20 mol so 2 l and 20 mol so 2 g 10 mol ch 4 g and 10

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2.0 mol SO 2( l ) and 2.0 mol SO 2( g ) 1.0 mol CH 4( g ) and 1.0 mol C 2 H 6( g ) 1.0 mol CH 4( g ) at 298 K and 1.0 mol CH 4( g ) at 273 K 1.5 mol Ar in 5.0 L and 1.5 mol Ar in 10.0 L 18 Reaction Entropy Standard reaction entropies ( Δ S ) Sign of reaction entropies ( Δ S ) Predict based on moles of gases Calculating standard reaction entropies ( Δ S o ) Δ S o = Σ nS m o P ( ) − Σ nS m o R ( ) Example 8 Calculating standard reaction entropy for N 2 H 4( l ) + 3 O 2( g ) 2 NO 2( g ) + 2 H 2 O ( l ) given the following standard molar entropies. S m o (H 2 O ( l ) ) = 69.91 J K –1 mol –1 S m o (N 2 H 4( l ) ) = 121.21 J K –1 mol –1 S m o (NO 2( g ) ) = 239.95 J K –1 mol –1 S m o (O 2( g ) ) = 205.03 J K –1 mol –1 19
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Reaction Entropy Problem 5 How much entropy change accompanies the combustion of 0.454 mol of propane (C 3 H 8 )? S m o (C 3 H 8( g ) ) = 270.3 J K –1 mol –1 S m o (CO 2( g ) ) = 213.8 J K –1 mol –1 S m o (H 2 O ( l ) ) = 69.91 J K –1 mol –1 S m o (O 2( g ) ) = 205.03 J K –1 mol –1 20 Global Changes in Entropy Entropy change of surroundings Process spontaneous if overall entropy change positive Δ S tot = Δ S + Δ S surr Δ S surr = q surr T Δ S surr = Δ H T 21
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Global Changes in Entropy Overall change in entropy Entropy change and spontaneity Δ S tot > 0 Spontaneous Δ S tot < 0 Reverse process spontaneous Δ S tot = 0 Not spontaneous in either direction 22 Δ S > 0 Δ S < 0 Exothermic Δ S surr ___ 0 Endothermic Δ S surr ___ 0 Global Changes in Entropy Overall change in entropy Entropy change and spontaneity 1. Determine type of system If isolated If open or closed 2. Calculate the entropy change for system 3. If necessary calculate the entropy change for the surrounding 4. If necessary, calculate the total entropy change 5. Use the sign of Δ S tot (or Δ S ) to determine spontaneity Spontaneous when Δ S tot (or Δ S ) _____ 0 23
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Global Changes in Entropy Overall change in entropy Entropy change and spontaneity Example 9 Is the reversible expansion of 0.755 mol of an ideal gas at room temp from 6.60 L to 8.05 L spontaneous? R is 8.314 J K –1 mol –1 . Explain briefly. 24 Global Changes in Entropy Problem 6 Is the expansion of 0.755 mol of an ideal gas at room temp from 6.60 L to 8.05 L at 1.50 atm spontaneous? Explain briefly. R is 8.314 J K –1 mol –1 and 1 L atm = 101.325 J. 25
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Global Changes in Entropy Problem 7 When 250. g copper block at 50 o C is placed next to 12.0 g (0.666 mol) ice cube at 0 o C, will the system spontaneously reach 5 o C? Explain briefly. Assume no heat loss. The heat capacities of copper, water and ice are 0.38 J o C –1 g –1 , 4.184 J o C –1 g –1 and 2.03 J o C –1 g –1 respectively. The heat of fusion of water is 6.01 kJ mol –1 .
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