Chem 102 Module 1: Entropy and Free Energy
Chem 102 Module 1: Entropy and Free Energy Chem 102
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This 3 page Class Notes was uploaded by Lyna Nguyen on Tuesday February 9, 2016. The Class Notes belongs to Chem 102 at Texas A&M University taught by Dr. Bethel in Spring 2016. Since its upload, it has received 31 views. For similar materials see General Chemistry 2 in Chemistry at Texas A&M University.
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Date Created: 02/09/16
Module 1: Entropy and Free Energy (18.1 – 18.6) 18.1: Spontaneity and Energy Transfer as Heat Common Spontaneous Changes that are endothermic or energy neutral o Solubility: dissolving NH H4 3 o Diffusion: expansion of gas into a vacuum o Phase Changes Temperature plays a part in spontaneity o Energy Transfer as heat Energy transfer from a hotter object to a cooler object is spontaneous o Chemical Reactions Equilibrium occurs spontaneously both directions Evolution of heat cannot be a sufficient criteria of spontaneity 18.2: Dispersal of Energy: Entropy In a spontaneous process, energy goes from being more concentrated to being more dispersed Entropy: allows quantification of dispersal of energy o Simply put: amount of disorder o State function o Second Law of Thermodynamics: a spontaneous process is one that results in an increase of the entropy of the universe o ΔS (universe) > 0 o ΔS (universe) = ΔS(system) + ΔS(surroundings) Energy dispersal is inversely proportional to the temperature o ΔS = q/T Q: reversible conditions; chemical equilibrium T: temperature in Kelvin ΔS: J/mol-K Entropy increases with increasing temperature 18.3: Entropy: A microscopic Understanding In a spontaneous process, energy changes from being localized or concentrated to being more dispersed or spread out In a spontaneous process, the change in entropy(ΔS) of the universe indicates the extent to which energy is dispersed Dispersal of Energy o Microstate: ways energy is distributed Another meaning: specific configuration If # of particles increase, # microstates increase More in liquids than solids o Increase in microstates, increase in entropy of system S = k*ln(W) W: accessible microstates K: Boltzmann’s constant Dispersal of Matter: Dispersal of Energy Revisited o Consequence of larger volume in gas expansion, increase in # of microstates, and those microstates are even more closely spaced Summary: Entropy, Entropy Change, and Energy Dispersal o ΔS = S final Sinital*ln(W /f) i 18.4: Entropy Measurement and Values Third Law of Thermodynamics: a perfect crystal at 0 K has zero entropy; S = 0 All substances have positive entropy values at temps >0K Entropy directly related to energy added as heat o Raising temp -> increase in entropy o Large input heat in phase change -> large increase entropy Standard Entropy Values, S o o o Standard molar entropy, S : entropy gained by converting 1 mol of it from a perfect crystal at 0K to standard state conditions (1 bar, 1 molal for a solution) Unites: J/ K-mol o Large molecules have larger entropies than small molecules o Molecules with more complex structures have larger entropies than molecules with simpler structures o Gas entropy > aqueous entropy > liquid entropy > solid entropy o For a given substance, a large increase in entropy accompanies change of state o Entropy is higher for: Higher temperatures Larger volumes More complex compounds Increase in moles of gas Heavier atoms Larger sample size Determining entropy changes in physical and chemical process o ΔS =rproducts – reactants For a system 18.5: Entropy Changes and Spontaneity Can be positive/negative ΔH(system) = -ΔH(surr) Summary predictions Reaction Type ΔH(system) ΔS(system) Spontaneous? Module 1 Page 2 1 Exo, <0 +, >0 @ all temps 2 Exo, <0 -, <0 @ low temps 3 Endo, >0 +, >0 @ high temps 4 Endo, >0 -, <0 none 18.6: Gibbs Free Energy G = H – TS o H: enthalpy o S: entropy o T: kelvin Free energy is a state function Change in the Gibbs Free Energy, ΔG o ΔG(system) = ΔH – TΔS Gibbs Free Energy, Spontaneity and Chemical Equilibrium o ΔG is related directly to ΔS(universe) o Can be used as criterion of spontaneity for physical and chemical changes o ΔG = G(products) – G(reactants) Units: kJ/mol o Free energy at equilibrium, where there is a mixture of reactants and products, is always lower than the free energy of the pure reactants and products. A reaction proceeds spontaneously toward the minimum free energy o Relationship between standard conditions and nonstandard ΔG = ΔG +RTlnQ A summary o Free energy decreases to a minimum as a system approaches equilibrium o When ΔG <0, proceeds spontaneity toward equilibrium and Q < K o When ΔG > 0, not spontaneous and Q > K o When ΔG = 0, @ equilibrium Q = K What is free energy? o Represents the maximum energy available to do useful work o “free” = available Free Energy depends on: o ΔH – enthalpy o TΔS – entropy If ΔH > TΔS, enthalpy favored If TΔS > ΔH, entropy favored Module 1 Page 3
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