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Chemistry 231

Chemistry 231 . Spontaneity of Chemical and Physical Processes: The Second and Third Laws of Thermodynamics. What Is Thermodynamics?. Study of the energy changes that accompany chemical and physical processes. Based on a set of laws.

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Chemistry 231

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  1. Chemistry 231 Spontaneity of Chemical and Physical Processes: The Second and Third Laws of Thermodynamics

  2. What Is Thermodynamics? Study of the energy changes that accompany chemical and physical processes. Based on a set of laws. A tool to predict the spontaneous directions of a chemical reaction.

  3. What Is Spontaneity? Spontaneity refers to the ability of a process to occur on its own! Waterfalls “Though the course may change sometimes, rivers always reach the sea” Page/Plant ‘Ten Years Gone’. Ice melts at room temperature!

  4. Thermodynamic Definition Spontaneous Process – the process occurs without outside work being done on the system.

  5. Physical Statements of the Second Law • Kelvin statement • Impossible to construct an engine the sole purpose of which is to completely convert heat into work • Clausius statement • Impossible for heat to flow spontaneously from low temperature to high temperature

  6. The First Law of Thermodynamics The First Law - conservation of energy changes. U = q + w The First Law tells us nothing about the spontaneous direction of a process.

  7. Entropy We will look at a new property (the entropy). Entropy is the reason why salts like NaCl (s), KCl (s), NH4NO3(s) spontaneously dissolve in water.

  8. The Solution Process Low entropy High entropy The formation of a solution is always accompanied by an increase in the entropy of the system! For the dissolution of KCl (s) in water KCl (s)  K+(aq) + Cl-(aq)

  9. The Carnot Engine An imaginary engine

  10. The Four Steps of the Carnot Engine • Isothermal Expansion • (P1, V1, Th)  (P2, V2, Th) • Adiabatic Expansion • (P2, V2, Th)  (P3, V3, Tc) • Isothermal Compression • (P3, V3, Tc)  (P4, V4, Tc) • Adiabatic Compression • (P4, V4, Tc)  (P1, V1, Th)

  11. First Law for the Carnot Engine Cyclic Process U = 0 qcycle = -wcycle qcycle = q1 + q3 wcycle = w1 + w2 + w3 + w4

  12. The Efficiency of Any Thermal Process The Carnot engine represents the maximum efficiency of a thermal process.

  13. The Efficiency of the Carnot Engine The thermal efficiency of the Carnot engine is a function of Th and Tc

  14. The Carnot Heat Pump Run the Carnot engine in reverse as a heat pump. Extract heat from the cold temperature reservoir (surroundings) and deliver it to the high temperature reservoir.

  15. The Coefficient of Performance • The coefficient of performance of the Carnot heat pump • quantity of heat delivered to the high temperature reservoir per amount of work required.

  16. The Coefficient of Performance Two definitions

  17. The Carnot Refrigerator Use a Carnot cycle as a refrigerator. Extract heat from the cold temperature reservoir (inside) and deliver it to the high temperature reservoir (outside).

  18. The Coefficient of Performance Again, two definitions

  19. Mathematical Definition of Entropy The entropy of the system is defined as follows

  20. Entropy Is a State Variable Changes in entropy are state functions S = Sf – Si Sf = the entropy of the final state Si = the entropy of the initial state

  21. The Fundamental Equation of Thermodynamics Combine the first law of thermodynamics with the definition of entropy.

  22. The Properties of S In general, we can write S as a function of T and V

  23. Isochoric Changes in S Examine the first partial derivative

  24. The Temperature dependence of the Entropy Under isochoric conditions, the entropy dependence on temperature is related to CV

  25. Entropy changes Under Constant Volume Conditions • For a macroscopic system For a system undergoing an isochoric temperature change

  26. Isothermal Changes in S Examine the second partial derivative

  27. Isothermal Changes in S • For a reversible, isothermal process From the first law

  28. The Ideal Gas Case For an isothermal process for an ideal gas, U = 0

  29. The Ideal Gas Case (Finally) The entropy change is calculated as follows

  30. The General Case We will revisit this equality later For a general gas or a liquid or solid

  31. Properties of S(T,P) In general, we can also write S as a function of T and P

  32. Rewriting the Entropy The entropy of the system can also be rewritten

  33. Rewriting (cont’d) From the definition of enthalpy

  34. Rewriting (cont’d) From the mathematical consequences of H

  35. Isobaric Changes in S Examine the first partial derivative

  36. The Temperature dependence of the Entropy Under isobaric conditions, the entropy dependence on temperature is related to CP

  37. Entropy changes Under Constant Pressure Conditions • For a macroscopic system For a system undergoing an isobaric temperature change

  38. Isothermal Changes in S(T,P) Examine the second partial derivative

  39. Isothermal Changes in S(T,P) Under isothermal conditions

  40. The Ideal Gas Case For an isothermal process for an ideal gas, (H/ T)p = 0

  41. The Ideal Gas Case (Finally) The entropy change is calculated as follows

  42. The General Case For a general gas or a liquid or solid

  43. Phase Equilibria At the transition (phase-change) temperature only tr = transition type (melting, vapourization, etc.) trS = trH / Ttr

  44. The Second Law of Thermodynamics • The second law of thermodynamics concerns itself with the entropy of the universe (univS). • univS unchanged in a reversible process • univS always increases for an irreversible process

  45. What is univS? univS = sysS + surrS sysS = the entropy change of the system. surrS = the entropy change of the surroundings.

  46. How Do We Obtain univS? We need to obtain estimates for both the sysS and the surrS. Look at the following chemical reaction. C(s) + 2H2 (g)  CH4(g) The entropy change for the systems is the reaction entropy change, rS. How do we calculate surrS?

  47. Calculating surrS Note that for an exothermic process, an amount of thermal energy is released to the surroundings!

  48. A small part of the surroundings is warmed (kinetic energy increases). The entropy increases!

  49. Calculating surrS Insulation Note that for an endothermic process, thermal energy is absorbed from the surroundings!

  50. A small part of the surroundings is cooled (kinetic energy decreases). The entropy decreases! For a constant pressure process qp = H surrS surrH surrS -sysH

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