Systems at Equilibrium Thermodynamics

The laws of thermodynamics are the cornerstones of any description of a system at equilibrium. The First Law (the energy conservation law) introduces H, the enthalpy, as a state function of a system. State functions depend only on the state of the system and not on how it managed to arrive at that state. 

The Second Law (the entropy law) introduces entropy, S, as a state function. More importantly, this law also describes the equilibrium state of a system as one of maximum entropy. For a system at constant temperature and pressure, the equilibrium condition is expressed as  

the free energy, or the Gibbs free energy, is related to enthalpy and entropy by  

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When the temperature of reactants and products are equal, AG is given by  

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The value of the equUibrium constant can be determined solely from thermochemical data. Additionally, the product of the fiigacities on the right-hand side of Equation (9.16) can be related to a single unknown variable, the extent of reaction, Thus, Equation (9.16) can be solved for it is then straightforward to solve for the composition of species in the system at equilibrium. Thermodynamics tells us the possible extent to which a given reaction will proceed however, the reaction kinetics also plays a role. Even when a reaction is thermodynamically favored, if its rate is very slow, it will not noticeably proceed. The analysis we learned in this chapter tells us nothing about reaction kinetics. 

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