Topic 7- Equilibrium
http://www.chem1.com/acad/webtext/chemeq/Eq02.html
•
We can distinguish between reactions that go to completion
and those that reach equilibrium.
a)Reactions that aren't reversible are represented by equations
that contain a single arrow.
2 Mg(s) + O2(g) => 2 MgO(s)
b) Reversible reactions that reach equilibrium are indicated by a
pair of arrows between the two sides of the equation.
ClNO2(g) + NO(g) < = > NO2(g) + ClNO(g)
Equilibrium Systems

A state of balance exists between the products
and the reactants

When the concentration of neither the
reactants nor the products is changing, the
system is in equilibrium.

Dynamic equilibrium
rate of forward reaction = rate of reverse reaction.
3

Once the equilibrium was attained, the composition
of the mixture is constant
Animation:
http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/kim2s2
_5.swf
H2+ I2 => 2HI
2HI => H2+ I2
Vapor Pressure

The process of evaporation in a closed container will proceed until there are
as many molecules returning to the liquid as there are escaping. At this point
the vapor is said to be saturated, and the pressure of that vapor (usually
expressed in mmHg) is called the saturated vapor pressure.

Vapor pressure is the pressure caused by a liquid's own vapor.
http://www.pkwy.k12.mo.us/west/teachers/anderson/pack5/boil/boil.html
5
Evaporation & Condensation

When placed in a closed container, water does evaporate until the air in
the container is saturated with water vapor. When the air is saturated with
water vapor, the molecules in the vapor condense to a liquid as fast as the
liquid evaporates, and the two processes (evaporation and condensation)
continue at equal rates.
Equilibrium Conditions
H2O + CO


H2 + CO2
7

Given the reaction at equilibrium,
aA + bB  cC + dD

At equilibrium these rates are equal
rate forward = rate reverse

The ratio of the rate constants is known as
the equilibrium constant, Kc
Kc
=
[C]c [D]d
[A]a [B]b
1
Writing Equilibrium Expressions
a.
N2 (g) + H2 (g)
b.
SO2 (g) +
c.
H2 (g) +
O2 (g)
Br2 (g)





NH3 (g)
SO3 (g)

HBr(g)
11
Equilibrium Constant x Reaction
Quotient

The equilibrium constant is a constant ratio
only when the system is in equilibrium.

If the system it not at equilibrium the ratio is
known as a Reaction Quotient

If the reaction quotient is equal to the
equilibrium constant then the system is at
equilibrium
13

If Q > K, the products are present in larger amounts as
compared to those present at the equilibrium. Therefore, the
reaction will proceed to the reverse direction (reactant side)
in order to attain equilibrium.

If Q = K, the concentration of reactants and products are
equal to those at the equilibrium.Hence the reaction is at
equilibrium.

If Q < K, the products are present in lesser amounts as
compared to those present at the equilibrium. The reaction
therefore will proceed to the forward direction, (products
side) in order to attain equilibrium.
Practice
For the reaction
H2(g) + I2(g) 2 HI(g)

Kc = 60
For each of the following sets of concentrations, determine
whether the reaction is at equilibrium. If it isn't, decide in
which direction it must go to reach equilibrium.
(a) (H2) = (I2) = (HI) = 0.010 M
(b) (HI) = 0.30 M; (H2) = 0.01 M; (I2) = 0.15 M
(c) (H2) = (HI) = 0.10 M; (I2) = 0.0010 M
Types of Equilibrium

The equilibrium between different chemical species present in the same or different phases is called chemical
equilibrium. There are two types of chemical equilibrium.

(1) Homogeneous equilibrium : The equilibrium reactions in which all the reactants and the products are in
the same phase are called homogeneous equilibrium reactions.
C2H5OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)
2SO2(g) + O2(g) 2SO3(g)

(2) Heterogeneous equilibrium : The equilibrium reactions in which the reactants and the products are
present in different phases are called heterogeneous equilibrium reactions.
2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(g)
Le Chatelier

http://www.cdli.ca/courses/chem3202/unit01_org03_ilo03/b_activity.html

http://www.wwnorton.com/college/chemistry/gilbert2/tutorials/interface.asp?chapt
er=chapter_15&folder=le_chateliers
Le Chatelier’s Principle
◦ Le Chatelier's Principle states: When a
system in chemical equilibrium is
disturbed by a change of temperature,
pressure, or a concentration, the system
shifts in equilibrium composition in a
way that tends to counteract this
change of variable.
◦ A change imposed on an equilibrium
system is called a stress
◦ The equilibrium always responds in such
a way so as to counteract the stress
18
Applications of
Le Chatelier’s Principle
N2 (g) +3 H2 (g)
 2NH (g) DH = -92 kJ
3

Haber’s process for the production of ammonia is an
example of an industrial equilibrium system. We will
use this equilibrium as a model to explain the how Le
Chatelier’s principle operates with the following
stresses:
• Change in the concentration of one of the components
• Changes in pressure
• Changes in temperature
• Use of a catalyst
19