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Lecture+16a

# Lecture+16a - Thermodynamics In thermodynamics we divide...

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Thermodynamics In thermodynamics, we divide the universe into a system (the material which are examining and measuring) and surroundings (everything else) The system may undergo some change from some initial state to some final state. We examine well-defined initial and final states. We look at changes between the final and initial states. Such changes typically involve a transfer of energy between system and surroundings. Typical changes examined are: H Change in enthalpy S Change in entropy G Change in free energy. Change always means “final minus initial” If any of the ’s are negative, this means the final state has a lower amount of the quantity measured than the initial state.

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We are most familiar with H, the change in enthalpy. As studied previously, H corresponds to q, the heat transferred to the system from the surroundings, when measured at constant pressure conditions. If q is positive, H is positive. Heat enters the system from the surroundings; the reaction is endothermic. If q is negative, H is negative. Heat leaves the system into the surroundings; the reaction is exothermic. Thermodynamics investigates reaction spontaneity . If we have well-defined initial and final states (states A and B) , will the reaction tend to from A to B or from B to A. Or perhaps not at all? We’ve already analyzed this question with the Q test. If we know the equilibrium constant for the reaction: Q < K Reaction is spontaneous to the right Q > K Reaction is spontaneous to the left Q = K Reaction is at equilibrium As we will be studying in this chapter, we will relate reaction spontaneity, (and indeed, even the value of K), to thermodynamic quantities, H, S, G. It is important to understand that “spontaneous” does not necessarily mean “fast.” A spontaneous reaction is one that will go, sooner or later. The thermodynamic functions, H, S, G, are all state functions . This means that their values depend only on final and initial states, not on the path or process of the change. Thermodynamics does not examine the path or process —this is studied in kinetics, where we have already examined, activation energies and mechanisms, which both relate to process and speed of reaction. Thermodynamics only looks at initial and final states, examines differences between these states ( H, S, G), and then calculates reaction spontaneity (will it go from A to B or from B to A) and also calculates the extent of the reaction (also measured by the equilibrium constant K).
Let’s look at some possible processes, listing initial and final states, and asking: is the reaction spontaneous? Initial State Final State Spontaneous? Ball on top of hill Ball at bottom of hill Yes ( ball rolls down) 4Fe + 3O 2 2Fe 2 O 3 Yes (iron rusts) 2H 2 + O 2 2H 2 O Yes (hydrogen burns) Yes (heat goes from hot to cold) Yes Gas will spread out into available volume H 2 O(s) H 2 O( ) if T > 273 K if T < 273 K if T = 273 K

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