OBJECTIVES
- give examples of reversible and
irreversible processes;
- explain the equilibrium condition in terms
of reaction rates of forward and
backward reactions and concentrations
of reactants and products;
• write the mass action expression for a given
balanced chemical equation for
homogeneous and heterogeneous equilibria;
and
 • predict the direction in which a reaction at
equilibrium will shift given the values of the
reaction quotients and the equilibrium
constant.;
Chemical thermodynamics answers the
question “Why do some reactions proceed
spontaneously while some or nonspontaneous?”
Chemical kinetics answers the
question “How fast a chemical
reaction proceeds?”
Chemical equilibrium, on the other hand,
basically answers the question “How far do
reactions proceed?”
Chemical thermodynamics answers the
question “Why do some reactions proceed
spontaneously while some or nonspontaneous?”
The Concept of Equilibrium
many chemical reactions do not proceed to
just one direction or proceed essentially to
completion. These are called reversible
reactions.
What happens in a reversible reaction?
In reversible reactions, the reactants are not
completely converted into products and
some of the products may be converted back
into reactants.
What happens in a reversible reaction?
In reversible reactions, the reactants are not
completely converted into products and
some of the products may be converted back
into reactants.
How do we distinguish an irreversible reaction from
a reversible reaction in a chemical equation?
Unlike in irreversible reactions where a single
headed arrow is used (), reversible reactions
use a double headed arrow () to indicate that
the forward and backward reactions are
occurring simultaneously. In general terms, a
reversible reactions may be represented as
follows:
aA + bB —> cC + dDwhere the lower case
letters represent the stoichiometric coefficients
of the reactants and products.
What is chemical equilibrium?
A state of balance is also referred to as
a state of equilibrium. In a reversible
reaction, when the reactants start to
form the products, the products would
then start to reform the reactants. The
two opposing processes happen at
different rates but a certain point in the
reaction will be reached where the rates
of the forward and backward reactions
are the same This is the state of
chemical equilibrium.
The state of chemical
equilibrium is a highly dynamic
state. This means that though
there are no change in the
composition of the reaction
mixture and no visible changes
taking place, the particles are
continuously reacting.
Also, a system at chemical
equilibrium can be easily disturbed
by changes in the reaction
conditions.
The Law of Mass Action Expression/Equilibrium
Constant Expression
The relationship between the concentrations of the
reactants and products may be expressed using
the law of mass action expression/equilibrium
constant expression. For the general equilibrium
reaction:
aA + bB =cC + dD
the law of mass action
expression is written as
where the [ ] is the concentration
expressed in molarity and Keq is the
equilibrium constant. If molar
concentrations are used, Keq may
also be referred to as Kc
The law of mass action is
basically the ratio of the
concentrations of the
products raised to their
respective stoichiometric
coefficients to that of the
reactants.
Equilibrium constants for
homogeneous
gaseous equilibria may also be
expressed in terms of partial
pressures. The expression is written in
much the same way as described,
only that the partial pressure is raised
to the coefficient instead of the
molar concentration.
 For equilibrium reactions where the reactants and products are in
different phases (heterogeneous equilibria), pure solids and pure
liquids are excluded in writing equilibrium constant expressions.
For example, consider the reaction:
Why do we omit pure solids and pure liquids in
the equilibrium constant expression?
When the mass of a certain pure solid
substance is doubled, its volume is also
doubled. Therefore, when the mass and
volume is related to get the concentration,
a constant value is obtained. Only reactants
and products whose concentration varies
during a chemical reaction are included in
the expression.
The Equilibrium Constant, K
The equilibrium constant, K, is the numerical
value that is obtained when equilibrium
concentrations are substituted to the
equilibrium constant expression. The value of K
may vary from very large to very small values.
This value provides an idea of the relative
concentrations of the reactants and products
in an equilibrium mixture.
How can the value of the equilibrium
constant be used to determine the
relative composition of the reaction
mixture at equilibrium?
 Consider the reaction for the production of phosgene (COCl2), a toxic gas used in
the manufacture of certain polymers and insecticides, at 100°C:
Examples of Reactions
If a certain fraction equates to a very large
value, then which has a larger value
between the numerator and the
denominator?.
The numerator should have the larger value.
The large value of 4.56 × 109 suggests that
the concentration of COCl2 must be very
large as compared to the individual
concentrations of CO and Cl2. The value
suggests that at equilibrium, the reactions
mixture contains more of the product
COCl2 than the reactants CO and Cl2.
This is experimentally verified. In other
words, the equilibrium lies to the right or
towards the product side based from the
chemical equation given.
In the second example, the value of Kc
is small (< 1). For this to happen, the
value of the denominator must be larger
compared to the numerator. In the case
of the given reaction, the equilibrium
concentration of HI is higher than the
equilibrium concentrations of the
decomposition products. This means
that the equilibrium lies on the left or on
the reactant side.
If K >> 1 (large K value), the
equilibrium lies to the right and the
products predominate in the
equilibrium mixture.
If K << 1 (small K value),
the equilibrium lies to the
left and the reactants
predominate in the
equilibrium mixture.
The Reaction Quotient, Q
What if the concentrations of reactant and
products given are not equilibrium
concentrations?
: If the concentrations given are not
equilibrium concentrations, we can calculate
for the reaction quotient.
What is a reaction quotient, Q
It is the value obtained when product and
reactant concentrations or partial pressures at
any point of the reaction is plugged in the
equilibrium constant expression. It is calculated
in the same way as K. Thus for a general
equilibrium reaction
What is the significance of the reaction
quotient?
The reaction quotient may be used to
determine if a particular reaction is at
equilibrium, and if not, in which direction the
reaction will proceed to attain the equilibrium.
Consider the following example:
How would we know if the reaction is already at
equilibrium?
We can calculate for the reaction quotient and
compare it to the reported equilibrium constant value.
If the two values are equal, then the mixture is already
in equilibrium.
In what direction will the reaction proceed in
order to attain the equilibrium?
To attain equilibrium, the quotient must
decrease to 0.150.
what should happen to the concentrations of
the products to the reactants in order for the
ratio to decrease from 0.500 to 0.150.
This will only happen if the
concentration of NH3 will decrease
and the concentrations of N2 and H2
will increase. Thus, the reaction must
proceed in the backward direction
until equilibrium is attained.
In general:
 If Q = K , then the system is already at
equilibrium;
 If Q > K, the products dominate the reaction
mixture so the products must react to form the
reactants; reaction proceeds in the backward
direction until equilibrium is attained;
 If Q < K, the reactants dominate the reaction
mixture so the reactants must react to form the
products; reaction proceeds in the forward
direction until equilibrium is attained.
THINK-PAIR SHARE
Think of at least two examples each
of a reversible and an irreversible
process that is evident in everyday
situations.
EVALUATION
A. Write the equilibrium constant
expression, Kc, for the following
reactions. Indicate also if the
equilibrium is homogeneous or
heterogeneous
For the following reactions at equilibrium,
identify which between the reactants and
products is dominant.
For the rxn
2CO (g) = C(s) + CO2 (g),
Keq= 7.7 x 10-15. At a particular time
, the following concentrations are
measured:
[CO]= 0.034 M, [CO2]= 3.6 x 10-17 M.
Is this reaction equilibrium? If not
which direction will the reaction
proceed?