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Chapter I
Chemical Equilibrium
Equilibrium Constant
It is probable that all chemical reactions can take place in both
directions, but in many cases the extent of the reverse reaction is so small asto
be negligible. Such chemical reactions may thus be regarded as proceeding
to completion in one direction. Consider, for example, the reaction between 
two parts of hydrogen and one part of oxygen; explosionof a mixture of these
gases at ordinary temperatures, by means of an electric spark, results in 
complete Conversion into water. There is no detectable residue of the 
reacting gases, provided they were present in the correct proportion. 
Nevertheless, at temperature above about 1500C0
, water vapor is 
decomposed to an appreciable extent into hydrogen and oxygen. The reverse
reaction thus definitely occurs at high temperatures, and it undoubtedly takes
place to some extent under ordinary conditions. Becausethis is so small as to
be virtually undetectable, the combination of hydrogen and oxygen is 
regarded as a reaction which proceeds to completion at normal temperatures 
and pressures.
When the conditions are such that forward and reverse reactions can
both occur to a noticeable extent, the process is described as a reversible
reaction.
If such a process takes place in a closed vessel, so that the products of
the forward reaction do not escape, the reactants cannot combine
completely. As the products accumulate, they tend to react so as to reverse
the process and regenerate the reacting substances; hence the reaction does
not go to completion in either direction. If hydrogen and oxygen were heate
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together in a closed space at a temperature of 2000°C, for example, some of
the reacting gases would remain unchanged, no matter how long the process
was allowed to continue. Similarly, to take the case of a reaction which is
appreciably reversible at much lower temperature, a mixture of hydrogen
gas and iodine vapor will not unit completely to form hydrogen iodide if
heated in a closed vessel at 450°C. In the vicinity of this temperature
hydrogen and iodine react to yield hydrogen iodide, but the latter
decomposes to an appreciable extent into its constituent elements; the
reversible nature of the reaction is indicated by writing.
Finally, reference nay be made to a reversible process occurring in a liquid
phase at ordinary temperatures; this is the esterification reaction between
ethanol and acetic acid leading to the formation of ethyl acetate and water.
If equal molecular
amount of the reactants is taken; the action apparently ceases when only
two-thirds of the initial amounts of alcohol and acetic acid have been used
up.
If has been found that after the lapse of a sufficient Interval of time all
reversible reactions reach a state of chemical equilibrium, that is a state in
which no further change in the composition with time can be detected
provided the temperature and pressure are not altered. If the conditions are
properly chosen, exactly the same state of equilibrium may be attained from
either direction for a given reversible reaction. For example, at a
temperature of 425°C, the equilibrium mixture consists of 12 molecular
percent of hydrogen 12 percent of iodine vapor and 76 percent of hydrogen
iodide irrespective of whether the starting point is hydrogen iodide or an
equivalent mixture of hydrogen and iodine