Chemical
Equilibrium
Quantitative Aspects Part 1
Green/Damji – Chapter 7.2
Chang - Chapter 14
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chemical Equilibrium – Part 2a
A+B
C+
D
Assume this reaction is first order with respect to each
species…
Forward rate = kf [A][B]
Reverse rate = kr [C][D]
At equilibrium the rates are equal
so
kf [A][B]
= kr [C][D]
rearranged to
kf
kr
=
[C][D]
[A][B]
since both kf and kr are constants (at a given temp)
their ratio must also be a constant…
called the equilibrium constant and symbolized Kc (or Keq)
14.1
Equilibrium Constant Expression
(aka Law of Mass Action)
aA + bB
Kc =
cC + dD
[C]c[D]d
[A]a[B]b
“expression” indicates you only put in
symbols… no actual values
14.1
When actual numbers are put into the equations, a
value for the Kc can be determined. That value
can reveal information about the reaction.
Equilibrium Will
If K >> 1
Lie to the right
Favor products
If K << 1
Lie to the left
Favor reactants
14.1
Derive the equilibrium constant expression
for the following homogenous equilibrium:
N2O4 (g)
Kc =
2NO2 (g)
= 4.63 x 10-3
14.2
Example: Ex 7.2 # 1
The equilibrium constant for a reaction that occurs
totally in the gas phase is given below. What is the
chemical equation for this equilibrium?
Kc = [CO2] [CF4]
[COF2]2
A. CO2 (g) + CF4 (g) ⇌ COF2 (g)
B. CO2 (g) + CF4 (g) ⇌ 2 COF2 (g)
C. 2 COF2 (g) ⇌ CO2 (g) + CF4 (g)
D. COF2 (g) ⇌ CO2 (g) + CF4 (g)
Ex: Ex 7.2 # 8e
Balance the equation below and write the Kc expression.
NO (g) + Cl2 (g) ⇌ NOCl (g)
Kc =
Ex: Ex 7.2 # 8f
Balance the equation below and write the Kc expression.
NH3 (g) + O2 (g) ⇌ H2O (g) + O2 (g)
Kc =
Ex: Ex 7.2 # 8h
Balance the equation below and write the Kc expression.
CH3OH (l) + CH3COOH (l) ⇌
CH3COOCH3 (l) + H2O (l)
Kc =
Derive the equilibrium constant expression
for the following heterogeneous equilibrium:
CaCO3 (s)
[
Kc‘ =
Kc = [
][
[
CaO (s) + CO2 (g)
[CaCO3 (s)] = constant
[CaO (s)] = constant
]
]
]=
Kc‘ x
[
[
]
]
14.2
If the concentration of a substance does not change, then
that substance is not included in the expression for the
equilibrium constant.
For instance…
• all solids
• pure liquids
• especially water … when it is a liquid involved in an
aqueous solution
Note: if water is a gas it must be included AND if water is
a liquid with mixed with other liquids (not an aqueous
situation)
14.2
CH3COO- (aq) + H3O+ (aq)
CH3COOH (aq) + H2O (l)
[
Kc‘ =
[
][
][
Kc =
]
]
[
[H2O (l)] = constant , so
][
[
]
]
= Kc‘ [H2O]
Units for an equilibrium constant vary by
situation. Sometimes they are not even
included.
14.2
Ex: Ex 7.2 # 8g
Balance the equation below and write the Kc expression.
CH3NH2 (aq) + H2O (l) ⇌ CH3NH3+ (aq) + OH – (aq)
Kc =
When the equation for a reversible reaction
is written in the opposite direction, the
equilibrium constant becomes the reciprocal
of the original equilibrium constant.
N2O4 (g)
K=
[NO2]2
[N2O4]
2NO2 (g)
= 4.63 x 10-3
2NO2 (g)
K‘ =
[
[
N2O4 (g)
]
1 = _______
=
] K
14.2
If a reaction can be expressed as the sum of
two or more reactions, the equilibrium
constant for the overall reaction is given by
the product of the equilibrium constants of
the individual reactions.
A+B
C+D
Kc‘
C+D
E+F
Kc‘‘
A+B
E+F
[C][D]
Kc‘ =
[A][B]
Kc
[E][F]
Kc‘‘ =
[C][D]
[E][F]
Kc =
[A][B]
Kc = Kc‘ x Kc‘‘
14.2
What?
H2O (g) + CO (g) + ⇌ H2 (g) + CO2 (g)
Kc‘
Kc‘‘
-----------------------------------------------------2 H2O (g) + C (s) + ⇌ 2 H2 (g) + CO2 (g)
Kc
H2O (g) + C (s) ⇌ H2 (g) + CO (g)
[H2 ][CO2]
Kc‘ =
[H2O][CO]
[H2 ][CO]
Kc‘ =
[H2O][C]
Kc =
[H2 ]2[CO2]
[H2O] 2
Kc = Kc‘ x Kc‘‘
14.2
SUMMARY: Writing Equilibrium Constant Expressions
1. The concentrations of the reacting species in the
condensed phase are expressed in M. In the gaseous
phase, the concentrations can be expressed in M or in atm.
2. The concentrations of pure solids and pure liquids often do
not appear in the equilibrium constant expressions.
3. The equilibrium constant is a dimensionless quantity (by
convention in the US… IB will occasionally ask for these)
14.2
Writing Equilibrium Constant Expressions
4. In quoting a value for the equilibrium constant, you must
specify the balanced equation and the temperature.
5. If a reaction can be expressed as a sum of two or more
reactions, the equilibrium constant for the overall reaction is
given by the product of the equilibrium constants of the
individual reactions.
14.2