CHAPTER 14 Thermodynamics: Spontaneous Processes, Entropy, and Free Energy A tiny fraction of the sun's energy is used to produce complicated, ordered, high- Useful energy is being "degraded" in energy systems such as life the form of unusable heat, light, etc. • 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 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 0C and ice melts above 0 0C • 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 Spontaneous chemical and physical changes are frequently accompanied by a release of heat (exothermic H < 0) - o C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) H = -2200 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 Some processes are accompanied by no change in enthalpy at all (Ho = 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: o C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) H = -2200 kJ 2. water melting: o H2O(s) H2O(l) H = 6.01 kJ 3. gas expansion: 4 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 Trends in Entropies Ssolid < Sliquid < Sgas Entropy is expected to INCREASE for these types of processes (S > 0) : 6 Changes in Entropy order S disorder S S = Sf - Si S > 0 S < 0 Suniverse 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 oC to 20 oC. 7 Entropy Changes in the System (Ssys) 0 The standard entropy of reaction (Srxn ) is the entropy change for a reaction carried out at 1 atm and 250C. aA + bB cC + dD from Appendix 0 0 0 0 0 Srxn = [c S (C) + dS (D) ] - [a S (A) + bS (B) ] 0 0 0 Srxn = S nS (products) - S mS (reactants) The Second Law of Thermodynamics: The total entropy of the universe increases in any spontaneous process Spontaneous process: Suniv = Ssys + Ssurr > 0 sys = system One can be negative but the surr = surroundings other will be even more positive Equilibrium process: Suniv = Ssys + Ssurr = 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 Entropy Changes in the Surroundings (Ssurr) Exothermic Process Endothermic Process Ssurr > 0 Ssurr < 0 The change in entropy of the surroundings can be calculated: if Hsys < 0 (exothermic), then Ssurr > 0 (entropy of the surroundings increases) Ssurr -Hsys if Hsys > 0 (endothermic), then Ssurr < 0 (entropy of the surroundings decreases) If the temperature of the surroundings is already high, Ssurr 1 then pumping heat in or out Tsurr causes less change in disorder than at lower temperatures 9 Combining the two: Ssurr -Hsys and Ssurr 1 so Ssurr = -Hsys Tsurr T (Tsurr usually = Tsys) e.g. N2(g) + 3H2(g) 2NH3(g) Ssys = -198.3 J/K * Hsys = -92.6 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 Suniv will decide the issue (next slide). Remember: for a spontaneous reaction the entropy of the universe increases. 0 0 0 *from Ch 5: Hrxn = S nH f (products) - S mHf (reactants) Is the reaction spontaneous at 25 oC? Suniv = Ssys + Ssurr 10 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 Suniv > 0. Suniv = Ssys + Ssurr > 0 Another way of stating the 2nd Law is that "You Can't Win!" 11 .