The reaction of hydrogen and oxygen
to form water only happens in one direction. Figure 6.13 illustrates why. It is almost
impossible for water to cross the activation barrier. Many chemical reactions happen in
both directions. Figure 6.14 shows the reaction of a glycine-glycine molecule with water
to yield two glycine molecules. Figure 6.15 shows how this reaction affects entropy.
Notice that the entropy change is very small. The small change in entropy means that the
reaction happens in both directions.
Figure 6.14: The formation of a glycine-glycine
Figure 6.15: Entropy Change Associated with a Peptide Bond
In this case, the energy barrier favors the formation of glycine + glycine, but the
reverse direction also happens as indicated by the reverse arrow in figure 6.14 and 6.15.
The number of available micro-states in this reaction is maximized when there is some
glycine-glycine and quite a bit of free glycine. To satisfy the second law, this chemical
reaction will find the point that maximizes the available micro-states.
The optimal mixture is the one that maximizes the available
micro-states and hence the entropy. Figure 6.15 should be compared to figure 6.13. The
change in entropy is so great in figure 6.13, that the number of available micro-states is
maximized when the universe exists only as water.
Notice that this chapter uses the term available micro-states as
opposed to micro-states. The number of micro-states is a property of a system and its
surroundings, and as such, in many reactions the total number of micro-states does not
change, but as unavailable micro-states become accessible to more atoms and electrons, the
number of available micro-states increases.
At equilibrium the concentration of the chemicals in a system no longer
changes. That is in figure 6.15, the concentration of glycine and glycine-glycine remains
constant once the system reaches chemical equilibrium. The forward and reverse reactions
still take place, but they cancel each other. Thus, no net change is observed.
next: closed vs. open systems
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