spano velasco (ds28282) – hw 09 – Fakhreddine – (52395)
1
This
printout
should
have
13
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
001
1.0 points
Under certain conditions, the reaction
3 A + 2 B
→
4 C
was
observed
to
proceed
at
a
rate
of
0
.
00199 M
·
s

1
. What was the corresponding
rate of change in reactant A?
1.
0
.
00398 M
·
s

1
2.

0
.
000663333 M
·
s

1
3.

0
.
00398 M
·
s

1
4.
0
.
00597 M
·
s

1
5.
0
.
000663333 M
·
s

1
6.

0
.
00597 M
·
s

1
correct
7.
0
.
000995 M
·
s

1
8.
0
.
00199 M
·
s

1
9.

0
.
00199 M
·
s

1
10.

0
.
000995 M
·
s

1
Explanation:
Reactant A is reacting at 3 times the stated
rate of the overall reaction. A is also a reac
tant, so the concentration is DECREASING
with time, which corresponds to a NEGA
TIVE rate.
002
1.0 points
The reaction
N
2
+ 3 H
2
→
2 NH
3
is proceeding under conditions that 0.150
moles of NH
3
are being formed every 20 sec
onds.
What is the rate of disappearance of
H
2
?
1.
1
.
125
×
10

2
moles/sec
correct
2.
The question cannot be answered from
the information given; we need to know the
volume of the container.
3.
The question cannot be answered from
the information given; we need to know the
rate law for the reaction.
4.
2
.
25
×
10

1
moles/sec
5.
7
.
5
×
10

3
moles/sec
Explanation:
n
NH
3
= 0.150 mol
t
= 20 s
The rate of appearance of NH
3
is
Rate =
1
2
Δ[NH
3
]
Δ
t
We can consider this to be
Rate =
1
2
·
0
.
150 mol
20 s
= 0
.
00375 mol
/
s
The rate of disappearance of H
2
is
Rate =

1
3
·
Δ[H
2
]
Δ
t
0
.
00375 mol
/
s =

1
3
·
Δ[H
2
]
Δ
t
Δ[H
2
]
Δ
t
=

0
.
01125 mol
/
s
Thus H
2
disappears at a rate of 1
.
125
×
10

2
mol/s.
003
1.0 points
What is the rate of the reaction
4 NH
3
+ 5 O
2
→
4 NO + 6 H
2
O
when 0.0500 mol/L of NH
3
is being consumed
per second?
1.

0.0125 M
·
sec

1
2.
0.200 M
·
sec

1
3.

0.200 M
·
sec

1
4.
0.0125 M
·
sec

1
correct
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spano velasco (ds28282) – hw 09 – Fakhreddine – (52395)
2
5.
One of the other answers has the right
number but the units are wrong.
Explanation:
Rate =

1
4
Δ[NH
3
]
Δ
t
parenleftbigg
=

1
5
Δ[O
2
]
Δ
t
=
1
4
Δ[NO]
Δ
t
=
1
6
Δ[H
2
O]
Δ
t
parenrightbigg
=

1
4
(

0
.
0500 M)
1
.
0 s
= 0
.
0125 M
·
s

1
004
1.0 points
The rate of the reaction
2 O
3
→
3 O
2
is equal to
1.

Δ[O
3
]
Δ
t
.
2.
+
1
2
Δ[O
3
]
Δ
t
.
3.

1
3
Δ[O
2
]
Δ
t
.
4.

1
2
Δ[O
3
]
Δ
t
.
correct
5.

Δ[O
2
]
Δ
t
.
Explanation:
The rate of a chemical reaction can either
be expressed as the rate of disappearance of
a reactant (as in this case) or as the rate
of appearance of a product.
However, this
rate is multiplied by the inverse of the coef
ficient of the species in question.
The nega
tive sign here reminds us that as the reaction
progresses, this species is consumed and de
creases in concentration.
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 Spring '09
 Fakhreddine
 Reaction, Chemical reaction, Rate equation, spano velasco

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