Chemistry 1011
TOPIC
Gaseous Chemical Equilibrium
TEXT REFERENCE
Masterton and Hurley Chapter 12
Chemistry 1011 Slot 5
1
12.1 The N2O4 - NO2 Equilibrium
YOU ARE EXPECTED TO BE ABLE TO:
• Distinguish equilibrium from steady state
situations.
• Recognize chemical equilibrium as a dynamic
process taking place in a closed system.
• Identify the changes taking place at the molecular
level in a chemical equilibrium process.
• Identify the equilibrium constant as a temperature
dependent constant related to the equilibrium
partial pressures of reactants and products.
Chemistry 1011 Slot 5
2
Review - Partial Pressure
• Total gas pressure is proportional to the
number of moles present; it is independent
of the identity of the material
• The partial pressure of a gas in a gas
mixture is the fraction of the total pressure
that is due to that gas
• The partial pressure of gas X2 in a mixture
is equal to the
mole fraction of X2 x Total Pressure
Chemistry 1011 Slot 5
3
Partial Pressure Example
• Air is approximately 80% N2 and 20%O2
• Total air pressure is approximately 100kPa
PN2 + PO2 = PTOTAL
PN2 = 80% x 100kPa = 80kPa
PO2 = 20% x 100kPa = 20kPa
• Partial pressures can be used to express the
concentrations of reactants and products in
chemical equilibria
Chemistry 1011 Slot 5
4
The N2O4 - NO2 Equilibrium
• At any given temperature, a sample of NO2
will exist as an equilibrium mixture of NO2
and N2O4
N2O4(g)
2NO2(g)
• The forward and reverse reactions are
taking place at the same rate
• The concentrations of the species remain
constant
Chemistry 1011 Slot 5
5
Chemistry 1011 Slot 5
6
Getting to Equilibrium
• Equilibrium can be approached starting
from either “reactants” or “products”
• In the N2O4 - NO2 example, we can
theoretically start from either 100%
reactants or 100% products
• The equilibrium position will be the same
Chemistry 1011 Slot 5
7
The Equilibrium Constant
• Equilibrium Measurements for the
N2O4 - NO2 System at 100oC
Original P (atm) Equilibrium P (atm)
Expt 1 N2O4
NO2
Expt 2 N2O4
NO2
Expt 3 N2O4
NO2
1.00
0.00
0.00
1.00
1.00
1.00
Chemistry 1011 Slot 5
0.22
1.56
0.07
0.86
0.42
2.16
8
The Equilibrium Constant
• The relationship
(PNO2 )2/ PN2O2
is a constant (P is the equilibrium partial pressure
of the species)
Experiment 1 (PNO2 )2/ PN2O2 = (1.56)2/0.22 = 11
Experiment 2 (PNO2 )2/ PN2O2 = (0.86)2/0.07 = 11
Experiment 3 (PNO2 )2/ PN2O2 = (2.16)2/0.42 = 11
• The constant is called the equilibrium constant K
for the system
Chemistry 1011 Slot 5
9
Characteristics of Equilibrium
• A dynamic equilibrium can only exist in a
closed system – neither reactants nor
products can enter or leave the system
• At equilibrium, the concentrations of
reactants and products remain constant
• At equilibrium, the forward and reverse
reactions are taking place at equal and
opposite rates
• Equilibrium can be approached from either
side of the reaction equation
Chemistry 1011 Slot 5
10
Steady State vs Equilibrium
• When a process takes place in an open
system, a steady state may be set up
• In a steady state, the concentration of a
product may be constant, but this will be
because it is being formed, and is leaving
the system at the same rate
• Consider the evaporation of water in closed
and open systems:
Chemistry 1011 Slot 5
11
The Evaporation of Water
• Water in an open beaker
– Water will evaporate at a rate dependent on its
temperature.
– The concentration of water vapour in the top of
the beaker will be constant, but water molecules
are entering the space and leaving it at the same
rate
– This is an open system – it is a steady state
– Eventually, all of the water will evaporate
Chemistry 1011 Slot 5
12
The Evaporation of Water
• Water in a closed container
– Water will evaporate at a rate dependent on its
temperature
– As the concentration of water vapour rises,
water molecules will condense
– Eventually a dynamic equilibrium will be
established at that temperature
– The rate of evaporation will equal the rate of
condensation
H2O(l)
H2O(g)
Chemistry 1011 Slot 5
13