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Topic 10: Equilibrium I
10.3 Equilibrium Constant (Kc)
10.3.1 Expression for Kc
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What is the first step in calculating
K
c
K_{c}
K
c
?
Write the balanced equation
The stoichiometric coefficients in a balanced equation represent the
molar ratios
between reactants and products.
For the reaction
N
2
(
g
)
+
N_{2}(g) +
N
2
(
g
)
+
3
H
2
(
g
)
⇌
2
N
H
3
(
g
)
3H_{2}(g) \rightleftharpoons 2NH_{3}(g)
3
H
2
(
g
)
⇌
2
N
H
3
(
g
)
, the coefficient for
H
2
H_{2}
H
2
is 3
Reactants are the substances formed during a chemical reaction.
False
P
r
o
d
u
c
t
s
Products
P
ro
d
u
c
t
s
are the substances formed from the reaction.Products
What is the relationship between stoichiometric coefficients and the
K
c
K_{c}
K
c
expression?
Exponents for concentrations
Stoichiometric coefficients are used as exponents in the
K
c
K_{c}
K
c
expression.
For the reaction
2
N
O
(
g
)
+
2NO(g) +
2
NO
(
g
)
+
O
2
(
g
)
⇌
2
N
O
2
(
g
)
O_{2}(g) \rightleftharpoons 2NO_{2}(g)
O
2
(
g
)
⇌
2
N
O
2
(
g
)
, the
K
c
K_{c}
K
c
expression is
K
c
=
K_{c} =
K
c
=
[
N
O
2
]
2
[
N
O
]
2
[
O
2
]
\frac{[NO_{2}]^{2}}{[NO]^{2}[O_{2}]}
[
NO
]
2
[
O
2
]
[
N
O
2
]
2
where
[
O
2
]
[O_{2}]
[
O
2
]
has an exponent of 1
Identifying reactants and products is crucial for calculating
K
c
K_{c}
K
c
.
What does the equilibrium constant
K
c
K_{c}
K
c
express?
Ratio of product to reactant concentrations
For the reaction
N
2
+
N_{2} +
N
2
+
3
H
2
⇌
2
N
H
3
3H_{2} \rightleftharpoons 2NH_{3}
3
H
2
⇌
2
N
H
3
, the exponent for
H
2
H_{2}
H
2
in the
K
c
K_{c}
K
c
expression is 3