Ch. 20 - Chem 1036 Chapter 20 1 of 28 Chem 1036 Chapter 20...

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Chem 1036 Chapter 20 1 of 28 Chapter 20 Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions Thermodynamics: Spontaneous Reactions: Nonspontaneous Reactions: If a reaction is spontaneous in one direction, it is not spontaneous in the reverse direction. Chem 1036 Chapter 20 2 of 28 Examples: Spontaneous reactions may be fast: 2H 2 (g) + O 2 (g) 2H 2 O(g) H = -483.6 kJ Or, spontaneous reactions may be slow: 2Fe(s) + 3/2O 2 (g) + 3H 2 O( l ) 2Fe(OH) 3 (s) H = -780.6 kJ Chem 1036 Chapter 20 3 of 28 FACTORS AFFECTING SPONTANEITY 1. Enthalpy, H: H = - : H = + : Most exothermic reactions are spontaneous at room temperature: BUT, some endothermic reactions are also spontaneous: Chem 1036 Chapter 20 4 of 28 2. Entropy (S) : Entropy of substances: how disordered are the atoms, molecules, etc.: Before Mixing After Mixing Gas A Gas B rinted with FinePrint - purchase at www.fineprint.com
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Chem 1036 Chapter 20 5 of 28 solid liquid liquid gas solute solvent solution + Melting: S > S liquid solid Vaporization: S > S vapor liquid Dissolving: S > (S + S soln solvent solute) Chem 1036 Chapter 20 6 of 28 S solid S liquid S gas Temperature (K) S o l id L iq u Vaporization Fusion G a s 100 200 300 50 100 150 200 Chem 1036 Chapter 20 7 of 28 Example Problem : Do the following processes lead to an increase in entropy ( S = +) or to a decrease in entropy ( S = -)? a. HCl(g) H(g) + Cl(g) b. 2H 2 (g) + O 2 (g) 2H 2 O(g) c. AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) Chem 1036 Chapter 20 8 of 28 How does entropy affect spontaneity? Which is more probable: a state of order or a state of disorder? Nature has a tendency to move from a state of lower probability to one of higher probability. In general, nature tends to move spontaneously from more ordered states (low entropy) to more random states (high entropy).
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Ch. 20 - Chem 1036 Chapter 20 1 of 28 Chem 1036 Chapter 20...

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