Lecture24 - Chem 162 Lect 24 Spring 2011 THERMODYNAMICS...

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Chem 162, Lect 24 , Spring 2011 THERMODYNAMICS Broadly: Thermodynamics deals with energy changes during chemical reactions and physical processes. It tries to answer questions such as : What is the driving force for a chemical reaction? What makes some reactions(processes) “go” and others not? And how far do they go (equilibrium constants)? What is the relationship between energy and equilibrium constant? Thermodynamics deals with the energy changes occurring from the initial (reactants) to the final (products) state .... doesn’t care so much about how we get to products or how fast we get there. That’s a matter of Kinetics . Thermodynamics determines the equilibrium state of the system. Kinetics determines the pathway to equilibrium. So far, we are familiar with H, the change in enthalpy. As studied previously, H equals q, the heat transferred to the system from the surroundings, when measured at constant P. q P > 0 => H > 0: Heat enters system from the surroundings; endothermic reaction q P < 0 => H < 0: Heat leaves from system into the surroundings; exothermic reaction Spontaneous process(reaction) : A process(reaction) that can occur in a system by itself (no external intervention). 1
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Statement says nothing about the time it may take for the process to occur --- that's a question for kinetics. Thermodynamics can tell us the direction a reaction will take (“left to right” or “right to left” or perhaps “not at all”) .... not when it will do so (kinetics). We have used the Q test to qualitatively analyze this issue: Q > K reaction goes spontaneously to the left Q < K reaction goes spontaneously to the right Q = K reaction is at equilibrium There will be a simple relationship between the value of K and ‘energy’. 2
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Examples of spontaneous processes (reactions) : Initial Final (a) (b) NaCl(s) + H 2 O( ) Na + (aq) + Cl - (aq) + H 2 O( ) (c) C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) (d) 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) (e) 2H 2 (g) + O 2 (g) 2H 2 O( ) or 2H 2 O(g) (f) (g) H 2 O(s) H 2 O( ) goes right if T > 273 K goes left if T < 273 K equilibrium if T = 273 K 3
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