Independent and Dependent Thermodynamic Properties

When the temperature is too high or the pressure too low, all molecules will eventually escape to the vapor and only that phase will exist at equilibrium. On the other hand, if the temperature is too low or the pressure too high, only liquid will exist. For a pure species, the dynamic process at which the rate of molecules that vaporize equals the rate at which they condense occurs at a unique pressure for a given temperature and is called the saturation pressure, As the temperature increases, more molecules enter the vapor phase, and the saturation pressure increases. Since the fraction of species at a given kinetic energy depends exponentially on temperature, the saturation pressure increases exponentially with temperature. 

Similarly, chemical reaction equilibrium represents a dynamic process on the molecular scale. Macroscopically, a reaction can proceed in the forward direction from reactants to products or in the reverse direction from products to reactants. A given reaction is said to be at chemical reaction equilibrium when there is no net reaction in either direction. However, again there is a dynamic process on a molecular scale. Reactant molecules will react to form products at the same rate that the product molecules form reactants. If we followed an individual molecule, it might indeed react. However, for each molecule that reacts in the forward direction, another molecule will be reacting in the reverse direction. On the other hand, if an excess of reactants is present, there will be a net macroscopic reaction in ihe forward direction, since more individual molecules will react in this direction than in the reverse direction. Reaction will occur until equilibrium is reached and there is no more tendency to react on a macroscopic scale. Conversely, if an excess of products is present, macroscopic reaction will occur in the reverse direction until the same equilibrium state is reached. 

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INDEPENDENT AND DEPENDENT THERMODYNAMIC PROPERTIES The State Postulate... 

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