CHAPTER 18: THERMODYNAMICS AND EQUILIBRIUM Part One: Heat Changes and Thermchemistry This aspect f Thermdynamics was dealt with in Chapter 6. (Review) A. Statement f First Law. (Sectin 18.1) 1. U ttal internal energy pssessed by a system. 2. q = heat absrbed by system during a prcess. 3. w = wrk dne n system during a prcess. 4. ΔU = q + w 5. Scenari: w = wrk invlved expanding against its surrundings wrk f expansin is calculated as: w = -PΔV P = ppsing pressure ΔV = vlume change f system 6. If system is heated but nt allwed t respnd in any way, then: ΔU = q n wrk is dne 7. S we say: ΔU = q v q v = heat absrbed in a prcess in which system is nt allwed t expand against its surrundings. (cnstant vlume!!) All energy input remains in the system. Chapter 18 Page 1
B. Enthalpy Changes. 1. Mst prcesses we care abut ccur at cnstant Pressure, nt cnstant Vlume. 2. Example: Heating an pen beaker f water. 3. q p = heat absrbed by system heated at cnstant pressure. 4. Enthalpy H = heat cntent f a system. ΔH = q p = enthalpy change = change in heat cntent f the system 5. ΔH slightly > ΔU. expands slightly against surrundings, wrk is dne Tiny amunt f energy input q p des nt end up increasing E 6. Enthalpy and Energy are nearly synnymus, but ΔH is mre directly measurable fr heat transfers under cnstant P cnditins. ΔH = ΔU + PΔV A. Spntaneity = Part Tw: Spntaneity f Reactins 1. Reactins are favred Chapter 18 Page 2
2. Reactins are favred by a. expansin f a gas int a vacuum b. disslving f a crystal int slutin Figure 18.6 c. A pendulum in the atmsphere will always cme t rest, never the ppsite. Figure 18.7 Chapter 18 Page 3
d. Heat flw frm high T regin t lw T regin. B. The Secnd Law f Thermdynamics. 1. Invlves Entrpy S = 2. Secnd Law: 3. 4. It is pssible fr entrpy f a system t be decreasing (system becming mre rderly): ΔS system < 0 but nly if Example: Crystallizatin f an insluble slid frm slutin is a decrease in disrder, but still can ccur spntaneusly, because heat is released int the surrundings 5. It can be shwn that the entrpy change f a system in a prcess is always: ΔS > q/t q=heat flw int the system at temp T 6. The Third Law f Thermdynamics: i.e., a substance that is perfectly crystalline at 0 K has zer entrpy. Chapter 18 Page 4
Therefre, there is an abslute zer n the entrpy scale, unlike energy, and yu can find S f a sample substance, whereas yu can t find its U, nly its ΔU Figure 18.8 7. Hess Law can be used t calculate ΔS f prcesses, using the data in Table 18.1 8. Entrpy changes in a reactin - Entrpy increases fr: a. a reactin in which a mlecule is brken dwn int tw r mre smaller mlecules. b. a reactin in which there are an increasing number f mles f gas. c. A prcess in which a slid changes t liquid r a liquid changes t a gas. C. Entrpy f phase transitins: ΔS trans = ΔH trans T trans Part Three: Free Energy A. Free Energy Change, ΔG, and Spntaneity. (Sectin 18.4) 1. It is incnvenient t mnitr S f universe as a criterin fr spntaneity. Chapter 18 Page 5
2. Mre cnvenient t fcus nly n. 3. Gibbs Free Energy, G, prvides the apprpriate indicatr. 4. Nw we have a new criterin fr spntaneity, invlving nly system variables. ΔG has tw factrs nw: 5. Nw we understand what we said abut tw factrs befre: ΔH negative (exthermic) helps ΔG t be negative ΔS psitive (increased disrder) als helps ΔG t be negative 6. Disslving NaCl(s) in H 2 O is endthermic: ΔH psitive but still is spntaneus because large psitive ΔS term causes ΔG t be verall negative. -TΔS ΔG = ΔH - TΔS (cnstant temperature and pressure) ΔH = - ΔS = + Rxns are spntaneus at all temperatures. ΔH = - ΔS = - Rxns becme spntaneus belw a definite temperature. ΔH = + ΔS = + Rxns becme spntaneus abve a definite temperature. ΔH = + ΔS = - Rxns are nnspntaneus at all temperatures. Chapter 18 Page 6
7. Can use Hess Law t calculate ΔG rxn frm standard free energies f frmatin, ΔG f. Therefre: ΔG rxn = nδg f nδg f prducts reactants just like we did fr heats f reactin: ΔH rxn = nδh f nδh f prducts reactants And als: ΔS rxn = ns ns prducts reactants Part Fur: Free Energy and the Equilibrium Cnstant A. Relatinship Between K and ΔG. (Sect. 18.6) 1. ΔG = 2. 3. 4. Nte that: ΔG f rxn K Prduct Frmatin if ΔG < 0 K > 1 if ΔG = 0 K = 1 if ΔG > 0 K < 1 5. The free energy change f reactin ccurring under nn-standard-state cnditins, use: Where Q = reactin qutient. Chapter 18 Page 7
6. Interpretatin f ΔG. Instantaneus indicatr f which directin a reacting system will g spntaneusly t reach an equilibrium state. Dependent upn instantaneus value f Q. Als: ΔG = maximum useful wrk that can be perfrmed by a prcess. B. Change f ΔG (and thus the Equilibrium Cnstant) with Temperature. (Sectin 18.7) 1. Can use the equatin: t quickly estimate hw ΔG changes with T if we simply plug in the values f ΔH and ΔS at 298 K, and put in T as the new temperature 2. Example prblem: A chemical reactin has a ΔH = -300 kj and an entrpy change f reactin ΔS = -2.5 kj/k. a. What is the ΔG f reactin at 298 K? b. Is the reactin spntaneus at 298K? c. At what temperature will the reactin switch ver t becming spntaneus? Chapter 18 Page 8
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NOTES: Chapter 18 Page 10