
What constitutes a reversible reaction?

Describe what it means when a reaction is in
a state of chemical equilibrium.

How do the rates of the forward and reverse
reactions change throughout a reaction?
16-2: The Law of Chemical Equilibrium

Development of the concept of equilibrium
was empirical and depended on the chemist
being able to measure the concentration or
reactants and products, since rate of reaction
depends upon concentration of reactants and
products

Guldberg and Waage proposed the law of mass
action
 Relates the concentrations of reactants and products
at equilibrium by using the equilibrium constant (Keq)
 Equation for a reversible reaction:
▪ aA + bB
cC + dD
▪ a, b, c, and d are coefficients for substances A,B,C, and D
▪ The equilibrium expression for this reaction is
▪
▪
▪
▪
Keq =([C]c[D]d )/([A]a[B]b)
The square brackets denote equilibrium concentrations (usually in M)
Concentrations are raised to the power of its coefficient
Products of forward reaction in numerator
Reactants of forward reaction in denominator

Every reversible reaction (at a given
temperature) proceeds to an equilibrium state
that has a specific ratio of the concentrations of
reactants and products expressed by Keq
 Regardless of initial concentrations each reaction will
reach equilibrium that can be expressed by the
equilibrium constant
 Equilibrium position- each set of different
equilibrium concentrations resulting from different
initial concentrations. There are an infinite number of
equilibrium positions but only one equilibrium
constant (fig. 16-7 p. 542)

Measure of the extent to which a reaction
proceeds to completion. It does not tell you the
amount of time needed to reach equilibrium
 Keq>>1, is read equilibrium constant is significantly
greater than 1 so the numerator must be larger than
the denominator (concentration of products must be
much greater than the concentration of the reactants)
 Keq<<1, denominator must be larger than the
numerator (concentration of reactants must be much
greater than the concentration of products)
 Keq~ 1, there are considerable concentrations of both
reactants and products present at equilibrium



Every example so far has been with gases
Homogeneous Equilibria- equilibrium
conditions for reactions where all reactants and
products are in the same state
Heterogeneous Equilibria- equilibrium
conditions for reactions with substances in more
than one state
 NH4Cl(s)
NH3(g) + HCl(g)
 We have been expressing concentrations of gases in
molarity. For pure solids or liquids we can leave the
concentration out, because it doesn’t change. So
equation for this type of reaction is Keq=[NH3][HCl]

Q is used to determine if a reaction is at
equilibrium
 Calculated like Keq except the concentrations used are
the concentrations taken at the time of
measurement, not the equilibrium concentrations
▪ Example: N2(g) +3H2(g) 2NH3(g) where Keq=.105
▪ When you measure the concentrations of each compound
they are: [NH3]=.15M, [N2]=.002M, and [H2]=.10 M
▪ Then plug them in:
 Keq= [NH3]2/[N2][H2]3
 =(.15)2/(.002)(.10)3 and Q= 1.1x104
 Since Q = Keq the reaction is not at equilibrium



If Q=Keq then the reaction is at equilibrium
If Q<Keq then the reaction will consume
reactants and form products to reach
equilibrium
If Q > Keq then the reaction will consume
products and form reactants to reach
equilibrium

Read through Sample Problem 3

Phosphorus pentachloride decomposes partially
to phosphorus trichloride and chlorine at
elevated temperatures. All the substances are
gases. At a given temperature the concentration
of PCL5 is .80 mol/L, the concentration of PCl3 is
.20 mol/L, and the concentration of Cl2 is 2.20
mol/L. The equilibrium constant for the reaction
is .055. Is the reaction at equilibrium? If not
which direction will it proceed?

At 740 degrees C, Keq=.0060 for the
decomposition of calcium carbonate, which is
described by the equation CaCO3 CaO +
CO2. Find Q and predict how the reaction will
proceed if [CO2]=.004M

For the reaction CO + H2O
H2 +CO2,
where Keq=5.10 at 527 degrees C. If
[CO]=.15M, [H2]=.25M and {CO2]=.37M,
calculate Q and determine how the reaction
will proceed.

Opposing Reactions in Equilibrium System