1 / 36

Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

The entropy change accompanying the dissolution of a salt. Open valve: gas spontaneously expands. More probable state. Expansion of a gas. SYSTEM CHANGES to state of HIGHER PROBABILITYFor entropy-driven reactions - the more RANDOM state. Ethanol. Water. Solution of ethanol and water. The small increase in entropy when ethanol dissolves in water..

ova
Download Presentation

Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

    1. Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

    3. Expansion of a gas

    7. Entropy Entropy, S, is a measure of the disorder of a system Entropy is a state function. ?S = Sfinal - Sinitial. If ?S > 0 the disorder increases if ?S < 0 the order increases.

    12. Spontaneous Processes Any process that occurs without outside intervention under specified conditions When two eggs are dropped they spontaneously break. For a non-spontaneous change to occur, the surroundings must supply the system with a continuous input of energy We can conclude that a spontaneous process has a direction. If a change is spontaneous in one direction, it is not spontaneous in the other.

    13. Spontaneous Processes Some classes of spontaneous processes: Phase transitions (melting, freezing) Mixing Expansion Heat transfer Movement towards chemical equilibrium

    14. Spontaneous Processes Which of the following are spontaneous? Spreading of the fragrance of perfume through a room Formation of CH4 and O2 from CO2 and H20 at room temp and 1 atm. Dissolution of sugar in a cup of hot coffee

    15. Spontaneous Processes Activity

    16. Spontaneous Processes Some reactions are spontaneous, others never occur. Why? A system tries to minimize its ENERGY A system tries to maximize its ENTROPY How can we predict whether a reaction will occur over time?

    17. Exothermic Spontaneous Processes In general, product-favored reactions are exothermic and spontaneous E.g. thermite reaction Fe2O3(s) + 2 Al(s) ? 2 Fe(s) + Al2O3(s) ?H = - 848 kJ

    18. Non-Exothermic Spontaneous Processes But many spontaneous reactions or processes are endothermic . . .

    21. 2nd Law Example ?Suniv = ?Ssys + ?Ssurr Is the decomposition of calcium carbonate spontaneous under standard conditions? Calculate ?Suniv to support your answer. CaCO3 (s) ? CaO (s) + CO2 (g)

    22. Entropy Changes in Chemical Reactions Used calorimetry to measure ?H for chemical reactions No comparable method for measuring S (variation of heat capacity at different temperatures) Standard molar entropy, S?: entropy of a substance in its standard state. Similar in concept to ?H?. Units: J/mol-K. See Appendix

    24. Entropy Changes in Chemical Reactions Can Calculate ?S for chemical reactions Calculate ?S? for the dissolution of ammonium nitrate, given the following entropy values: NH4NO3 (s) NH4+ (aq) + NO3- (aq) 151.04 J/mol*K 112.8 146.4

    25. Problem Solving Calculate the standard entropy change for the following reaction: Al2O3 (s) + 3H2 (g) 2Al (s) + 3 H2O (g)

    26. 2nd Law Example ?Suniv = ?Ssys + ?Ssurr Is the decomposition of calcium carbonate spontaneous under standard conditions? Calculate ?Suniv to support your answer. CaCO3 (s) ? CaO (s) + CO2 (g)

    27. Entropy Changes in the Surroundings We can calculate Entropy changes in the system, what about the surroundings? ?Ssurr = -?H/T

    28. You Try! Is the decomposition of Be(OH)2 spontaneous at 25 C? Be(OH)2 (s) ? BeO (s) + H2O (g)

    30. What is Free about Free Energy? The change in free energy for a process equals the maximum useful work that can be done by the system on the surroundings in a spontaneous process occurring at constant temperature and pressure It is the portion of the energy change of a spontaneous process that is free to do useful work For a process that is not spontaneous, the free energy change is the measure of the minimum amount of work that must be done to cause the process to occur.

    31. Problem Solving N2 (g) + 2O2(g) ? 2NO2 (g)

    33. Problem Solving Sulfur dioxide in the effluent gases from coal-burning electric power plants is one of the principal causes of acid rain. One method for reducing SO2 emissions involves partial reduction of SO2 to H2S, followed by catalytic conversion of the H2S and the remaining SO2 to elemental sulfur: 2H2S (g) + SO2 (g) ? 3S (s) + 2H2O (g) a. Using the charts, calculate ?G. Is this reaction spontaneous at 25C? 100C? b. Estimate the temperature at which the reaction becomes non-spontaneous.

    34. Life is Ordered Does this violate the second law of thermodynamics?

    36. Recap Do you agree with the following statements? A. Spontaneous reactions are always fast. B. In any spontaneous process, the entropy of the system always increases. C. An endothermic reaction is always non-spontaneous.

    37. Conceptual Question Which substances in each of the following pairs would you expect to have the higher standard molar entropy? Why? a. C2H2 (g) or C2H6 (g) b. CO2 (g) or CO (g) c. I2 (s) or I2 (g) d. CH3OH (g) or CH3OH (l)

More Related