Page 25 - Basic physical chemistry for the atmospheric sciences

Chemical equilibrium 11

Now let us apply LeChatelier's principle to predict the effects on a
chemical reaction of changing the concentrations of any of the species
involved in the reaction. Consider, for example, the reaction between
Hi(g) and Nz(g) to form NH3(g) at high temperature (see Exercise 1.4).
If either H2(g) or N2(g) s added to an equilibrium system more NH 3 (g)
i
will form, since by doing so the concentration of H2(g) or N2(g) will
tend to be returned closer to its original concentration. Alternatively,
if NH3(g) is added to the system, it will tend to decompose into Hz(g)
and Nz(g).
Consider next the effect of changing the volume of a chemical
system. For example, suppose there is a decrease in the volume in
which the equilibrium Reaction (I. I I) occurs but with temperature
remaining constant. This will cause an increase in pressure ; therefore,
the system will react in such a way as to relieve this increase, which
can be accomplished if the number of moles of gas in the system is
reduced. Inspection of Reaction (I. I I) shows that 3 moles of gases on
the left produce 2 moles of gas on the right. Hence, when the volume
of this system is reduced at constant temperature, more S0 (g) is pro­
3
duced.
Finall , Jet us apply LeChatelier's principle to determine the effect
y
of temperature on an equilibrium chemical reaction. Raising the tem­
perature of a system is equivalent to adding heat. Therefore, if the
temperature of a system is raised at constant pressure, the chemical
reaction will proceed in the direction that absorbs heat (i. . , in the
e
direction of the so-called endothermic reaction). For example , the
reverse reaction of Reaction (I. II) is endothermic, but the forward
reaction is exothermic (i.e. , heat is released by the forward reaction).
s
Therefore, the equilibrium h ifts in the forward direction if the temper­
ature is lowered, and in the reverse direction if the temperature is
raised at constant pressure.

Exercises
Answer, interpret, or explain the following in light of the
1.7.
principles presented in this chapter:
(a) Why do chemists prefer to use the mole as a unit of
mass rather than, say, I kg?
(b) A gram -atomic weight can be defined in an analogous
way to a gram-molecular weight. A gram-atomic weight
of any element contains Avogadro's number of atoms of
that element.