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Spontaneity, Entropy, and Free Energy

Spontaneous Processes and Entropy

Thermodynamics lets us predict whether a process will occur, but gives no information about the amount of time required for the process. A spontaneous process is one that occurs without outside intervention.  For entropy, the driving force for a spontaneous process is an increase in the entropy for the universe.  Entropy, S, can be viewed as a measure of randomness, or disorder. 

Entropy and the second law of thermodynamics

The second law of thermodynamics states that for any spontaneous process there is always an increase in the entropy of the universe.  Using entropy, thermodynamics predicts the direction in which a process will occur.  However, it cannot predict the rate at which the process will occur; this must be done by using the principles of kinetics.

The effect on temperature on spontaneity

For a process at constant temperature and pressure, DSsys is dominated by changes in positional entropy, and DSsurr is determined by heat flow: DSsurr = -DH/T.  The sign of DSsurr depends on the direction of the heat flow: DSsurr positive for an exothermic process and negative for an endothermic process.  The magnitude of DSsurr however, depends on both the quantity of energy that flows as heat and the temperature at which the energy is transferred.  This the significance of exothermicity as a driving force for a process depends on the temperature at which the process occurs.

free energy and chemical reactions

For a chemical reaction, entropy changes in the system are dominated by the change in the number of gaseous molecules.  Fewer gaseous molecules on the product side means a decrease in entropy, and vice versa.  Molecular stricture also plays one.

free energy and equilibrium

The equilibrium position for a chemical reaction occurs where the free energy is the minimum value to the system.  The relationship between DGand the equilibrium constant is DG = -RT ln(K). 

free energy and work

The maximum possible useful work obtainable from a process at constant temperature and pressure is equal to the change in free energy: Wmax = DG.  In any real process, the actual work obtained is less than Wmax.  When energy is used to do work the total energy of the universe remains constant, but it is rendered less useful.  After being used to do world, concentrated energy is spread out in the surroundings as thermal energy, and this is the crux of our energy problem.