Ch20_Entropy - Entropy and Free Energy Entropy and Free...

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Unformatted text preview: Entropy and Free Energy Entropy and Free Energy How to predict if a reaction can occur, given enough time? THERMODYNAMICS Thermodynamics Thermodynamics • Is the state of a chemical system such that a rearrangement of its atoms and molecules would decrease the energy of the system? • If yes, system is favored to react — a product-favored system. system. • Most product-favored reactions are exothermic. How to predict if a reaction can occur at a reasonable rate? KINETICS Product-Favored Reactions In general, productfavored reactions are exothermic . • Often referred to as spontaneous Often spontaneous reactions. • Spontaneous does not imply anything about time for reaction to occur. Product-Favored Reactions But many spontaneous reactions or processes are endothermic or even have H = 0. Fe2O3(s) + 2 Al(s) ---> 2 Fe(s) + Al2O3(s) H = - 848 kJ Thermodynamics and Kinetics Diamond is thermodynamically favored to convert to graphite, but not graphite, not kinetically favored . Paper burns — a product-favored reaction. Also reaction. Also kinetically favored once reaction is begun. Entropy, S One property common to product-favored processes is that the final state is more DISORDERED or more DISORDERED RANDOM than the original. Spontaneity is related to an increase in randomness. NH4NO3(s) + heat ---> NH 4NO3 (aq) Page 1 The thermodynamic property related to randomness is ENTROPY, S . Reaction of K with water Directionality of Reactions The entropy of liquid water is greater than the entropy of solid water (ice) at 0˚ C. How probable is it that reactant molecules will react? PROBABILITY suggests that a product-favored reaction will result in the dispersal result dispersal Directionality of Reactions Probability suggests that a productfavored reaction will result in the dispersal of energy or of matter or both. Matter Dispersal * of energy energy * or of matter matter * or of energy & matter. or energy Directionality of Reactions Probability suggests that a productfavored reaction will result in the dispersal of energy or of matter or both. Energy Dispersal Directionality of Reactions Entropy, S Entropy, Energy Dispersal Exothermic reactions involve a release of stored chemical potential energy to the surroundings. The stored potential energy starts out in a few molecules but is finally dispersed over a great many molecules. The final state—with energy dispersed—is more probable and makes a reaction product-favored. Page 2 So ((J/K•mol) So J/K• mol) H22O(liq) H O(liq) 69.91 69.91 H22O(gas) 188.8 H O(gas) 188.8 188.8 S (gases) > S (liquids) > S (solids) Entropy, S Entropy of a substance increases with temperature. Molecular motions of heptane, C7H16 Entropy, S Entropy, S Increase in molecular complexity generally leads to increase in S. Molecular motions of heptane at different temps. So ((J/K•mol) So J/K• mol) CH44 CH Entropy usually increases when a pure liquid or solid dissolves in a solvent. 336.1 336.1 336.1 C33H8 C H8 So ((J/K•mol) So J/K• mol) 248.2 248.2 C22H6 C H6 Entropy, S Entropies of ionic solids depend on coulombic attractions. 419.4 419.4 Entropy Changes for Phase Changes For a phase change, For S = q/T q/T where q = heat transferred in phase change For H2O (liq) ---> H2O(g) H = q = +40,700 J/ mol MgO MgO 26.9 26.9 NaF NaF 51.5 51.5 Calculating S for a Reaction So = So = So ((products) -So products) So ((reactants) So reactants) Consider 2 H 2(g) + O2(g) ---> 2 H2O(liq) So = 2 So (H2O) - [2 So (H2) + So (O2)] So = 2 mol (69.9 J/K•mol) [2 mol (130.7 J/K• mol) + [2 1 mol (205.3 J/K• mol)] So = -326.9 J/K q 40,700 J/mol S= = = + 109 J/K•mol 373.15 K T Page 3 Note that there is a decrease in S because Note because 3 mol of gas give 2 mol of liquid. 2nd Law of Thermodynamics A reaction is spontaneous (productfavored) if S for the universe is positive. Suniverse = Ssystem + Ssurroundings 2nd Law of Thermodynamics Dissolving NH 4NO3 in water—an entropy driven process. Suniverse > 0 for product-favored process 2nd Law of Thermodynamics 2 H2(g) + O 2(g) ---> 2 H2O(liq) Sosystem = -326.9 J/K So surroundings = qsurroundings - H system = T T Can calc. that Horxn = Hosystem = -571.7 kJ that First calc. entropy created by matter dispersal ( Ssystem) So surroundings = Next, calc. entropy created by energy dispersal ( Ssurround) Sosurroundings = +1917 J/K 2nd Law of Thermodynamics 2 H2(g) + O 2(g) ---> 2 H2O(liq) Sosystem = -326.9 J/K Sosurroundings = +1917 J/K Souniverse = +1590. J/K +1590. The entropy of the universe is increasing, so the reaction is productfavored. Page 4 - (-571.7 kJ)(1000 J/kJ) 298.15 K ...
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