Chem 1120 1st Edition Lecture 26 Outline of Last Lecture I Crystal Field Splitting and Magnetism a Ligand Field Stabilization II Tetrahedral Square Planar and Octahedral Complexes III Complex Color and Spectroscopy Outline of Current Lecture I Intro to Entropy Spontaneity II Spontaneity and Reversibility III Entropy and the Second Law of Thermodynamics Current Lecture I When a chemist is considering an unfamiliar chemical reaction they focus on two questions 1 Will the reaction yield a significant quantity of products K very large products favored K very small reactants favored 2 Will the reaction proceed at a reasonable rate K very large fast reaction K very small slow reaction These two questions focus on energy and entropy 1st Law of Thermodynamics energy change in E q w 2nd Law of Thermodynamics entropy change in S Larger S more spontaneous process Spontaneous process a chemical or physical change that is thermodynamically favored by nature 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 Physical example A ball going down a hill is more thermodynamically favorable therefore spontaneous whereas a ball going uphill is not favorable and therefore non spontaneous Chemical example Spontaneous hydrogen and oxygen being put together to form water Nonspontaneous water being broken down into oxygen and hydrogen A spontaneous reaction need not proceed at an observable rate spontaneous does not always mean fast for example iron to rust is very slow yet spontaneous II Spontaneity is a thermodynamics issue vs reaction rate which is a chemical kinetics issue Spontaneity can be temperature dependent Reversibility usually we think of a reversible process as being a process that can proceed in either direction but thermodynamics has a much more strict definition Reversible Process a process that can be reversed by an infinitesimal change in a variable Example At 0oC liquid water and ice are in equilibrium adding an infinitesimal amount of heat will melt a tiny amount of ice removing an infinitesimal amount of heat will freeze a tiny amount of water reversing the process returns the system and the surroundings to their original state no real world process is truly reversible since it would require an infinite amount of time only ideal processes can result in reversible changes in a system Irreversible Process a process which is not a reversible process Irreversible does not mean the process cannot be made to proceed in the opposite direction reversing an irreversible process can only return the system to its original state not the surroundings At equilibrium reactants and products can interconvert by a reversible process but at equilibrium no net change occurs For any spontaneous process the interconversion must be by an irreversible process Irreversible Process III Many spontaneous reactions are exothermic change in H is negative which means they lower a system s energy but not all work this way and some end up being endothermic change in H is positive The Second Law of Thermodynamics For any spontaneous process the entropy S of the universe must increase For an isolated system Ssurroundings 0 and S universe 0 Entropy can be defined in more than one way the most accurate definitions often do little to help you understand it Entropy a measure of the disorder of a system Entropy a measure of the extent to which a system s energy is not available for work Entropy a measure of the dispersal of energy Entropy S k ln W Entropy S qrev T Entropy is a state function so the final minus the initial will give the overall change Entropy as disorder entropy S a measure of the randomness or disorder of a system Example For the following predict whether the change in entropy is or a gaseous water liquid water b N2 2N gas change in S 0 less disorder change in S 0 more moles of highly disordered