MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Department of Chemistry
5.60 Physical Chemistry
Problem Set #5
Readings: Silbey…., Chapter 5.1, 5.3, 5.45.8
1. Consider the dissociation of N
2
O
4
(g) into NO
2
(g) described by
N
2
O
4
(g) = 2NO
2
(g)
Assuming that we start with
n
0
moles of N
2
O
4
(g) and no NO
2
(g), show that the extent of
reaction,
eq
, at equilibrium is given by
eq
n
0
=
1/2
4
p
p
K
Kp
Plot
eq
/
n
0
against
p
given that
p
K
= 6.1 at 100
o
C. Is your result in accord with Le
Châtelier’s principle?
2
. In Problem 1 you plotted the extent of reaction at equilibrium against the total pressure for
the dissociation of N
2
O
4
(g) to NO
2
(g). You found that
eq
decreases as
p
increases, in
accord with Le Châtelier’s principle. Now let’s introduce
n
inert
moles of an inert gas into the
system. Assuming that we start with
n
0
moles of N
2
O
4
(g) and no NO
2
(g), derive an
expression for
eq
/
n
0
in terms of
p
and the ratio r =
n
inert
/
n
0
. Let
K
P
= 6.1 and plot
eq
/
n
0
verses
p
for
r
= 0,
r
= 0.50,
r
=1.0, and
r
=2.0. Show that introducing an inert gas into the
reaction mixture at constant pressure has the same effect as lowering the pressure. What is
the effect of introducing an inert gas into a reaction system at constant volume (your answer
should be qualitative, no need for a calculation)?
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 Fall '08
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