Rates and Equilibrium
Recall: the rate of any reaction depends on the
CHEMICAL SYSTEMS AND
EQUILIBRIUM
Dynamic Equilibrium in Chemical
Systems
Chemical (dynamic) Equilibrium
concentration of the reactants and products is
not changing with time because the forward
and reverse reactions are occurring at equal
rates
concentration of the reacting chemicals.
As a reaction proceeds, the concentrations of the
“product” chemicals increases, and the reverse
reaction may re-form “reactants”.
Under certain conditions, the rate of the reverse
reaction increases as the rate of the forward
reaction decreases.
Eventually, the rate of the forward reaction
equals the rate of the reverse reaction.
Equilibrium occurs when opposing changes are
occurring simultaneously at the same rate.
For example: (in a closed
container)
this graph
represents
the rxn:
N2O4 ↔ 2 NO2
THE FOUR CONDITIONS THAT APPLY
TO ALL EQUILIBRIUM SYSTEMS:
1. Equilibrium is achieved in a reversible process
when the rates of opposing changes are equal. A
double arrow indicates this reversible change.
2. The observable (macroscopic) properties of a
system at equilibrium are constant. Examples
include colour, pressure, concentration and pH.
3. Equilibrium can only be reached in a closed system.
4. microscopic properties ARE NOT constant → i.e.
there are reactants changing into products and
products into reactants
1. when reaction
starts, there is a
high [] of reactants
and low [] of
products
*if NO REACTIONS are occurring in the
forward or reverse direction, this is called
STATIC equilibrium → not the same as
chemical equilibrium (boring ☺)
2. as reaction proceeds, [R] and [P] change
Initially, forward rxn > reverse rxn
as reaction continues, [P] becomes high
and more products are reacting to produce
reactant molecules → THE REVERSE
REACTION
eventually forward and reverse reactions
occur at EQUAL rates
forward reaction = reverse reaction, ∴
concentrations don’t change