CHEM 480 1nd Edition Lecture 8 Outline of Last Lecture I. Chemical ChangesII. Energy from biological fuelsIII. Energy is DispersedIV. Heat of formation, ΔfHϴV. Calculate average C-H bond enthalpy in CH4 given heats of formationVI. Potassium metal added to waterOutline of Current Lecture I. SpontaneityII. Entropy, SIII. Trouton’s RuleIV. ΔS associated w/ phase changesV. ΔS for surroundingsVI. Second LawCurrent LectureI. Spontaneitya. A spontaneous change has a natural tendency to occur; this change doesn’t require work to make it happenb. A nonspontaneous change has no natural tendency to occur. It is brought about only by doing workc. Usually, a process that is spontaneous in one direction is nonspontaneous in the opposite directiond. Why are some changes spontaneous?i. Driving force of spontaneous change is dispersal of matter or energyii. Expansion of gas to fill volume available to itiii. A hot object coolingiv. Isothermal expansion of ideal gas, ΔU=0 but occurs spontaneouslyII. Entropy, Sa. Measure of disorderb. Extensive property (depends on amount of matter present)c. State function (independent of how you got there)i. State functions = capital lettersii. Path functions = lower case letters These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.d. Second law of thermodynamics:i. Entropy of isolated system tends to increase ii. ΔS = qrev/T (units: J/K)iii. Reversible transfer of heat1. Thermal equivalent of mechanical equilibriumiv. Proportional to heat, not work1. Heat associated w/ random motion, increase in disorderv. Inversely proportional to temperature1. Considers random motion already present in the form of thermal energye. Upon heatingi. Recall that q=CΔTii. Then ΔS = CΔT/T which (assuming C is independent of temperature) becomes:1. ΔS=C lnTf/Ti 2. Can reference constant volume or constant pressure3. Consider sign and magnitude of ΔTa. Tf > Ti then ln > 0 and ΔS > 0 versus decreasing temperature, ΔS < 0 (decreasing entropy or disorder)4. Consider magnitude of Cf. ΔS for phase change i. When phase change occurs at transition temperature, at constant pressure, process occurs reversiblyii. ΔS = qrev/T = ΔtransitionH/Tiii. For example, at boiling point1. ΔvapS=ΔvapH/Tb III. Trouton’s Rulea. ΔvapS = ΔvapH/Tbb. ΔvapS = 10.5 R ~87-88 J/mol K for most liquidsc. Can be used to estimate heat of vaporization for liquid whose BP is knownIV. ΔS associated w/ phase changesa. When phase change occurs at some other temperature, Tb. Ex: ΔS when H2O (l) => H2O (g) @310 Ki. Consider reaction occuring in three steps1. Liquid water at body temperature heated to liquid water at boilingpoint (373 K)a. q1=C ln(Tf/Ti)2. Convert liquid to gasa. q2=ΔvapH/Tb3. Cool gas to 310 K (body temperature)a. q1=C ln(Tf/Ti)b. q1=C ln(Tf/Ti)V. ΔS for surroundingsa. ΔSsurr=qsurr,rev/T=qsurr/Tb. Then qsurr = -qsysc. At constant pressure, qsys = ΔrHd. Thereforei. ΔSsurr = -ΔrHsys/Tii. If ΔH < 0, then ΔS > 0 (disorder increases) VI. Second Lawa. In the course of a spontaneous change, ∆Stotal > 0 where Stotal is the entropy of an isolated system that contains the system of interestb. Entropy of isolated system = the