Thermodynamic “pain point” results – here are your responses

I believe that a better understanding of thermodynamics is available by explaining the connections between the micro-world of moving and colliding atoms that attract and repel each other and the macro-world of classical thermodynamics. My goal is to identify and clarify such micro-to-macro connections. To ensure that I’m addressing true needs of the science community, I reached out to you all at the beginning of this year (here) to seek your personal “pain points” with thermodynamics. I asked, what are the stumbling blocks you encounter when trying to teach or learn the physical meanings behind thermodynamic equations and phenomena? Presented below are your responses. My thanks to those who engaged in this exercise.

If you feel you can address any items on the list with supportive references, could you please let me know?

Based on a review of these responses, and subsequent discussions with some of you, I have decided to begin this journey by focusing on a single, specific phenomenon, the Joule-Thomson effect, number 13 in the list below. I hope to have some results to share with you in my next post.

A final note. One responder asked me, how can you animate thermodynamic concepts so that students can understand them? How can you translate physical chemistry and thermodynamics into practical real-world audiovisual content? If any of you has ideas on this, please let me know.

_ _ _ _ _

  1. Explain the physical meaning of not only temperature, but also energy, entropy, enthalpy, exergy, Gibbs energy, and the dreaded fugacity.
  2. Speaking of which, what exactly is fugacity and how does it relate to the material world?
  3. What is the physical mechanism behind the existence of the critical point?
  4. What does the concept of energy minimization physically mean, and how is this applied in the form of Gibbs free energy minimization of protein folding?
  5. Reversibility: What is it (really) and why is it important?
  6. What is the fundamental physical cause of the temperature effects that result when you depressurize a gas cylinder?
  7. Explain the presence of heterogeneous azeotropes.
  8. How deep a vacuum on steam turbines is worthwhile to pursue? How can this be more quickly understood and appreciated?
  9. Van der Waals equation. Why does long range attraction and short range repulsion give a liquid (ie phase transition), beyond just saying, “It’s in the math”? Does this same phenomenon hold with colloids and polymers in solution, that they undergo a “vapor-liquid type phase transition”?
  10. Column of gas in a gravitational field. Is it isothermal or is there a temperature gradient, and why? James Clerk Maxwell and Ludwig Boltzmann assumed the former, Josef Loschmidt the latter. Who was right?
  11. Where does thermodynamics begin? Is a perfect vacuum really a thermodynamic system? Without molecules, do we have pressure, temperature, Q, W, S & H?
  12. Gas Phase Behavior – ideal and non-ideal. Given that all atoms/molecules attract all atoms/molecules via London dispersion forces, what is it that makes a gas behave ideal in which such attraction has negligible influence? What is the dividing line between “ideal gas” and “non-ideal gas”? Some have suggested that it is the formation of dimers the causes of deviation from ideal gas law, but I have yet to find conclusive evidence of this in the literature.
  13. Joule-Thomson Effect – explain this. This effect is naturally related to the combined effects of intermolecular attraction and repulsion. But how exactly does this work at the molecular level? How does this explain, for example, no effect for ideal gas, heating effect for hydrogen, and the presence of an inversion temperature?
  14. Gas – flow. Explain in plain English Bernoulli’s equation, especially the trade-off between pressure and flow velocity.
  15. Photons. When are photons released? Solely with the acceleration of charges? Does this always release photons? As an unbound electron accelerates towards a single proton, are photons released? How about during chemical reactions? Because chemical reactions involve a change in energy level of electrons, are photons always released? If so, should photons be included in reaction equations? When photons are absorbed, heat is generated in the form of an increase in temperature. What is the energy balance around photon absorption (and emission)? What is the physical event that leads to an increase in kinetic energy of the atoms that absorb the photon? When does the presence of photons influence reaction equilibrium?
  16. Explain the micro-physics behind the Stefan-Boltzmann T4 law of radiation.
  17. Explain the micro-physics behind the existence of a supercritical fluid.
  18. Explain the Clausius-Clapeyron Equation in plain English. Why is it what it is?
  19. Gas Phase Reactions – Walk through exactly what happens at the atomic scale during reaction. For example, picture two hydrogen atoms. Long-range attraction draws each towards the other. But up close, the strong electron repulsion pushes them apart. How is this repulsive force overcome so that reaction occurs? Also, “heat” is generated when two hydrogen atoms combine to form molecular hydrogen. What exactly does this mean? What specific physical events lead to an increase in the kinetic energy of the atoms comprising the H-atom gas system when they react? Also, are photons emitted as a result of this reaction?
  20. What exactly does the change in Gibbs energy of a chemical reaction quantify? Is it simply the total change in energy of the orbital electrons?
  21. Why isn’t the distribution of orbital electrons included in the Boltzmann definition of entropy? If a chemical reaction is really the distribution of orbital electrons into their most probable distribution, shouldn’t the change in entropy account for this?
  22. Phase Change – Vapor/Liquid (similar discussion for liquid/solid). How does phase change occur? Walk through each step involved in energy balance. Also, walk through condensation. How does an atom/molecule slow down enough to be ‘captured’ by another atom/molecule? Do the slow atoms/molecules at the left end (slower speed) of the statistical distribution condense first? (Same could be asked of chemical reactions. Do the fast atoms/molecules at the right end of the statistical distribution react first?) When an atom escapes from liquid to vapor, what velocity does it end with? Is the resulting vapor initially at a very low temperature due to escape and then is this why some thermal energy is needed to bring the escaped gas up to temperature of the liquid? Also, does the average velocity of particles in vapor fall short of their average velocity in liquid, especially in case the liquid is a solution and the vapor pressure at a given temperature of the liquid is hence reduced?
  23. Absolute zero. Yes, the entropy of a pure crystal is zero at absolute zero. But aren’t the electrons still in motion? And wouldn’t this mean that the polarity of the atoms is not constant at absolute zero, and instead varies, and wouldn’t this result in a variation of attraction and hence result in motion, which is inconsistent with the concept of absolute zero? So what does matter physically look like at absolute zero?


Published by Robert T Hanlon

I earned my Sc.D. in chemical engineering from the Massachusetts Institute of Technology and subsequently conducted post-doctoral research at Karlsruhe University in Germany. My professional career took me to Mobil Oil Research & Development Corporation, the Rohm and Haas Company, and then back to MIT where I am currently involved with their School of Chemical Engineering Practice.

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