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Chapter 18 Thermodynamics Week 1 2009

Chapter 18 Thermodynamics Week 1 2009 - Chapter 18...

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Chapter 18 Thermodynamics: Entropy and Free Energy Section 18.2 Spontaneous Change and Equilibrium . We must now address why reactions occur, and what controls the equilibrium position of a reaction. To do this, we must introduce the idea of a spontaneous process. “spontaneous means occurs by itself a spontaneous reaction or change occurs by itself (until it reaches equilibrium), whereas a “non-spontaneous” reaction is in a direction away from equilibrium and can thus occur only if we force it to happen by inputting energy. ** Let’s consider reactions that lie ~100% to one side or the other to help get the points across, and then we will be more general later.** Examples. 1) Sodium metal, Na(s), will spontaneously react with Cl 2 gas to give NaCl, but NaCl will not spontaneously give Na metal and Cl 2 gas unless we input energy. 2) Iron metal will spontaneously rust when exposed to water and oxygen gas, but rust will not spontaneously give Fe metal. These examples are obvious, but what about: Which is the spontaneous direction in which the reaction will go by itself, and which is the direction we must force it to go??? Similarly, consider something falling, e.g. a ball or chalk-board eraser Note : “spontaneousness” (or “spontaneity”) does not tell us anything about rates of reaction, only about whether it will occur by itself. What factors determine which direction of a reaction is spontaneous ? We shall see in Chapter 18, and involves the 2 nd Law of Thermodynamics. First, let’s refresh our memories of the 1 st Law. 1 Pb (s) + 2 Ag + (aq) Pb 2+ + Ag (s) ?
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E = q + w E = internal energy of system (sum of potential and kinetic energies) q = heat w = work As chemists, our “system” is usually a reaction in a beaker or flask - everything else is “surroundings” E universe = E system + E surroundings The universe consists of everything its energy is constant ( E univ = 0) E univ = E sys + E surr = 0 i.e., E sys and E surr cancel each other out energy (E) cannot be created or destroyed. This is the First Law of Thermodynamics NOTE : First Law does not by itself help us to understand and predict the spontaneous direction, however. How is E, the internal energy of the system, related to H, the enthalpy we learned about in Chapter 6? H = E + PV (P = pressure, V = volume) H = E + P V (at constant pressure) For most reactions (in solution), V is insignificant H E H = enthalpy change = the heat gained or lost at constant pressure. Can we predict spontaneity from the sign of H? i.e., are all exothermic rxns ( H < 0) spontaneous? are all endothermic rxns (
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Chapter 18 Thermodynamics Week 1 2009 - Chapter 18...

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