Chemical Thermodynamics

Closed vs. Open Systems

Chemical equilibrium is a hard state to maintain. Almost anything that changes will alter equilibrium. If the temperature rises or falls, a new equilibrium will have to be found. If energy is put into the system, the temperature will rise. Any chemicals that enter or leave the system will also change equilibrium. Because of these factors, only closed systems reach equilibrium. A closed system is one that is completely isolated from its surroundings. Chemicals in the system are not allowed to leave. New chemicals are not allowed to enter, and no heat can be transferred to or absorbed from the surroundings.

   The earth is an open system. The sun continually transfers energy into the earth’s system, and the amount of energy varies with the time of year. On a smaller scale, the earth’s oceans are open systems. They continually receive new water and chemicals from rivers, and lose water to evaporation. Lakes and ponds are also open systems. Closed systems are very rare.

   The implication is that most chemicals do not ever reach chemical equilibrium. This is why the second law is often stated as follows: all spontaneous processes tend to increase the entropy of the universe. The second law does not state that all spontaneous processes must instantly maximize the entropy of the universe. This is very fortunate for life. It allows the chemicals in life to exist in a state very far from equilibrium. This is why life is possible. This topic will be discussed in the next chapter.

References:

Vemulapalli, Physical Chemistry, Prentice Hall 1993
Energy Flow in Biology, Morowitz, Ox Bow Press, 1979.

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