UNC-Chapel Hill CHEM 480 - Chapter 2 (3 pages)

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CHEM 480 1nd Edition Lecture 8 Outline of Last Lecture I Chemical Changes II Energy from biological fuels III Energy is Dispersed IV Heat of formation fH V Calculate average C H bond enthalpy in CH4 given heats of formation VI Potassium metal added to water Outline of Current Lecture I Spontaneity II Entropy S III Trouton s Rule IV S associated w phase changes V S for surroundings VI Second Law Current Lecture I Spontaneity a A spontaneous change has a natural tendency to occur this change doesn t require work to make it happen b A nonspontaneous change has no natural tendency to occur It is brought about only by doing work c Usually a process that is spontaneous in one direction is nonspontaneous in the opposite direction d Why are some changes spontaneous i Driving force of spontaneous change is dispersal of matter or energy ii Expansion of gas to fill volume available to it iii A hot object cooling iv Isothermal expansion of ideal gas U 0 but occurs spontaneously II Entropy S a Measure of disorder b Extensive property depends on amount of matter present c State function independent of how you got there i State functions capital letters ii 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 III IV V d Second law of thermodynamics i Entropy of isolated system tends to increase ii S qrev T units J K iii Reversible transfer of heat 1 Thermal equivalent of mechanical equilibrium iv Proportional to heat not work 1 Heat associated w random motion increase in disorder v Inversely proportional to temperature 1 Considers random motion already present in the form of thermal energy e Upon heating i Recall that q C T ii Then S C T T which assuming C is independent of temperature becomes 1 S C lnTf Ti 2 Can reference constant volume or constant pressure 3 Consider sign and magnitude of T a Tf Ti then ln 0 and S 0 versus decreasing temperature S