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# chapter 7 slides - THERMODYNAMICS: The 2nd and 3rd Laws...

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1 THERMODYNAMICS: The 2 nd and 3 rd Laws Chapter 7 CHEM 6B Dr. DiPasquale CHEMICAL PRINCIPLES: Spontaneity c A spontaneous change occurs without being driven by an outside influence. c A spontaneous change need not be fast. c A non-spontaneous change can be brought about only by doing work. c How can we tell if a reaction is going to be spontaneous or not???

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2 Entropy and Disorder c Energy and matter tend to become more disordered. c Entropy, S , is a measure of disorder (in units of J K -1 ). Low entropy – low disorder; high entropy – high disorder. c The entropy of an isolated system increases in the course of any spontaneous change – 2 nd Law. c Entropy is a state function. Increasing Disorder c Breaking chemical bonds to form a greater number of smaller molecules/atoms. c Solid to liquid phase change, liquid to gas phase change, sublimation c Increasing temperature (esp. with a gas) c Mixing of pure materials; dissolving c Big → small
3 Example Problem c What sign would you expect for Δ S for the following reaction? (A=positive, B=negative, C=neither or cannot be determined) c N 2 O 4 (g) → 2 NO 2 (g) Example Problem c What sign would you expect for Δ S for the following reaction? (A=positive, B=negative, C=neither or cannot be determined) c 2 H 2 (g) + O 2 (g) → 2 H 2 O (l)

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4 Example Problem c What sign would you expect for Δ S for the following reaction? (A=positive, B=negative, C=neither or cannot be determined) c CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (l) Example Problem c What sign would you expect for Δ S for the following reaction? (A=positive, B=negative, C=neither or cannot be determined) c H 2 O (l) → H 2 O (s)
5 Change in Entropy c Δ S = q rev /T c where q rev = energy transferred reversibly as heat c T is the temperature at which the transfer takes place c If a lot of energy is transferred as heat, then we would expect a big change in the entropy. c A greater change in the disorder is expected at lower temperatures than higher temperatures Disorder c Thermal disorder results from the natural thermal motion of the molecules. c Positional disorder results from translation of the position of molecules. c Expansion of a gas, mixing of two different gasses together, heating of a gas all serve to increase the disorder of a system.

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6 Heating a gas at under constant volume c Constant volume, variable temperature c Where C is the heat capacity of the system c and T 2 / T 1 is the ratio between the initial and final T . 2 1 ln T S C T Δ = Example Problem c A sample of nitrogen gas of volume 50.0 L at 20 atm pressure is heated from 20.0 °C to 500 °C at constant volume. What is the change in the entropy of the nitrogen? ( C v,m N 2 = 20.81 J K -1 mol -1 )
7 Isothermal Gas Expansion 2 1 ln V S nR V Δ = c Constant temperature, variable volume c Where R is 8.31447 J K -1 mol -1 c and V 2 / V 1 is the ratio between the initial and final V . Example Problem

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## This note was uploaded on 01/25/2011 for the course CHEM 6BL taught by Professor Berniolles during the Spring '08 term at UCSD.

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chapter 7 slides - THERMODYNAMICS: The 2nd and 3rd Laws...

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