18.1- Equilibrium: A state of dynamic
balance
1) When a reaction results in almost
complete conversion of reactants to
products, chemists say that the reaction
goes to completion.
A+BC
2) But most reactions do not go to
completion. They appear to stop. This is
because the reactions are reversible. The
reaction can occur in both the forward
and the reverse direction.
Forward: A + B  C
Reverse: C  A + B
Reaction: A + B C
Example:
Forward: N2(g) + 3H2(g)  2NH3(g)
Reverse:
N2(g) + 3H2(g)  2NH3(g)
Reaction: N2(g) + 3H2(g)
2NH3(g)
Chemical equilibrium = a dynamic state in
which the forward and reverse reactions
balance each other because they take place
at equal rates
Rateforward reaction = Ratereverse reaction
Law of chemical equilibrium = at a given
temperature, a chemical system may reach a
state in which a particular ratio of reactant
and product concentrations has a constant
value.
The equilibrium constant, Keq, is the
numerical value of the ratio of product
concentrations to reactant concentrations.
Reactants
aA + bB
Products
cC + dD
Keq = [C]c[D]d
[A]a[B]b
If [products] > [reactants], then Keq > 1.
If [reactants] > [products], then Keq < 1.
The state of matter of the reactants and
products is significant. Only gases and
aqueous solutions are included in the Keq
expression.
Solids and liquids are pure substances and
have a concentration of 1, so they are not
included in the Keq expression.
If all the reactants and products are in the
same physical state, the reaction is a
homogeneous equilibrium.
If there is more than one physical state, the
reaction is a heterogeneous equilibrium.
18.2- Le Chatelier’s principle: If a stress,
or change, is applied to a system at
equilibrium, the system shifts in the
direction that relieves the stress (change).
The 3 sources of stress are:
1) Changes in concentration of a reactant
or product.
2) Changes in temperature to an
endothermic or exothermic reaction.
a) Endothermic reaction: heat is
added as a reactant and the overall
change in energy is positive
(ΔH= +).
Heat + A + B C + D
b) Exothermic reaction: heat is
released as a product and the
overall change in energy is
negative (ΔH= -).
A + B C + D + heat
3) Changes in pressure due to a change in
volume. (If V increases, P decreases. If
V decreases, P increases.)
The equilibrium system will shift as follows
in response to:
1) changes in concentration, [ ]
a) if [ ] increases on one side, favor
the opposite side
b) if [ ] decreases on one side, favor
the same side
2) changes in temperature (heat)
a) if heat increases on one side, favor
the opposite side
b) if heat decreases on one side, favor
the same side
3) changes in pressure (volume)
a) if pressure increases (volume
decreases), then favor side with
less moles
b) if pressure decreases (volume
increases), then favor side with
more moles