Equilibrium Notes
Dynamic Equilibrium

Particles in constant motion

Reactants collide to from products

At some point amount of products has increased and they collide to form
reactants

rate of the forward rxn = rate of reverse rxn.
aA + bB  cC + dD
 = reversible rxn
Reversible reaction

Products collide to re-form reactants

Example Rxn:
o

2H2(g) + O2(g)  2H2O(g)

Forward: 2H2(g) + O2(g) 2H2O(g)

Reverse: 2H2O(g)  2H2(g) + O2(g)
Example Phase change:
o
H2O (l)  H2O(g)
Complete reaction

Some rxns are said to go to completion but always, there is an equilibrium
established

Products do not re-form reactants in significant amount

Ex:
o
Precipitation rxn
o
Gas forms and escapes
Equilibrium Constant, Keq

Used to determine
o
If a rxn is at equilibrium
o
What will happen if conc. are changed
o
If reactants or products are favored (equilibrium position)


If Keq >> 1, products favored

If Keq << 1, reactants favored

If Keq = 1, than neither side favored

If Keq between 10-2 and 102, both reactant and product will be
present in significant amt.
Symbol = Keq
o
Kc (for rxns in sol’n, c = conc.)
o
Kp (for gas rxns, p = pressure)
o
Ka (for acid dissociation rxn)

o
Kw (for water dissociation)

o
NaOH(s)  Na+(aq) + OH-(aq)
Ksp (for solubility)


2 H2O  H3O+1 + OH-1
Kb (for base dissociation rxn)

o
HCl(s)  H+(aq) + Cl-(aq)
NaCl(s)  Na+(aq) + Cl-(aq)
What affects Keq ?
Factor
Affects Keq?
Temperature
Yes
Starting concentration
No
Catalysts
No
Pressure
No
How to write equilibrium expressions:

Given the following reaction
aA + bB  cC + dD
[C ]c [ D]d
Keq 
[ A]a [ B]b

Given the following reaction:
2H2O(g)  2H2(g) + O2(g)
[ H 2 ]2 [O2 ]
Keq 
[ H 2O]2

Homogeneous Equilibrium: products and reactants are all in same phase

Heterogeneous Equilibrium: products and reactants are in different phases
o

Ex: AgNO3 + NaCl  AgCl (s) + NaNO3(aq)
Pure solids and liquids are not included in equilibrium expressions
[ NaNO3 ]
Keq 
[ AgNO3 ][ NaCl ]

Given the concentrations of reactants and products at equilibrium, you can put
them into the equation above and calculate the same number – this is what
makes it a constant.
Example:
(Example 1)
Concentration of
substance after
equilibrium is reached
(Example 2)
Concentration of
substance after
equilibrium is reached
[H2]
10 M,
20 M
[O2]
5M
1.25 M
[H2O]
2M
2M
Keq

Keq 
102 5
=125
22
Keq 
This example is
showing that even
when the
concentrations at
equilibrium have
changed (because
starting amounts were
changed), the value
of Keq is the same
202 1.25
=125
22
You can also use Keq to find out of a reaction is at equilibrium

Lets say that you start a reaction by mixing H2 and I2 gas. You need to know
when the reaction has reached equilibrium so you periodically measure the
concentration of H2, I2, and HI that are present

After 1 minute the concentrations are as follows
[H2]
8 M,
[O2]
4M
[H2O]
1M
Keq
Keq 
82 4
=256
12
o
The reaction is not at equilibrium yet because the concentrations do not
show an equilibrium constant of 125.
o
The reaction will proceed in a direction that increases the amount of
products (forward direction)
Le Chatelier’s principle

A reaction at equilibrium will proceed in a direction that relieves the stress put
upon it.

Possible stresses:
o
Temperature
o
Volume
o
Pressure
o
Concentration
Example: 2H2(g) + O2(g) + energy  2H2O(g)

If [H2] increases,
o

If [H2] is decreased
o

Because energy is a reactant, reaction will proceed to left to make up
some of the energy that was lost
When pressure is increased (or Volume decreased)
o

Because energy is a reactant, reaction will proceed to right to use up the
added energy
If temperature is decreased
o

Reaction will proceed to left to make up some of the H2 that was lost
If temperature is increased,
o

reaction will proceed to right to produce more product and used up the
excess [H2]
Reaction will proceed to make fewer moles of gas, will shift to right
When pressure in decreased (or Volume increased)
o
Reaction will proceed to make more moles of gas, will shift to left