CHAPTER 14. Thermodynamics: Spontaneous Processes, Entropy, and Free Energy

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1 CHAPTER 14 Thermodynamics: Spontaneous Processes, Entropy, and Free Energy Useful energy is being "degraded" in the form of unusable heat, light, etc. A tiny fraction of the sun's energy is used to produce complicated, ordered, highenergy systems such as life Our observation is that natural processes proceed from ordered, high-energy systems to disordered, lower energy states. In addition, once the energy has been "degraded", it is no longer available to perform useful work. It may not appear to be so locally (earth), but globally it is true (sun, universe as a whole). 1

2 Thermodynamics - quantitative description of the factors that drive chemical reactions, i.e. temperature, enthalpy, entropy, free energy. Answers questions such as- will two or more substances react when they are mixed under specified conditions? if a reaction occurs, what energy changes are associated with it? to what extent does a reaction occur to? Thermodynamics does NOT tell us the RATE of a reaction Spontaneous Processes A spontaneous process is one that is capable of proceeding in a given direction without an external driving force A waterfall runs downhill A lump of sugar dissolves in a cup of coffee At 1 atm, water freezes below 0 0 C and ice melts above 0 0 C Heat flows from a hotter object to a colder object A gas expands in an evacuated bulb Iron exposed to oxygen and water forms rust 2

3 Spontaneous chemical and physical changes are frequently accompanied by a release of heat (exothermic H < 0) - C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) H o = kj Although many spontaneous processes are exothermic (e.g., the previous combustion reaction), not true for all spontaneous reactions: Endothermic (ΔH > 0), but reaction is spontaneous. 3

4 Some processes are accompanied by no change in enthalpy at all ( H o = 0.0), as is the case for an ideal gas spontaneously expanding: spontaneous nonspontaneous There's another factor promoting spontaneity in these processes, and that's the increasing randomness or disorder of the system: 1. propane combustion: C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) H o = kj 2. water melting: H 2 O(s) H 2 O(l) H o = 6.01 kj 3. gas expansion: 4

5 Thermodynamics: Entropy Second Law of Thermodynamics: The total entropy of the universe increases in any spontaneous process Entropy (S): A measure of the amount of disorder (qualitative), or unusable energy in a system at a specific temperature (quantitative). Entropy is affected by molecular motion, or disorder from volume changes (e.g. the previous gas expansion example). Types of Molecular Motion Three types of motion: Translational movement through space. Rotational spinning motion around axis to bond. Vibrational movement of atoms toward/away from each other. As temperature increases, the amount of motion increases. 5

6 Trends in Entropies S solid < S liquid < S gas Entropy is expected to INCREASE for these types of processes ( S > 0) : 6

7 Changes in Entropy order S disorder S S = S f - S i S > 0 S < 0 S universe Example - Predict whether the entropy change is greater than or less than zero for each of the following processes: (a) freezing ethanol (b) evaporating a beaker of liquid bromine at room temperature (c) dissolving sucrose in water (d) cooling nitrogen gas from 80 o C to 20 o C. 7

8 Entropy Changes in the System ( Ssys) The standard entropy of reaction ( S 0 rxn ) is the entropy change for a reaction carried out at 1 atm and 25 0 C. aa + bb cc + dd from Appendix S 0 rxn = [ cs 0 (C) + ds 0 (D)] - [ as 0 (A) + bs 0 (B)] S 0 rxn = S ns 0 (products) - S ms 0 (reactants) The Second Law of Thermodynamics: The total entropy of the universe increases in any spontaneous process Spontaneous process: sys = system surr = surroundings S univ = S sys + S surr > 0 One can be negative but the other will be even more positive Equilibrium process: S univ = S sys + S surr = 0 At equilibrium the forward and reverse rates are equal, e.g vapor pressure (l) = (g) So there is no net change in entropy 8

9 Entropy Changes in the Surroundings ( Ssurr) Exothermic Process S surr > 0 Endothermic Process S surr < 0 The change in entropy of the surroundings can be calculated: S surr - H sys S surr 1 T surr if H sys < 0 (exothermic), then S surr > 0 (entropy of the surroundings increases) if H sys > 0 (endothermic), then S surr < 0 (entropy of the surroundings decreases) If the temperature of the surroundings is already high, then pumping heat in or out causes less change in disorder than at lower temperatures 9

10 Combining the two: S surr - H sys and S surr 1 T surr T so S surr = - H sys (T surr usually = T sys ) e.g. N 2 (g) + 3H 2 (g) 2NH 3 (g) S sys = J/K H sys = kj * The two main driving forces are in opposition to each other - the release of heat favors a spontaneous reaction while the decrease in entropy does not. Calculating S univ will decide the issue (next slide). Remember: for a spontaneous reaction the entropy of the universe increases. *from Ch 5: H 0 rxn = S n H 0 f (products) - S m Hf 0 (reactants) Is the reaction spontaneous at 25 o C? S univ = S sys + S surr 10

11 The previous example with ammonia illustrated that maybe entropy will decrease in the system, but this will always be accompanied by a greater increase in the entropy of the surroundings such that S univ > 0. S univ = S sys + S surr > 0 Another way of stating the 2nd Law is that "You Can't Win!" 11