 Many reactions can proceed in both the forward
direction (towards products- to the right) and the
reverse reaction (towards reactants- to the left)
Recall weak acids and bases!
Dynamic Equilibrium
 Molecules are constantly moving, so in chemistry we
always have dynamic equilibriums (not static)
 Dynamic equilibriums occur when the rate of the
forward reaction is equal to the rate of the reverse
Which is at equilibrium?
 Salt in water?
4 Conditions for ANY Equilibrium
 1. Equilibrium is achieved in a reversible process when
the rates of opposing changes are equal (represented by
the double arrow)
 2. Macroscopic properties are constant. No change in
properties that depend on the total quantity of matter
(ex. colour, pressure, concentration, pH, etc.)
Dynamic change at the molecular level, but no change at
the macroscopic level
 3. Equilibrium can only be reached in closed systems,
including constant temperature (energetically closed too)
 4. Equilibrium can be approached from either direction
and proportions will still be the same.
Equilibrium Concentrations
 Although the rates are the same for either direction,
it is important to remember that the concentrations
are not!
Don’t be fooled by a steady state!
 Reactants are constantly added and products are
removed as they form- appears at equilibrium
Le Chatelier’s Principle
 Henri Louis Le Chatelier was the first
to describe how chemical systems at
equilibrium adjust to changes:
 If a system at equilibrium is subjected
to an external stress, the equilibrium
will shift to minimize the effects of
that stress
 If you think about it- we do that too!
Le Chatelier
 External stresses are factors that cause either the
forward or reverse reaction to change, throwing the
system out of balance.
 Le Chatelier allows us to predict how stress will
affect the system
 4 factors to consider:
1. Concentration
2. Pressure and volume
3. Temperature
4. Catalyst
Le Chatelier- Concentration
 Consider the following equilibrium:
Fe3+(aq) + SCN-(aq) ↔ FeSCN2+(aq)
 If more Fe3+ is added to the reaction, what will happen?
The system needs to react to minimize the stress
Since Fe3+ is on the reactant side, the rate of the forward reaction will
increase to “use up” the additional reactant
This causes a shift to the right (to produce more FeSCN2+)
 What color will the solution become?
 When we added more Fe3+ we could say any of the
following and they all mean the same thing:
Equilibrium shifts to the right
Equilibrium shifts to products
The forward reaction is favoured
 Conversely, if we add more FeSCN2+
 The equilibrium will shift to the left
 The equilibrium will shift to reactants
 Reactants will be favoured
Removing a Substance
 Concentration of one of the substances can also be
changed by removing that substance
This is usually done by adding another substance to react with
one of the compounds/ions that is already in the reaction
Ex. Pb2+ reacts with SCN-. If we add Pb2+ ions it will remove
SCN- ions and the equilibrium will shift to replace these ions
(shift left)
Recall: Pressure
 Pressure is caused by molecules colliding
Changing Pressure and Volume
 If you increase the pressure, you typically do this by
decreasing the volume
 Reaction will favour the side with the least number of
mols of a gas
Conversely, if you decrease the pressure the reaction will shift to
favour the greater number of moles of gas
 If both sides of the equilibrium have the same number of
moles, the equilibrium will not shift.
For Example:
 N2O4 is colorless, NO2 is brown
 Decreasing the volume increases the pressure,
equilibrium shifts left
Changing Temperature
 Easiest way to see the affect of temperature is to
include heat in the reaction equation
Exothermic reactions give off heat as a product
Endothermic reactions require heat (include in reactants)
For N2O4(g) ↔ 2NO2 (g) the ΔH = + 53kJ
So we can write this as:
N2O4(g) + heat ↔ 2NO2 (g)
Now it is much easier to see how heat will affect the
-addition of heat will cause a shift to the right
Addition of a Catalyst
 Catalysts speed up both the forward and reverse
reactions- the no not affect the position of an

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