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UI CHEM 1120 - Entropy and Spontaneity
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Chem 1120 1st Edition Lecture 26Outline of Last Lecture I. Crystal Field Splitting and Magnetisma. Ligand Field StabilizationII. Tetrahedral, Square Planar, and Octahedral ComplexesIII.Complex Color and SpectroscopyOutline of Current Lecture I. Intro to Entropy/SpontaneityII. Spontaneity and ReversibilityIII.Entropy and the Second Law of ThermodynamicsCurrent LectureI. 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 favoredK very small ——> reactants favored2. Will the reaction proceed at a reasonable rate?K very large ——> fast reactionK very small ——> slow reactionThese two questions focus on energy and entropy,1st Law of Thermodynamics = energy change in E = q + w2nd Law of Thermodynamics = entropy change in S = ???Larger S = more spontaneous processSpontaneous process = a chemical or physical change that is thermodynamically favored by natureThese 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 favorableand therefore non spontaneous Chemical example: Spontaneous = hydrogen and oxygen being put together to form water, Nonspontaneous = water being broken down into oxygen and hydrogenA 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 issueSpontaneity 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 definitionReversible Process = a process that can be reversed by an infinitesimalchange in a variableExample: 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 systemIrreversible Process = a process which is not a reversible processIrreversible 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 surroundingsAt equilibrium, reactants and products can interconvert by a reversible process, but at equilibrium no net change occursFor any spontaneous process, the interconversion must be by an irreversible processIrreversible 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 > 0Entropy can be defined in more than one way, the most accurate definitions often do little to help you understand itEntropy = a measure of the disorder of a systemEntropy = a measure of the extent to which a system’s energy is not available for workEntropy = a measure of the dispersal of energyEntropy = S = k*ln(W)Entropy = S = (qrev)/TEntropy is a state function, so the final minus the initial will give the overall changeEntropy as disorder: entropy = S = a measure of the randomness or disorder of a systemExample: For the following, predict whether the change in entropy is + or -a) gaseous water —> liquid water change in S < 0 , - , less disorderb) N2 —> 2N change in S > 0 , + , more moles of highly disordered


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UI CHEM 1120 - Entropy and Spontaneity

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