Chem 125
Quiz #4 Calvin
Dr. Retsek
Spring 2008
NAME:
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KEY
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Computer:
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1. (10 points)
At 25°C, K
c
for the following reaction is 4.0 x 10
7
.
N
2
O
4
(g)
⇔
2 NO
2
(g)
In an experiment, 1.5 moles of N
2
O
4
are placed in a 10.0L flask.
Calculate the
concentrations of N
2
O
4
and NO
2
when this reaction reaches equilibrium.
[N
2
O
4
] = 1.5 moles/10.0 L = 0.15 M
Need “ICE” table to find out what is happening at equilibrium
N
2
O
4
⇔
2 NO
2
Initial
0.15
0
Change
 x
+ 2 x (since 2 moles in balanced equation)
Equilibrium 0.15 – x
2x
K
c
= 4.0 x 10
7
= [NO
2
]
2
/[N
2
O
4
] = [2x]
2
/[0.15 – x]
Since K is so small, there will be very little product (x will be small).
Assume that
0.15 – x
≈
0.15
K
c
= 4.0 x 10
7
= [2x]
2
/[0.15]
x = 1.2 x 10
4
Check assumption:
K’ = (2 x 1.2 x 10
4
)
2
/(0.15 – 1.2 x 10
4
) = 3.84 x 10
7
Within 5%? K’/K = (3.84 x 10
7
/4.0 x 10
7
) x 100 = 96% (assumption ok!)
[N
2
O
4
]
equilibrium
= 0.15 – x = 0.15 – 1.2 x 10
4
= 0.15 M
[NO
2
]
equilbrium
= 2x = 2(1.2 x 10
4
) = 2.4 x 10
4
M
2. (5 points)
In which direction (if any) will the position of the equilibrium for
2 HI (g)
⇔
H
2
(g) + I
2
(s)
∆
H = 100 kJ
be shifted for each of the following changes?
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 Spring '08
 Retsek
 Mole, Reaction, Chemical reaction, Dr. Retsek

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