ch19d1_reduced

Of randomness or disorder in a system or extent to

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Unformatted text preview: rder in a system or Extent to which energy is distributed among the various motions of the molecules in the system. State Fnc: )S = Sfinal ! Sinitial )S > 0 Sf > Si inc. in disorder )S < 0 Sf < Si dec. in disorder 24 A) Isothermal Process Constant temp. process qrev )S = -----T qrev = heat which would be transferred if process were reversible S is a state function - Thus this eqn. can be used to calc. )S for any isothermal process (whether reversible or irreversible). 25 1) )S and Phase Changes Phase changes occur at constant T qrev = )Hphase change )Hphase change )Sphase change = --------------T a) Ex: boiling water at 100 °C )Hvap = 44.01 kJ/mol )Hvap 44.01 kJ/mol )Svap = --------- = ----------------T 373.15 K = 0.1178 kJ/molCK = + 117.8 J/molCK 26 nd B) 2 Law of Thermodynamics and S Irreversible Process Results in inc. in Stotal ()S > 0) Reversible Process Results in no change in Stotal ()S = 0) 2nd Law of Thermodynamics Total entropy of a system & its surroundings always inc. for a spont. process Irreversible (Spont.) Process )Suniv = )Ssys + )Ssurr > 0 Reversible Process )Suniv = )Ssys + )Ssurr = 0 27 1) Ex: 1 mole of Liquid water is left outside where it’s -10.0 °C and it freezes. Calc. )Ssys & )Ssurr. Will it spontaneously freeze? )Hfus = + 6.01 kJ/mol )Hfreezing = ! )Hfus = ! 6.01 kJ/mol )Hfrz (1 mol)(- 6.01 kJ/mol) )Ssys = -------- = ----------------------------T 273 K = ! 22.0 J/molCK )Hsurr (1 mol)(+ 6.01 kJ/mol) )Ssurr = --------- = ----------------------------T 263 K = + 22.9 J/molCK )Suniv = )Ssys + )Ssurr = (! 22.0 J/molCK) + (+ 22.9 J/molCK) = + 0.9 J/molCK Spont. 28 This shows that even though the entropy of the system DEC. the process was still spont. because the entropy of the surroundings increased more. )Suniv > 0 Spont. processes occur w. an overall INC. in )Suniv Practically speaking we can’t always easily determine )Suniv. Want to focus on the system. Will see how to determine spontaneity based on )Ssys. Simplify notation and refer to )Ssys simply as )S 29 III) Molecular Inerpretation of Entropy A) Expansion of Gas at Molecular Level Have the following system: Open stopcock: We know gas expands spont. Why? Look at possible arrangements of the particles 30 Probability that blue molecule is in left flask is ½. Same for the red particle. Probability that both molecules remain in the left flask is: (1/2)2 = 1/4 For 3 particles it would be 3 (1/2) = 1/8 31 Consider a mole of gas: Prob. all the molecules are in the left flask at the same time is: N (1/2) where N = 6.02 x 10 23 Infinitesimally small!!! Essentially ZERO prob. of all molecules in left flask at same time. - Gas SPONT. expands to fill both flasks - Does NOT spont. go back to left flask 32 Most probable arrangements are those with essentially equal numbers of particles in both flasks. Gas spreads out and the arrangement of gas particles is more disordered than when confined to one flask Greater entropy Just seen disorder on the molecular...
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