You will learn some of the terminology that is used in this chapter. It is very important for you to refresh your mind on the topics in Chapter 6.
We will use the term spontaneous to mean something that occurs without any outside (out of the system) intervention. Thus a reaction could spontaneously proceed either toward products or toward reactants. No speed of event occurrence is implied by the use of the term spontaneous. Thermodynamics does not have time as a variable. [Kinetics allows us to introduce time as a variable.] It is also important to remember that if we say that a reaction proceeds spontaneously in the product direction (or product-favored reaction), that does not mean that the reaction only proceeds in that direction. Similarly if we say that a reaction proceeds spontaneously in the reactant direction (or reactant-favored reaction), that does not mean that no products are formed. Remember that very few reactions actually proceed in only one direction.
We know that H2O does not spontaneously change into H2 and O2. We have to do work on a system to cause a nonspontaneous change to occur.
This use of the term spontaneous is also consistent with the manner in which it is used by the medical profession. You may be told that you had a spontaneous hemorrhage, which means that there was no (apparent) outside influence that caused the hemorrhage. You may also be told that you have a spontaneous remission of some disease, and that would mean that the disease reversed its direction without outside intervention.
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