CHEMICAL EQUILIBRIUM
Everything you need to
know!!
CHEMICAL EQUILIBRIUM
A state when the rate of the
forward reaction is equal to the
rate of the reverse reaction.
4A + B  C + 3D
4A + B  C + 3D
4A + B ⇄ C + 3D
CHEMICAL EQUILIBRIUM
Dynamic process. Both forward
and reverse reactions continue,
but there is NO NET CHANGE in
the amount of reactants and
products.
CHEMICAL EQUILIBRIUM
Products
M
Reactants
 Equilibrium
Time
CHEMICAL EQUILIBRIUM
The equilibrium point is actually a
point in a reaction where a specific
mixture of reactants and products
has obtained a minimum energy
state – the most stable state. This
is a temperature dependent
condition.
LE CHATELIER’S PRINCIPLE
When a system at equilibrium
experiences a stress, the system
responds to relieve that stress by
changing the ratio of reactants and
products. It shifts right or left!
STRESSORS
 Concentration
Changes of R or P
 Temperature Changes
 Pressure/Volume Changes
Concentration Changes
 Concentration
changes only affect
reactants or products in the
aqueous (aq) or gaseous (g) state
– pure solids (s) or liquids (l) don’t
have concentrations.
 System will consume what is
added or replace what is removed.
Example 1:
Consider the following reaction:
4A (g) + B (g) ↔ C (s) + 3D (g)
 Adding
more A will cause the
reaction to shift to the right, the
direction that consumes A.
Example 2:
Consider the following reaction:
4A (g) + B (g) ↔ C (s) + 3D (g)
 Removing
some B will cause the
reaction to shift to the left, the
direction that replaces B.
Example 3:
Consider the following reaction:
4A (g) + B (g) ↔ C (s) + 3D (g)
 Adding
or removing C has no effect
on the equilibrium position, since it
is in the solid state.
Example 4:
Consider the following reaction:
4A (g) + B (g) ↔ C (s) + 3D (g)
 Which
direction do you think it will
shift if some D is removed?
Yes, that’s right! To the Right!!
Temperature Changes
 Effect
depends on whether the
reaction is endothermic or
exothermic.
 Increasing the temperature is like
adding energy.
 Decreasing the temperature is like
removing energy.
Temperature Changes
 In
endothermic reactions, energy is
written on the reactant side:
Energy + 4A + B  C + 3D
 In exothermic reactions, energy is
written on the product side:
4A + B  C + 3D + Energy
Example 5:
Consider the following reaction:
4A + B ↔ C + 3D + Energy
Is this endothermic or exothermic?
Which way will it shift if the
temperature is increased?
Yes, that’s right! To the Left!!
Example 6:
Consider the following reaction:
25 kJ + 4A + B ↔ C + 3D
Is this endothermic or exothermic?
Which way will it shift if the
temperature is decreased?
Yes, that’s right! To the Left!!
Pressure/Volume Changes
 Pressure/Volume
changes only
affect reactants or products in the
gaseous (g) state.
 Systems that differ in the number of
moles of gaseous reactants and
products respond by shifting to a
more or less compact state.
Pressure/Volume Changes
Consider the following reaction:
4A (g) + B (g) ↔ C (s) + 3D (g)
How many moles of gaseous
reactants/products are represented on
each side of the arrow?
5 moles on left; 3 mol on right
Example 7:
4A (g) + B (g) ↔ C (s) + 3D (g)
Which way will it shift if the volume
is decreased? (Pressure increased)
Yes, that’s right! To the Right!!
Factors That DO NOT Affect
Equilibrium Positions
 Adding
a Catalyst
 Adding an Inert substance, like
a noble gas.
Affect of Catalyst
Speeds up the rate of establishing
equilibrium, but will NOT affect the
equilibrium position.
Affect of Inert Substance
Does not change any reactant or
product concentration, does not change
temperature, does NOT affect any
individual gas pressure – thus, NO
EFFECT on equilibrium position.
EQUILIBRIUM EXPRESSIONS
An equilibrium expression computes
an equilibrium constant, Keq. This
constant is the ratio of products to
reactants raised to their stoichiometric
coefficients once equilibrium has been
established. Again, pure solids (s) and
liquids (l) are OMITTED from the
expression.
EQUILIBRIUM EXPRESSIONS
For the following reaction:
4A (g) + B (g) ⇄ C (s) + 3D (g)
Keq =
[D]3
[A]4 [B]
EQUILIBRIUM EXPRESSIONS
Meaning of K:
If K > 1 then it’s products favored.
If K = 1 then it’s neither.
If K < 1 then it’s reactants favored.