Chrystal D. Bruce, Erskine College

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Presentation transcript:

Chrystal D. Bruce, Erskine College Teaching Students to Think Using Topics in Thermodynamics: The Virtual Substance Program Chrystal D. Bruce, Erskine College and Carribeth Bliem and John Papanikolas, University of North Carolina at Chapel Hill We all want our students to be intelligent, thinking individuals. We have content to teach, but we want our students to use the content to solve problems. What is it about Pchem, and thermo in particular, that sometimes turns students off? Math

Outline Virtual Substance Program Lab Assignments at Erskine Ideal Gas Law Real Gases Thermodynamic Properties Independent Projects Student Comments Additional Modules

Modeling with Virtual Substance What is Virtual Substance? A numerically accurate, inquiry driven, molecular dynamics software program. Go over each box and the options; run simulation (Constant volume fixed wall) Advantages of VS are NA, ID Compare results with known values (Provides feedback to counteract tendency to just “generate numbers”) (Test expts) Test theory; ID – Connection to Teaching students to think

Go over each box; options – script and graphics, output Transition – What can we do with VS in Pchem specifically?

Virtual Substance is Numerically Accurate Ideal Gas Law Calculate Gas Constant, R Measure pressure at series of volumes Slope = RT Argon: R = 0.082 L atm/mol K Xenon: R = 0.083 L atm/mol K Measure pressure at series of temperatures Slope = R/Vmolar Argon: R = 0.083 L atm/mol K Xenon: R = 0.082 L atm/mol K T = 100K First is Boyle’s Law. Second is variation on Charles Law which relates V and T, not P and T Obviously, many alternatives available After this lab, they have some trust in VS. **Test’s experiments** Virtual Substance is Numerically Accurate

Real Gas Behavior NVT simulations of Argon at 165 K using periodic boundary conditions Collect LJ, soft sphere obtain NISt calculate vdW + Hard Sphere Pushes students to think about what makes a gas real; limitations of vdw; why density dependent? Z = P/Pideal

Thermodynamic Properties Calculate expansion coefficient, a, and isothermal compressibility, kT, for Argon behaving as an ideal gas. Most independent. Students must determine how to design the experiment to obtain the correct data. Left-– Must use NPT Graphs courtesy of jessica martin. NPT simulations NVT simulations

For an Ideal Gas: Virtual Substance Error: < 2% Format Tables/ Add exciting color Isothermal compressibility: 0 - 2 % error for ideal gas; 2 – 12% error for LJ data compared to NIST from earlier

Topic Measurement Kinetic Theory of Gases Velocity Distribution Speed Distribution Collision Frequency Mean Free Path Determine average and RMS speeds from data Real Gases P(V) Identify limit(s) of ideal behavior Determine critical constants Energy and Enthalpy E(T) H(T) E(V) H(P) Determine CV and CP from data Determine ∆Hvap and ∆Hfus Internal Pressure Combine with CP to get Joule-Thomson coefficient Entropy CP(T) Calculate Sm(T) Calculate ∆Sfus and ∆Svap P(T) Confirm that πT = T(∂P/∂T)V – P Phase Diagrams Construct phase diagram from Tfus and Tvap measurements Phase transitions Identification of phase boundaries Comparison with Clausius-Clapeyron equation

Using Virtual Substance in Independent Projects Submit Proposal Revise Proposal Conduct Research Submit Report and Present Results to Department Quality Clearly varied Proposal Must include experimentally determined quantity for comparison Funny cartoon here too – Scientific Process in action

Sample Student Projects Determination of Joule-Thomson coefficient values with Virtual Substance Calculating Internal Pressure (pT) for Ideal and Real Gases Evaluation of the Redlich-Kwong Equation of State Determination of (CP – CV) for Argon

Student Comments Virtual Substance … made [concepts] real and concrete. It gave the visual/tactile learner in me a way to grab hold of the topics [The independent project] was probably the most valuable assignment … I had to think of a problem …, consider the capabilities of Virtual Substance, write a proposal …, and use critical thinking skills to execute and analyze the project. It wasn't just following a simple recipe to obtain some data for a lab report. [the independent project] was … good preparation for graduate studies. The most important thing was a good understanding of the chemistry. 1st one – Z vs. P is a picture in nearly all textbooks; Actually generating it made it real

Phase Transitions Measure E, P, and V at different temperatures Combine to get H(T) using H = E + PV Temperature (K) H (kJ/mol) P = 8.5 atm Ar (actual) Bp (Argon) ~ 115 K @ 8.5 atm DH vap (Argon)= 6.5 kJ/mol Design experiment to measure Hvap 200 K 80 K 10 K

Real Gas Isotherms Vm (L/mol) P (atm) 151 K (Tc) 130 K NIST data Measure Pressure for series of Volumes at constant Temperature for Ar Output agrees with actual data  Numerically Accurate Obtain P-V isotherms, Compare to actual data (NIST) Van der Waals loops present – replaced with straight line; page 22 Atkins Crtitical temperature Argon = 150.9 K

Polymer Statistics LJ: Lennard Jones – Non-bonded atoms Or FENE – finite extensible (sion) nonlinear elastic potential used to treat polymers instead of harmonic potential <R2>1/2 = Nv v = ½ for ideal random walk; v = 0.6 for self-avoiding random walk; LJ: Lennard Jones – Non-bonded atoms FENE: Finite Extension Nonlinear Elastic Potential – Bonded Atoms

Acknowledgements For More Information: BCCE Symposium Organizers Our Students For More Information: cbruce@erskine.edu www.unc.edu/~jpapanik/VirtualSubstance/VGMain.htm

More Student Comments VS could be running in the dorm room while you were doing other homework or laundry. I will use [Virtual Substance] as a teaching tool when I become a professor. © Sidney Harris

Forms of Various Potentials Hard Sphere Billiard Balls of radius s Soft Sphere Repulsive forces; typically n=12 Lennard – Jones Repulsive and attractive forces FENE – Use instead of harmonic oscillator or Morse Potential to treat polymer binding