Chapter 8 Thermodynamics

Chapter 8 Thermodynamics - L25: Chapter 8: Spontaneous...

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L25: Chapter 8: Spontaneous Processes and Thermodynamic Equilibrium Review Thermochemistry (Ch. 7) - Spontaneity of chemical processes - Entropy (work, heat, probability) - 2nd Law of Thermodynamics - Gibbs Free energy Back to equilibrium (Ch. 9): Equilibrium constant and Free energy Derive the Law of Mass action
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Tornado demo: Hg(NO 3 ) 2 + KI Hg 2+ (aq) + 4I - (aq) <-> HgI 4 2- (aq) K f = 1.0 x 10 30 What is the concentration of Hg 2+ (aq) after 0.78 M of KI is added to 0.010 M Hg 2+ (aq)? Strategy: 1. Because K f is so large, we can reverse the reaction, making K -f = 1.0 x 10 -30 assuring that x will be very small. 2. We can let the reaction first go entirely to creating the complex and then go back to establish equilibrium: [HgI 4 2- ] 0 = 0 + 0.01 M; [Hg 2+ ] 0 = 0.010 – 0.010 M = 0; [4I - ] 0 = 0.78 – 4x0.01 = 0.74 M HgI 4 2- (aq) <-> Hg 2+ (aq) + 4I - aq) I 0.010 0 0.74 C -x x 4x E 0.010 -x x 0.74 +4x
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Tornado demo: Hg(NO 3 ) 2 + KI 1. HgI 4 2- (aq) <-> Hg 2+ (aq) + 4I - aq) E 0.010 -x x 0.74 +4x x = [Hg 2+ ] = 3.3 x 10 -32 M. Assumptions are good. x is so small that it makes sense to assume that the reaction goes first to completion and then re-establishes equilibrium. 44 30 (0.74 4 ) (0.74) 1.0 10 (0.01 ) (0.01) f xx x Kx x + ==
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Kinetics: rates, pathway of rxn Equilibrium: yield of rxn Thermochemistry: energy released Thermodynamics: Can the process happen? Will explain equilibrium Define laws for machines and chemical reactions
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This is an example of perpetuum mobile , a device based on mechanical, chemical, electrical or other physical processes which, when started, will remain in operation forever and provide additional work as well. Only the natural wear of the components will eventually stop its operation.
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First Law of Thermodynamcis: “Energy cannot be created or destroyed.” or E univ =const Internal energy of a system: Δ E=q + w Perpetual motion museum: http://www.lhup.edu/~dsimanek/museum/unwork.htm
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Consider: CH 4 + O 2 -> 2H 2 O + CO 2 - is energy absorbed or released? Thermochemistry : Δ H <-> heat The first law of thermodynamics: conservation of energy. - how fast does it go? Kinetics: E a & mechanisms. Thermodynamics: - can the rxn go at all in a certain direction? - would it be spontaneous under certain conditions? Spontaneous process: can occur by itself without outside intervention once conditions have been established for its initiation. Spontaneous does not mean fast!
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Examples of spontaneous processes: - combustion - diffusion: ink in water - dissolving: salt in soup - gas expansion - ice melting at room T - rock going downhill - heat flowing from hot to cold objects: - rusting of metals
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What is the driving force for spontaneity? Exothermicity (from high to low energy)? - rock downhill -combustion - rusting - cooling But: - melting of ice: endothermic - evaporation: endothermic Entropy increase: an increase of disorder in the Universe
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Different arrangements of 4 molecules By how many ways each arrangement can be achieved? Entropy is related to a number of arrangements available to a system in a given state of energy The larger is the number of ways a particular state of the system can be reached, the larger is the probability to encounter this state
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Each of the possible arrangements is called microstate The sum of all arrangements makes a state There are a total of 16 microstate for 4 particles : (1/2) 4 4 6 4 1
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Chapter 8 Thermodynamics - L25: Chapter 8: Spontaneous...

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