Equilibrium
Chapter 18 Part 1
POINT > Describe equilibrium
POINT > Define the equilibrium constant, Keq
POINT > Interpret equilibrium constants
POINT > Calculate reactant concentrations and Keq
POINT > Describe equilibrium
Some reactions go in one direction to completion, such
as the dissociation of a strong acid:
HCl + H2O  H3O+ + ClMany reactions, however, do not run to completion,
such as the dissociation of a weak acid:
HF + H2O ⇌ H3O+ + FWe say that the reaction reaches equilibrium
POINT > Describe equilibrium
As a reaction approaches equilibrium, both the
forward and reverse reactions are occurring
N2O4(g) ⇌ 2 NO2(g)
Chemical equilibrium occurs when a reaction and its
reverse reaction proceed at the same rate
When this dynamic equilibrium is achieved, the
amount of each reactant and product remains constant
POINT > Define the equilibrium constant, Keq
N2O4(g) ⇌ 2 NO2(g)
The rate law for the “forward” reaction is:
Rate = kf [N2O4]
The rate law for the “reverse” reaction is:
Rate = kr [NO2]2
If the two rates are equal, then…
POINT > Define the equilibrium constant, Keq
Ratef = Rater
kf[N2O4] = kr[NO2]2
Rearranging, we get the expression for the
equilibrium constant, Keq
, Keq
POINT > Define the equilibrium constant, Keq
, Keq
N2O4(g) ⇌ 2 NO2(g)
Keq is given by the ratio of product concentrations over
reactant concentrations, each raised to its respective
stoichiometric coefficient
POINT > Define the equilibrium constant, Keq
Given:
aA + bB⇌ dD + eE
The equilibrium expression for this reaction
would be
Keq
Keq is expressed as Kc when using concentration
WB CHECK:
Show the Keq expression for the reaction:
hydrogen and oxygen gases combine to form
water
2H2 + O2  2H2O
Keq = [H2O]2
[H2]2[O2]
WB CHECK:
Show the Keq expression for the reaction:
hydrogen and nitrogen gases combine to form
ammonia gas
3H2 + N2  2NH3
Keq = [NH3]2
[H2]3[N2]
POINT > Interpret equilibrium constants
At a given temperature, the same equilibrium constant,
Keq, will result regardless of starting concentrations:
Ex. N2O4 ↔ 2NO2
POINT > Interpret equilibrium constants
If Keq >>1, the reaction favors products; equilibrium
“lies to the right”
If Keq <<1, the reaction favors reactants; equilibrium
“lies to the left”
Keq of a reaction in the reverse direction is the
reciprocal of Keq of the forward direction:
N2O4 ↔ 2NO2
vs.
2NO2 ↔ N2O4
POINT > Interpret equilibrium constants
As with Hess’s law, reaction equations can be
combined and a new, single equilibrium determined
The equilibrium constant of the new reaction is the
product of the two constants:
K3 = K1 × K2
Example:
2 NOBr ⇌ 2 NO + Br2 K1 = 0.014
Br2 + Cl2 ⇌ 2 BrCl
K2 = 7.2
2 NOBr + Cl2 ⇌ 2 NO + 2 BrCl K3 = ?
K3 = K1 × K2 = 0.014 × 7.2 = 0.10
WB CHECK:
At 298K,
2NO ↔ N2 + O2
Kc = 2.1 x 1030
At equilibrium, will the container have more
N2 and O2 gas or more NO gas?
Keq = [N2][O2]
[NO]2
WB CHECK:
At 298K, given: 2NO + Br2 ↔ 2NOBr
and
2NO ↔ N2 + O2
Kc = 2.1 x 1030
Determine Kc for the reaction:
N2 + O2 + Br2 ↔ 2NOBr
Kc = 9.5 x 10-31
Kc = 2.0
WB CHECK:
Show the equilibrium expression for the reaction when
oxygen gas combines with solid aluminum to form
solid aluminum oxide
Keq = [Al2O3]2
[Al]4[O2]3
WB CHECK:
Show the chemical equation for the reaction: equal
amounts of H2 and I2 combine to form hydroiodic acid
H2 + I2 ↔ 2HI
Show the equilibrium expression
Keq =
[HI]2
[H2][I2]
If Keq = 54.8 and [HI] = 3.3 x 10-2M, what is [H2]?
54.8 = (3.3 x 10-2)2
(x)(x)
[H2] = 4.5 x 10-3M
Homework:
Read Ch. 18 pages 555-561
Complete Practice Exercises!
F.A. #1-9 pg. 561