Page 24 - Basic physical chemistry for the atmospheric sciences

1 0 Basic physical chemistry

Hence the equilibrium constant for this reaction is

2
[S03(g)]
Kc = [S0 (g)] [0z(g)]
2
2
2
Since Kc= . 8 x 1 0 at to00°C, we have
2

Rearranging and simplifying yields
2
-70y 3 + 1 5 3y - ll5y+ 25=0
The reader may verify by substitution that an approximate solution to
this cubic equation is y = 0.37. Hence, the equilibrium concentrations
0
of S0 (g), Oz(g), and S 0 3(g) are approximately . 2 1 M, 0 . 0 57 M, and
2
M
0.89 , respective y .
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1.4 LeChatelier's principle
The way in which a s y stem at equilibrium will respond to an imposed
change can be predicted in a qualitative sense by LeChatelier' s princi­
e
pl , which states that if a system at equilibrium is subjected to a
disturbance that changes any o f the f a ctors that determine its state o f
equilibrium, the system will react in such a way as to minimize (i.e.,
relieve) the ef f e ct of the disturbance.
Before applying LeChatelier's principle to chemical systems, let us
app y it to evaluate the effect of pressure on the melting point of ice.
l
At a pressure of I atm the melting point of ice is 0°C ; under these
conditions liquid water and ice can coexist in equilibrium. According
to LeChatelier's principle, if the pressure is increased the ice-water
system will react in such a way as to tend to relieve the increase in
pressure . Since the specific volume of water is less than that of ice,
this is accomplished by the ice melting. Hence, when the pressure
exceeds l atm, 0°C is no longer a sufficiently low temperature for ice
and water to exist in equilibriu m . In other words, the melting point of
e
s
l
the y stem is lowered by app y ing pressur .