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1aa3-2017-tut2-a.pdf-CHEMISTRY 1AA3...
1aa3-2017-tut2-a.pdf-CHEMISTRY 1AA3 Week of JANUARY 16,
##### Page 5
5
Again, for
NH
4
+
+
H
2
O
NH
3
+
H
3
O
+
pH
= p
K
a
+
log ([NH
3
]/[NH
4
+
])
8.74 = 9.255
2
+ log ([0.30]/[Y])
and we assume x is small
- 0.5152 = log ([0.30]/[Y])
This solves to give Y = 0.9825 M. When multiplied by volume, this also gives us 1.4
74
mol
of NH
4
Br, which will lead to the same mass value.
Any of these three approaches works, and with sufficient extra digits in our calculation will
Kinetics
- Chapter 14
6.
Consider the reaction:
N
2
(g) + 3 H
2
(g)
à
2 NH
3
(g)
(a)
Write expressions for the rate of the reaction in terms of the changes of
concentrations of N
2
, H
2
, and NH
3
.
(b)
Suppose that at some moment the rate of consumption of H
2
is 0.074 M
-1
s
-1
.
What is the rate at which N
2
is being consumed?
SOLUTION:
(a)
Reaction rates are defined in terms of the reaction stoichiometry,
as written
.
rate
=
Δ
[N
2
]
=
1
Δ
[H
2
]
=
+1
Δ
[NH
3
]
Δ
t
3
Δ
t
2
Δ
t
(b)
Rate of consumption or production is defined in terms of the specified reactant or
product.
-
Δ
[H
2
] =
0.074 M
1
s
1
= 3 (-
Δ
[N
2
] )
Δ
t
Δ
t
-
Δ
[N
2
]
=
-1
Δ
[H
2
]
= 1 (0.074 M
1
s
1
)
=
0.025 M
1
s
1
Δ
t
3
Δ
t
3

##### Page 6
6
7.
Consider the reaction:
2 NO(g) + O
2
(g)
2 NO
2
(g)
(a)
Write expressions for the rate of the reaction in terms of the changes of
concentrations of NO, O
2
, and NO
2
.
(b)
Suppose that at a particular moment the rate of reaction with respect to NO is
0.066 M s
-1
. What is the rate at which O
2
is being consumed?
SOLUTION:
(a)
rate
=
1
Δ
[NO]
=
−Δ
[O
2
]
= +1
Δ
[NO
2
]
2
Δ
t
Δ
t
2
Δ
t
(b)
Δ
[NO] =
0.066 M
1
s
1
Δ
t
1
Δ
[NO]
=
−Δ
[O
2
]
2
Δ
t
Δ
t
Δ
[O
2
]
=
(0.066 M s
1
) =
0.033M s
1
Δ
t
2
8.
What are the overall orders of the reactions to which the following rate laws apply?
(a)
rate = k[Cl
2
][H
2
]
(b)
rate = k[NO
2
]
2
[O
2
]
(c)
rate = k[H
2
]
½
[C
2
H
4
]
SOLUTION:
The overall order of a reaction comes from the sum of the exponents in the rate law,
i.e.,
for
rate = k[A]
x
[B]
y
overall order = x + y
(a)
1 + 1 =
2
(b)
2 + 1 =
3
(c)
0.5 + 1 =
1.5
9.
The rate for the reaction:
NH
4
+
(aq) + NO
2
-
(aq)
N
2
(g) + 2 H
2
O (l)

##### Page 7
7
is given by the rate law:
rate = k[NH
4
+
][ NO
2
-
]. At 25
°
C, the rate constant for the
reaction is 3.0
×
10
-4
M
-1
s
-1
. Calculate the rate of the reaction at 25
°
C if [NH
4
+
] = 0.26
M and [NO
2
-
] = 0.080 M.
SOLUTION:
rate = k [NH
4
+
][ NO
2
]
and
k = 3.0
×
10
4
M
1
s
1
rate
=
(3.0
×
10
4
M
1
s
1
)(0.26 M)(0.080 M)
rate
=
6.24
×
10
6
M/s
( =
Δ
[N
2
]
/
Δ
t )
10.
The rate law for the reaction:
F
2
(g) + 2 ClO
2
(g)
2 FClO
2
(g)
is
rate = k [F
2
][ClO
2
]. At 25
°
C, the rate constant for the reaction is 1.2 M
1
s
1
.
Calculate the rate of the reaction at 25
°
C if [F
2
] = 0.010 M and [ClO
2
] = 0.020 M.
SOLUTION:
rate = k [F
2
][ClO
2
]
and
k = 1.2 M
1
s
1
rate
=
(1.2 M
1
s
1
)(0.010 M)(0.020 M)
rate
=
2.4
×
10
4
M/s

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