Dynamic Equilibrium
Chapter 15
Example
liquid + heat vapour
an endothermic physical change
Le Chatelier’s Principle
• When a dynamic equilibrium is upset by a disturbance, the
system responds in a direction that tends to counteract the
disturbance and, if possible, restore equilibrium
liquid + heat  vapour
Increase the temperature (add heat)
Le Chatelier’s Principle
• When a dynamic equilibrium is upset by a disturbance, the
system responds in a direction that tends to counteract the
disturbance and, if possible, restore equilibrium
liquid
+ heat 
vapour
Liquid evapourates to absorb the added heat and the
position of the equilibrium shifts to the right
Chemical Equilibrium
CH3COOH + H2O H3O++CH3COO10
Concentration of reactant
9
Concentration
8
Equilibrium established
7
6
5
4
3
2
1
Concentration of product
0
0
50
100
150
200
Time
N2O4(g) 
2NO2(g)
Cinit
0.0350 mol
0
or
0
0.0700 mol
Cequil
0.0292 mol
0.0116 mol
For a given composition of
a system, equilibrium is
independent of which side
you approach it
from…..reactions are
REVERSIBLE
Equilibrium Law
simple relationship between molar concentrations (or pressures) of
reactants and products at equilibrium
H2(g) +
I2(g)

Mass action expression:
Where Q is the reaction quotient
2HI(g)
(at 440C)
HI 2  Q
H 2 I 2 
LAW: at equilibrium the reaction quotient is equal
to the equilibrium constant, K.
In general for
dD + eE  gG + hH
G g H h
Dd E e
 K C (at a given T)
Heterogeneous Equilibria
(involving more than one phase)
2 NaHCO3 ( s )  Na2CO3 ( s )  H 2O( g )  CO2 ( g )
(in a sealed container)
K
CO2 H 2ONa2CO3 
NaHCO3 2
BUT : for any pure liquid of solid, the ratio of the amount of substance
to the volume of substance is a constant
1 mol
2mol
for example : NaHCO3

 25.7 mol / L
0.0389L 0.0778 L
therefore
CO2 H 2O  K NaHCO3   K C
Na2CO3 
2
Equilibrium Law for Gaseous Reactions
PV  nRT
n
 P  RT
V
 P  GasRT
For gaseous reactions use partial
pressures, P, to give an
equilibrium constant (Kp)
N 2 ( g )  3H 2 ( g )  2 NH 3 ( g )
2

NH 3 
Kc 
N 2 H 2 3
Kp 
2
PNH
3
PN 2  PH32
OR K p 
2
PNH
3
PN 2  PH32
2
2
2


NH 3  RT 
NH 3 
2





RT
N 2 RT   H 2 3 RT 3 N 2 H 2 3
ng
Kp  K c RT 
The Magnitude of Equilibrium Constants
• The equilibrium constant, K, is the ratio of products to reactants.
• Therefore, the larger K the more
products are present at equilibrium.
• Conversely, the smaller K the more
reactants are present at equilibrium.
• If K >> 1, then products dominate
at equilibrium and equilibrium lies
to the right.
• If K << 1, then reactants dominate
at equilibrium and the equilibrium
lies to the left.
Predicting direction of equilibrium changes
(Le Chatelier redux)
Adding or removing reactant or product
Cu(H2O) 42+ (aq.) + 4Cl-(aq.) CuCl 42- (aq.) + 4H2O
Changing volume (or pressure)-gases only
as liquids and solids are incompressible
N 2 ( g )  3H 2 ( g )  2 NH 3 ( g )  HEAT
Decrease V=>increase P
System opposes this by reducing the number of molecules
Predicting direction of equilibrium changes
(Le Chatelier redux)
Changing temperature (heat)
System opposes this by trying to
absorb the added heat
N 2 ( g )  3H 2 ( g )  2 NH 3 ( g )  HEAT
ONLY factor that actually changes the value of K
Effect of catalyst
Speeds up approach to equilibrium, but does not
alter K or the system (kinetics)