chap19_bch1049_07MCWC3a

chap19_bch1049_07MCWC3a - Chemical Thermodynamics (Chapter...

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1 Chemical Thermodynamics Chemical Thermodynamics (Chapter 19) What’s Ahead: 1. Spontaneous Processes and Entropy Spontaneous Processes and Entropy 2. Second and Third Laws of Thermodynamics Second and Third Laws of Thermodynamics 3. 3. Gibbs Free Energy Gibbs Free Energy 4. Relationship of Changes in Free Energy, Enthalpy, Entropy Relationship of Changes in Free Energy, Enthalpy, Entropy and Temperature and Temperature Chemical Thermodynamics Reminder: First Law of Thermodynamics • Energy cannot be created nor destroyed. • Therefore, the total energy of the universe is a constant. • Energy can, however, be converted from one form to another or transferred from a system to the surroundings and vice versa.
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2 Chemical Thermodynamics Spontaneous Processes • Spontaneous processes are those that can proceed without any outside intervention. • The gas molecules in vessel B will spontaneously effuse into vessel A , but once the gas is in both vessels, it will not spontaneously move back into B. Processes that are spontaneous in one direction are non-spontaneous in the reverse direction. Chemical Thermodynamics Spontaneous Processes and Temperature • Processes that are spontaneous at one temperature may be non-spontaneous at other temperatures. • Above 0 ° C it is spontaneous for ice to melt. • Below 0 ° C the reverse process is spontaneous.
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3 Chemical Thermodynamics Reversible Processes In a reversible process the system changes in such a way that the system and surroundings can be returned to their original states by exactly reversing the process. Chemical Thermodynamics Irreversible Processes • Irreversible process: work by surroundings must be done to reverse the change to the system. • Spontaneous processes are irreversible. Spontaneous (irreversible) expansion Restoring system to original state after irreversible process changes surroundings.
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4 Chemical Thermodynamics For a isothermal process (occurring at constant temp.), the change in entropy is equal to the heat that would be transferred if the process was reversible, divided by the temperature: Entropy Entropy ( S ) is ratio of heat given and the associated temperature: q T Entropy can be thought of as a measure of the randomness or disorder of a system. It is related to the various modes/degree of motion in molecules. Like total energy E , and enthalpy H , entropy is a state function. • Therefore, Δ S = S final S initial Δ S = q rev T Chemical Thermodynamics In other words: For reversible processes: Δ S univ = Δ S system + Δ S surroundings = 0 For irreversible processes: Δ S univ = Δ S system + Δ S surroundings > 0 Second Law of Thermodynamics The entropy of the universe increases for spontaneous (irreversible) processes, and the entropy of the universe does not change for reversible processes.
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This note was uploaded on 01/11/2011 for the course BCH 1049 taught by Professor Michaelcwchan during the Spring '06 term at City University of Hong Kong.

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chap19_bch1049_07MCWC3a - Chemical Thermodynamics (Chapter...

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