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7 Entropy Changes and Processes

# 7 Entropy Changes and Processes - Entropy Changes Processes...

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Section 4.3, 7th edition; 3.3, 8th edition Entropy of Phase Transition at the Transition Temperature Expansion of the Perfect Gas Variation of Entropy with Temperature Measurement of Entropy Chapter 4 of Atkins: The Second Law: The Concepts Last updated: Oct. 24, 2006; minor change to slide 1

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Entropy at a Phase Transition Changes in molecular order occur when a substance freezes or boils Consider the phase transitions of water, at transition temperatures T trs . For ice water, T trs = 273 K, ice in equilibrium with liquid water at 1 atm and boiling water, T trs = 373 K, liquid water in equilbrium with vapour at 1 atm The external pressure is constant for a glass of ice water, and in order to match attractive forces between ice molecules, energy must come from kinetic energy of the water molecules or the surroundings At T trs , any transfer of heat between the system and surroundings is reversible since the two phases in the system are in equilibrium (the forces pushing the ice towards melting are equal to those pushing the water towards freezing) - so a phase transition is reversible It does not matter how the ice melts (what path it takes) since entropy is a state function. What does matter for this particular expression is that the system be isothermal . If it was not isothermal, one would have a problem examining the process in steps – as we shall see.
Entropy at a Phase Transition, 2 For exothermic and endothermic phase transitions: exothermic endothermic freezing, condensing melting, boiling phase transition ) trs H > 0 ) trs S > 0 disordered ) trs H < 0 ) trs S < 0 ordered ) trs S ' ) trs H T trs At constant pressure, q = ) trs H , and the change in molar entropy is Consistent with decreasing disorder: gases > liquids > solids Example: when compact condense phase vapourizes into a widely dispersed gas, one can expect an increase in the disorder of the molecules

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Trouton’s Rule Some Standard Entropies of Vaporization ) vap H o (kJ mol -1 ) 2 boil ( o C) ) vap S o (J K -1 mol -1 ) Benzene +30.8 80.1 +87.2 CCl 4 +30.00 76.7 +85.8 Cyclohexane +30.1 80.7 +85.1 H 2 S +18.7 -60.4 +87.9 Methane +8.18 -161.5 +73.2 Water +40.7 100.0 +109.1 Trouton’s Rule : This empirical observation (see Table 4.2) states that most liquids have approximately the same standard entropy of vaporization , ) vap S o . 85 J K -1 mol -1 : So, ) vap H o = T
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7 Entropy Changes and Processes - Entropy Changes Processes...

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