is minimized.
Free Energy vs. Temperature Graphs
T
G = H – TS
Teq
G = 0
reactants
products
Equilibrium is the state where G = 0.
Free Energy vs. Temperature Graphs
T
G = H – TS
Teq
G = 0
reactants
products
Equilibrium is the state where G = 0.
ΔH < 0
Free Energy vs. Temperature Graphs
T
G = H – TS
Teq
G = 0
reactants
products
Equilibrium is the state where G = 0.
ΔH < 0
slope = -S
Sproducts < Sreactants
ΔS < 0
Applications: Use of Classroom Demonstrations
Demonstration 1: “Balloon in an Erlenmeyer”
H2O (l) H2O (g)
Applications: Use of Classroom Demonstrations
Demonstration 1: “Balloon in an Erlenmeyer”
Qualitative illustration of the interdependence between vapor pressure and temperature.
Questions discussed during class include, “What is inside the flask?” before and after boiling.
Applications: Use of Classroom Demonstrations
Demonstration 1: “Balloon in an Erlenmeyer”
T
G = H – TS
H2O(g)
H2O(l)
T1
(a)
P = 1 atm
Applications: Use of Classroom Demonstrations
Demonstration 1: “Balloon in an Erlenmeyer”
T
G = H – TS
T1 T2
P2 > 1 atm
(b)
H2O(g)
H2O(l)
Applications: Use of Classroom Demonstrations
Demonstration 1: “Balloon in an Erlenmeyer”
T
G = H – TS
H2O(g)
H2O(l)
T1
(a)
P = 1 atm
Applications: Use of Classroom Demonstrations
Demonstration 1: “Balloon in an Erlenmeyer”
T
G = H – TS
P << 1 atm
Troom T1
(c)
H2O(g)
H2O(l)
Demonstration 2: “Can of Beans”
H2O (l) H2O (g)
T
G = H – TS
H2O (l)
PH2O < 1 atm
H2O (g)Will the can of beans survivethe extreme heat of the hotplate?Get ready to duck!
Demonstration 2: “Can of Beans”
H2O (l) H2O (g)
T
G = H – TS
H2O (l) PH2O = 1 atm
H2O (g)Will the can of beans survivethe extreme heat of the hotplate?Get ready to duck!
Demonstration 2: “Can of Beans”
H2O (l) H2O (g)
T
G = H – TS
H2O (l)PX > 1 atm
H2O (g)Will the can of beans survivethe extreme heat of the hotplate?Get ready to duck!
Note: stirring, not heat turned on for the purpose of thisdemonstration -- works every time!
Demonstration 3: Low Pressure
Evaporation CH2Cl2 (l) CH2Cl2 (g)
T
G = H – TS
X(g)
X(l)
P << 1 atm
Teq Troom
Boiling observed with no application of heat.
Demonstration 3: Low Pressure
Evaporation CH2Cl2 (l) CH2Cl2 (g)
T
G = H – TS
X(g)
X(l)
P << 1 atm
Teq T2
Boiling observed with no application of heat. Frost formation on flask discussed as result of the endothermic (liquid gas) reaction.
Demonstration 4: Relative Humidity and Dew Point
Current weather data from Faribault airport is taped and played for the class.
G = H – TS
H2O (g)
H2O (l)
T
Pvap = P100%
Relative humidity relates the ratio of the actual water vapor pressure in the air to the equilibrium vapor pressure for the current air temperature.
G = H – TS
H2O (g)
H2O (l)
Tcurrent
Pvap = 0.30 x P100%
Demonstration 4: Relative Humidity and Dew Point
Relative humidity <100% indicates liquid water is not generally at equilibrium with atmospheric water vapor; vaporization is thermodynamically favored.
G = H – TS
H2O (g)
H2O (l)
Tcurrent
P100%
Tdp
Pvap = 0.30 x P100%
Demonstration 4: Relative Humidity and Dew Point
The dew point (Tdp) indicates where the actual pressure water vapor curve crosses the liquid curve.
Demonstration 5: Triple Point of CO2
T
G = H – TS
–78 oC
CO2 (l)CO2 (s)
CO2 (g)
P = 1 atm
Liquid phase is always higher in free energy than solid or gas and therefore is not observed.
Demonstration 5: Triple Point of CO2
T
G = H – TS
–56 oC
CO2 (l)CO2 (s)
CO2 (g)
P = 5.11 atm
Liquid phase is always higher in free energy than solid or gas and therefore is not observed. Increasing the pressure results in a flatter gas curve which crosses the solid and liquid curves at the melting point.
Demonstration 6: Solubility or “Hot Lemonade Tastes Better”
Sugar (s) Sugar (aq)
Endothermic dissolution of solids emphasized.
Demonstration 6: Solubility or “Hot Lemonade Tastes Better”
Sugar (s) Sugar (aq)
Endothermic dissolution of solids emphasized. Curves cross when saturated.
G = H – TS
sugar (aq)
sugar (s)
[sugar] low
0 oC
Demonstration 6: Solubility or “Hot Lemonade Tastes Better”
Sugar (s) Sugar (aq)
Endothermic dissolution of solids emphasized. Curves cross when saturated. Increase in temperature results in dissolution.
G = H – TS
sugar (aq)
sugar (s) [sugar] high
70 oC
Demonstration 7: Supersaturated Sodium Acetate Solutions
NaOAc (s) NaOAc (aq) The molar free energy of the aqueous solute is higher than that of the solid in a supersaturated solution. Precipitation is thermodynamically favored to equalize the molar free energy of the species; external stimulus required.
G = H – TS
NaOAc (aq)
NaOAc (s)
TsatTact
Demonstration 8: Dissolution of Gases or “Dead Fish”
O2 (g) O2 (aq)
G = H – TS
O2 (g)
O2 (aq)
T > TeqTeq
Increase in temperature favors solute gas; solution is degassed.
Demonstration 9: Freezing Point Depression or “Fool the Waitperson”
As the solute dissolves liquid water becomes impure and its entropy increases.
T
G = H – TS
0 oC
H2O (l)H2O (s)
H2O (g)
Demonstration 9: Freezing Point Depression or “Fool the Waitperson”
T
G = H – TS
Tmp < 0 oC
H2O (l)H2O (s)
H2O (g)
The liquid water curve’s slope becomes more steep and establishes a lower crossing temperature.
T
G = H – TS
Tmp lower
H2O (l)H2O (s)
H2O (g)
Tbp higher
Note that boiling point elevation arises from the same phenomenon. Increasing molar entropy of the liquid will push the boiling point to the right.
Demonstration 9: Freezing Point Depression or “Fool the Waitperson”
Laboratory Exercise: “Chemical Magic: Free Energy”
Why should chemistry professors have all the fun?
Students perform a demonstration in groups of 2-3,
answer 4-6 questions about the demonstration then
move on to the next station.
G-T graphs are drawn for each laboratory exercise.
Laboratory Exercise: “Chemical Magic: Free Energy”
Laboratory demonstrations include:“Inside-Out Balloon”
“Fog Chamber”“Cold Boiling”“Liquid Air”“Cool Glue”“Hot Ice”“Crystals from not-so-thin-air”“Instant Snowflakes”“Instant Hot; Instant Cold”“Magic Rope”
Upon completing the third demonstration (45 minutes) the studentsprepare to present a magic show for their classmates.
Laboratory Exercise: “Chemical Magic: Free Energy”
Laboratory Exercise: “Chemical Magic: Free Energy”
“Chemical Magic is fun!”
Conclusion
Free energy vs. temperature graphs can be utilized to discuss a variety of common classroom demonstrations.
“Chemical Magic” laboratory exercise can effectively supplement the discussion initiated during “Demo-Days.”