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Entropy

Entropy. Time’s Arrow. Objectives. Explain the tendency of matter and energy to spread out over time. Identify entropy changes in familiar processes. Poll Question. I think I know what “entropy” means. True. False. The Flow of Matter. particles disperse. Gas Molecule in a Box.

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Entropy

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  1. Entropy Time’s Arrow

  2. Objectives • Explain the tendency of matter and energy to spread out over time. • Identify entropy changes in familiar processes.

  3. Poll Question I think I know what “entropy” means. • True. • False.

  4. The Flow of Matter particles disperse

  5. Gas Molecule in a Box No energy transfer to walls: elastic collisions

  6. Gas Molecule in a Box Double the size of the box!

  7. Gas Molecule in a Box Double the size of the box!

  8. a. Allb. Half c. noned. 75% Poll Question What portion of the time will the molecule spend in the original volume (left half of the box)?

  9. Poll Question What portion of the time will the molecule spend in the original space if we quadruple the volume of the box?

  10. a. Allb. Half c. 1/3d. 1/4 Poll Question What portion of the time will the molecule spend in the original space if we quadruple the volume of the box?

  11. a. 1b. 1/2 c. 1/3d. 1/4 Poll Question What is the probability that the molecule will be in the original space at any given time? V V/4

  12. a. 1b. 1/2 c. 1/4d. 0 Two Molecules (Poll) What is the probability that both will be in the left half of the container at the same time?

  13. a. 1/2b. 1/3 c. 1/4d. 1/8 Three Molecules (Poll) What is the probability that all three will be in the left half of the container at the same time?

  14. Whiteboard Work • What is the probability that all N will be in the given sector of the container at the same time? xV V

  15. Expansion Summary Random motions cause particles to spread out. The chance that they will randomly come back together decreases tremendously as the number of molecules increases. Spreading out is irreversible.

  16. Group It! Discuss all succeeding poll questions with your group before answering.

  17. The Flow of Energy energy disperses

  18. Collision! A moving object rams a stationary object. Before impact:Kinetic Energy of projectile > 0 Kinetic Energy of target = 0

  19. Group Poll Question What happens to the kinetic energy after the collision? • All of it goes to the target. • The projectile and target have equal kinetic energies. • The projectile keeps it all. • It depends; there isn’t enough information to know for sure.

  20. I did the math! I calculated the kinetic energies of object 1 (projectile) and object 2 (target) as a function of • Offset • Relative masses

  21. Energy Transfer: Mass Effect

  22. Energy Transfer: Mass Effect

  23. Energy Transfer: Offset Effect

  24. Energy Transfer: Offset Effect

  25. Energy Transfer: Offset Effect

  26. Collision Summary • Before impact, all the kinetic energy is in the motion of the projectile. • At impact, the kinetic energy almost always distributes to motion of both the projectile and target. • When two objects collide, their kinetic energies are usually closer after the collision than before. • Spreading out is irreversible.

  27. Kinetic Energy Randomizes • Spreads out over more objects • Spreads out in more directions • Work becomes internal energy

  28. Example • How does entropy increase when a ball is dropped, bounces, and eventually stops? • How is energy conserved? • Bouncing ball example applet www.chem.uci.edu/undergraduate/applets/bounce/bounce.htm PEKErandom molecular KE

  29. Heat Transfer multiple interactions

  30. Heat Flow Two solids with different temperatures (average molecular kinetic energies) are brought into contact.

  31. Heat Flow Two solids with different temperatures (average molecular kinetic energies) are brought into contact. What happens to the atoms’ kinetic energies (temperatures)?

  32. Heat Flow Summary Molecular kinetic energy flows from high temperature objects to low temperature objects, but not the other way around. This is because kinetic energy tends to even out between colliding objects. There are more ways to distribute energy among many molecules than among few molecules.

  33. Temperature Difference Hot Cold heat Until Warm Warm Equilibrium

  34. Temperature Difference Hot Heat flows until total entropy stops increasing • Thermal equilibrium • Same temperature Cold heat DS DS high low DU DU

  35. Thermodynamic Temperature Hot 1/T = DS/DU DS = q/T Cold heat DS DS high low DU DU

  36. Overall Summary • Particles and energy tend to become spread out uniformly. • Entropy is a measure of how many different ways a state can be arranged. • libraryanalogy • Total entropy increases in all processes that actually occur.

  37. What It Means examples

  38. Interesting but vital point We cannot see most of the motion that occurs in our world.

  39. The Funnel and the Ice Pack • How can matter ever become localized? • Stars form • Rain falls • How can thermal motion ever decrease? • Refrigerators and heat pumps • First aid cold packs • Any time one thing becomes localized, something else spreads out more

  40. Entropy, Technically • W = number of “configurations” of a state • S = kBln(W) = entropy of the state • DS = entropy change • DS = S2− S1 • = kBln(W2) − kBln(W1) • = kBln(W2/W1)

  41. Free Expansion Example • W2/W1 = = xN N xV V • DS = kNln(x) xV V

  42. Heat Generation • Any energy molecular motion • Raises entropy • Energy less constrained • Effect more important at low temperatures • DS = q/T • Greater proportional increase in thermal energy at low T

  43. Ice Melting Example • Solid  liquid • disperses matter DSc > 0 • constrains energy DST < 0 DSc DS DS 0 temp DST melting temperature

  44. Add Salt • Salt dissolves in liquid only • Raises S of liquid (+ salt) • Raises DSc lower melting temperature DSc DSc DS DS DS 0 temp DST original melting temperature

  45. Real Processes • How can matter ever become localized? • Stars form • Rain falls • How can temperature ever decrease? • Refrigerators and heat pumps • First aid cold packs • Whenever one thing becomes localized, something else spreads out more

  46. Group Work Explain how your process increases entropy. Think about: • Could the reverse process occur? • What spreads out: matter, energy or both? How? • Why does total entropy increase?

  47. Chemical Thermodynamics • Enthalpy (DH) is heat transfer to surroundings • Spontaneous if DG = DH – TDS < 0 • Equivalent to DS – DH/T > 0 • DS is entropy change of system • DH/T is entropy change of surroundings • If a state changes spontaneously, entropy increases.

  48. Entropy and Evolution • Darwinian evolution is perfectly consistent with thermodynamics. • Energy from the sun powers life processes. • Energy from earth radiates into space. • Material order on earth can increase because energy is dispersed. • Entropy always increases!

  49. Congratulations! “[Asking someone to] describe the Second law of thermodynamics is about the scientific equivalent of: Have you read a work of Shakespeare’s?” – C.P. Snow, Rede Lecture, Cambridge, May 7, 1959.

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