Page 15 - Basic physical chemistry for the atmospheric sciences

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Chemical equilibrium

One of the major goals of chemistry is to predict what will happen
when various substances come into contact. Will a chemical reaction
occur, or will the substances just exist side by side? One to way to
approach this problem is through the concept of chemical equilibrium,
which is the focus of this chapter.

1.1 Some introductory concepts
In a balanced equation for a chemical reaction, there are the same
number of atoms of each element on the left side of the equation as
there are on the right side. For example, the balanced equation for the
chemical reaction representing photosynthesis is 1
6C02(g) + 6H20(1)­
C H 2 06(s) + 60z(g) ( l . l a )
6 1
In a balanced chemical equation (which we will often call a reaction),
the relative numbers of the molecules involved in the reaction are
given by the numerical coefficients preceding the chemical symbol for
l
the molecule. Thus, Reaction (I. a ) indicates that six molecules of
carbon dioxide, COz(g), react with six molecules of water, H20(1),
to form one molecule of glucose, C H 20 (s), and six molecules of
6
6 1
oxygen , 02(g)
6 molecules of COz(g) + 6 molecules of H20(1)-
1 molecule of C H1206(s) + 6 molecules of 02(g) ( l . l b )
6
Reaction ( 1. 1 a) does not necessarily mean that if six molecules of
C0 (g) are mixed with six molecules of HzO(l) they will react com­
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pletely and produce one molecule of C6H 20 (s) and six molecules of
6
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