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CHEM 11B (26)
Lecture 24

# CHEM 11B Lecture 24: Chem17 Premium

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School
Brandeis University
Department
Chemistry
Course
CHEM 11B
Professor
Novak Claudia
Semester
Spring

Description
Thermodynamics Spontaneity - Things that are nonspontaneous is not impossible First law of thermodynamics - You can’t win - Law of conservation of energy - Energy is a constant Second law of thermodynamics - You can’t break even - Something pays the price - Lose a “transaction fee” - Entropy - All spontaneous processes are accompanied by an increase in the entropy of the universe. - Some of your energy investment must be lost so that the entropy of the universe increases. - Can’t get as much out as you put in - All spontaneous processes are accompanied by an increase in the entropy of the universe. Third law of thermodynamics - You can’t quit - Perfect crystalline solid at absolute zero is the only thing with zero entropy System, surroundings, universe Enthalpy (ΔH): the measure of heat exchanged under conditions of constant pressure Spontaneous physical processes Phase changes and entropy Increasing entropy: solid, liquid, gas S = k ln W S - entropy K - Boltzmann’s constant: R/NA= 1.38 × 10-23 J/K W - The number of energetically equivalent ways to arrange the components of a system A state in which a given amount of energy is more highly dispersed (or more highly randomized) has more entropy than a state in which the same energy is more highly concentrated. Entropy: Symbols and Signs In thermodynamic calculations, the entropy term is given the symbol S. Entropy is a state function: the change in entropy for a process can be calculated as the final state minus the initial state: - ΔS = Sfinal – Sinitial or - ΔS = Sproducts – Sreactants Increase in entropy: ΔS > 0 Decrease in entropy: ΔS < 0 Calculating the Standard Entropy Change (ΔS°rxn) for a Reaction ΔS°rxn = Σ np S° (products) - Σ nr S° (reactants) The sum of the standard molar entropies of the products, each multiplied by its stoichiometric coefficient (np), minus the sum of the standard molar entropies of the reactants, each multiplied by its stoichiometric coefficient (nr). All spontaneous processes are accompanied by an increase in the entropy of the universe. ΔSuniv = ΔSsys + ΔSsurr an exothermic process increases the entropy of the surroundings (ΔHsys < 0 g ΔSsurr > 0) an endothermic process decreases the entropy of the surroundings (ΔHsys > 0 g ΔSsurr < 0) When water freezes at temperatures below zero °C the entropy of the universe increases, and the process is spontaneous Gibbs free energy: G = H − TS G = free energy ΔGsys = ΔHsys − TΔSsys → the gibbs equation The lower the temperature, the more the entropy of the surroundings is affected by a given amount of heat emitted or absorbed by the system. In order for a
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