M1 Entropy & Gibbs Free Energy

M1 Entropy & Gibbs Free Energy - S olomon’s Study...

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Unformatted text preview: S olomon’s Study Notes General Chemistry II Spring 2010 Thermodynamics Solomon Weiskop PhD [ Entropy & Gibbs Free Energy ] Thermodynamics (2) Study Notes & Practice Problems are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD © Copyright 2010 1 1. Spontaneous vs. Non-Spontaneous For specified conditions of T, P and [ ] (temperature, pressure and concentration) every process (chemical or physical) has a “preferred” direction called the “spontaneous” direction. Consider a generic chemical reaction For specified conditions (of T P [ ]) one direction will be Spontaneous, say For the same specified conditions, the reverse direction will then necessarily be Non-Spontaneous Spontaneous means the process will “ go ” on its own (unless it is explicitly blocked from going). Non-Spontaneous means the process will not “ go ” on its own (unless it is explicitly forced to go) As an example, under standard conditions (T=298°K, P=1 atm) ?? ? ¡¢ + £ ? ¡¢ → ?? ? £ ( ) is Spontaneous whereas, for these same conditions, the reverse reaction ?? ? £ ¢ → ?? ? ¡¢ + £ ? ( ¡ ) is Non-Spontaneous Note: Saying that a process is spontaneous only means that the process will “go”. It says nothing about how fast or slow the process goes (i.e. the rate). Whereas spontaneity is determined by Thermodynamics (as will be explained here in these Notes) reaction rates, on the other hand, are determined by other issues (as will be explained later in this course when we discuss Kinetics .) 2 The idea of Spontaneous vs. Non-Spontaneous directions applies to physical processes too: In a fridge at – 15°C Eq.(1) H 2 O ( at 0°C) → H 2 O ( at 0°C) is Spontaneous Eq.(2) H 2 O ( at 0°C) → H 2 O ( at 0°C) is Non-Spontaneous In a room at 25°C Eq.(2) H 2 O ( at 0°C) → H 2 O ( at 0°C) is Spontaneous Eq.(1) H 2 O ( at 0°C) → H 2 O ( at 0°C) is Non-Spontaneous So it is clear that changing the conditions can change which direction is Spontaneous and which is Non-Spontaneous. Note that H 2 O ( at 0°C) ????? ?? −?¡℃ ¢¢¢¢¢¢¢¢¢£ H 2 O ( at 0°C) is Exothermic ( ΔH neg) whereas H 2 O ( at 0°C) ?¤¤? ?? ¥¡℃ ¢¢¢¢¢¢¢£ H 2 O ( at 0°C) is Endothermic ( ΔH pos) yet both are Spontaneous. So clearly ΔH (Enthalpy change) does not determine Spontaneity (although we will later see that it plays a role). What then does determine Spontaneity? ΔS (Entropy change) Terminology: For a reaction Reactants → Products “Product- Favored” is equivalent to “Spontaneous” “Reactant- Favored” is equivalent to “Not Spontaneous” Th ese ‘equivalences’ are not really true. Product/Reactant Favored and Spontaneous/Not Spontaneous are not interchangeable. But it is what your textbook says, so I’ll go with it too. 3 Entropy (S) unit: J/°K Entropy is a thermodynamic state function that is a measure of the disorder (or randomness) of a system...
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This note was uploaded on 11/14/2010 for the course CHE 131 taught by Professor Kerber during the Spring '08 term at SUNY Stony Brook.

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M1 Entropy & Gibbs Free Energy - S olomon’s Study...

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