Chapter 15
Chemical Equilibrium
BLB 12th
2 NO2(g) → 2 NO(g) + O2(g)
15.1 The Concept of Equilibrium
N2O4(g) ⇌ 2 NO2(g)
colorless
brown
Chemical equilibrium
occurs when a reaction
and its reverse reaction
proceed at the same
rate.
The Concept of Equilibrium
N2O4(g) ⇌ 2 NO2(g)


As a system approaches
equilibrium, both the
forward and reverse
reactions are occurring.
At equilibrium, the
forward and reverse
reactions are proceeding
at the same rate.
A System at Equilibrium
N2O4(g) ⇌ 2 NO2(g)


Once equilibrium is
achieved, the concentrations of reactants and
products remain
constant.
Ratio of concentration
values equals a constant.
The Concept of Equilibrium





The reaction system is closed.
Opposing reactions occur at equal rates, that
is, the rates of forward and reverse reactions
are equal.
Dynamic process (never stops)
Concentrations of reactants and products are
constant.
Equilibrium can be reached from either
direction, reactants or products.
15.2 The Equilibrium Constant
aA + bB ⇌ cC + dD
c
d
[C ] [ D]
K
[ A]a [ B]b
@ at some T
no units ( see p. 618)
[ ] equilibrium concentrations
15.2 The Equilibrium Constant







K is constant for a reaction, regardless of the [A]0.
K is temperature dependent.
K depends on stoichiometry, not on the mechanism.
Pure solid or liquid reactant & products are not included
for heterogeneous equilibria. (15.4, p. 623)
Water (or other pure solvent) is not included if the
reactant and product concentrations are low. (p. 625)
Kc – concentrations (M) of solutions
Kp – partial pressures of gases; p. 617 to convert between
Kp & Kc
Write the expression for K.
a)
N2(g) + O2(g) ⇌ 2 NO(g)
b)
2 SO2(g) + O2(g) ⇌ 2 SO3(g)
Evaluating K
N2O4(g) ⇌ 2 NO2(g)
15.3 Understanding and Working with
Equilibrium Constants
The Magnitude of K
 K >> 1 Essentially all products; lies to right
K > 1 product-favored
K < 1 reactant-favored
K << 1 Essentially all reactants; lies to left
Manipulating K (pp. 620-622)

Reverse reaction:
1
Kf 
Kr

Adding reactions:
K net  K1  K 2

Multiplying reaction by some factor:
#1: C(s) + ½ O2(g) ⇌ CO(g)
#2: 2 C(s) + O2(g) ⇌ 2 CO(g)
2
[CO]
[
CO
]
23
47
K1 

4
.
6

10
K


2
.
1

10
1
2
2
[O2 ]
[O2 ]
K 2  K1 
2
15.4 Heterogeneous Equilibria




Reactant components are in different phases.
When a pure solid or liquid is involved in a
heterogeneous equilibrium, its concentration
is not included in the K expression.
The concentration of a pure solid or pure
liquid doesn’t change.
Write the K expression:
SnO2(s) + 2 CO(g) ⇌ Sn(s) + 2 CO2(g)
15.5 Calculating Equilibrium Constants




Must plug in the equilibrium concentrations
of reactants and products
A concentration table with initial, change, and
equilibrium concentrations is set up.
See Sample Exercise 15.9.
More in lab and in Chapter 16 with weak
acids and bases.
Calculating K
N2(g) + 3 H2(g) ⇌ 2 NH3(g) p. 642 @ 472°C
2.46
7.38
0.166 ← eq. partial pressures (atm)
15.6 Applications of Equilibrium Constants


Predicting the direction a reaction must proceed
to establish equilibrium
The reaction quotient, Q (p. 627)


Q has the same form as K, but for non-equilibrium
conditions.
Comparing Q and K:
Q < K achieves equilibrium by shifting to right
Q = K @ equilibrium
Q > K achieves equilibrium by shifting to left
Sample problem 46a
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
45 atm 55 atm
98 atm
KP = 4.51 x 10-5 at 450°C
15.7 Le Châtelier’s Principle
If a system at equilibrium is disturbed by a
change in concentration, pressure, or
temperature, the system will shift its
equilibrium position to counter the effect of
the disturbance. (p. 631)
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
Le Châtelier’s Principle
Disturbance
More reactant or product
Volume decrease
(total pressure increase)
Inert substance added
Temperature increase
Effect
Shifts to consume
Shifts to reduce pressure
(fewer moles)
None
If exothermic, shifts left and
K decreases.
If endothermic, shifts right
and K increases.
N2O4(g) ⇌ 2 NO2(g)
15.1 The Concept of Equilibrium
N2O4(g) ⇌ 2 NO2(g)
colorless
brown
Chemical equilibrium
occurs when a reaction
and its reverse reaction
proceed at the same
rate.
Sample problem 61
2 SO2(g) + O2(g) ⇌ 2 SO3(g) ΔH < 0
Effect on equilibrium mixture when…
(a) O2(g) is added?
(b) the reaction mixture is heated?
(c) the volume of container is doubled?
(d) a catalyst is added?
(e) the total P is increased by adding a noble gas?
(f) SO3(g) is removed?
6 CO2(g) + 6 H2O(l) ⇌ C6H12O6(s) + 6 O2(g) ΔH = 2816 kJ
Write the K expression.
Effect on equilibrium mixture when…
(a) PCO2 is increased?
(b) the reaction is heated?
(c) some CO2(g) is removed?
(d) the total P is decreased?
(e) part of C6H12O6(s) is removed?
(f) a catalyst is added?