CHEMISTRY
Matter and Change
Chapter 17: Chemical Equilibrium
CHAPTER
Table Of Contents
17
Section 17.1
A State of Dynamic Balance
Section 17.2
Factors Affecting Chemical
Equilibrium
Section 17.3
Using Equilibrium Constants
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SECTION
17.1
A State of Dynamic Balance
• List the characteristics of chemical equilibrium.
• Write equilibrium expressions for systems that are at
equilibrium.
• Calculate equilibrium constants from concentration
data.
free energy: the energy that is available to do work—the
difference between the change in enthalpy and the product
of the entropy change and the absolute temperature
SECTION
17.1
A State of Dynamic Balance
reversible reaction
chemical equilibrium
law of chemical equilibrium
equilibrium constant
homogeneous equilibrium
heterogeneous equilibrium
Chemical equilibrium is described by an
equilibrium constant expression that relates
the concentrations of reactants and products.
SECTION
17.1
A State of Dynamic Balance
What is equilibrium?
• Chemical reactions often reach a balancing point,
or equilibrium.
Forward: N2(g) + 3H2(g)  2NH3(g)
Reverse: N2(g) + 3H2(g)  2NH3(g)
SECTION
17.1
A State of Dynamic Balance
What is equilibrium? (cont.)
• A reversible reaction is a chemical
reaction that can occur in both the forward
and reverse directions, such as the
formation of ammonia.
N2(g) + 3H2(g)
2NH3(g)
SECTION
17.1
A State of Dynamic Balance
What is equilibrium? (cont.)
• How does reversibility affect the production of
ammonia?
• Decreases in the concentrations of N2 and H2
cause the reaction to slow.
• As soon as ammonia is present, the reverse
reaction can occur, slowly at first, but at an
increasing rate as the concentration of ammonia
increases.
SECTION
17.1
A State of Dynamic Balance
What is equilibrium? (cont.)
• This continues until the two rates, the
forward slowing and reverse increasing,
are equal
• At that point, the system has reached a
state of equilibrium, figure d.
SECTION
17.1
A State of Dynamic Balance
What is equilibrium? (cont.)
• Chemical equilibrium is a state in which the
forward and reverse reactions balance each other
because they take place at equal rates.
• Equilibrium is a state of action, not inaction.
SECTION
17.1
A State of Dynamic Balance
Equilibrium Expressions
• Some chemical systems have little
tendency to react, others go to completion.
• The majority reach a state of equilibrium with
some of the reactants unconsumed.
SECTION
17.1
A State of Dynamic Balance
Equilibrium Expressions (cont.)
• The law of chemical equilibrium states that at a
given temperature, a chemical system might reach
a state in which a particular ratio of reactant and
product concentrations has a constant value.
SECTION
17.1
A State of Dynamic Balance
Equilibrium Expressions (cont.)
• The value of Keq is constant only at a
specified temperature.
Keq > 1: Products are favored at equilibrium
Keq < 1: Reactants are favored at equilibrium
SECTION
17.1
A State of Dynamic Balance
Equilibrium Expressions (cont.)
H2(g) +I2(g)
2HI(g)
• This reaction is a homogeneous
equilibrium, which means that all the
reactants and products are in the same
physical state.
SECTION
17.1
A State of Dynamic Balance
Equilibrium Expressions (cont.)
• When the reactants and products are
present in more than one physical state,
the equilibrium is called a heterogeneous
equilibrium.
• Ethanol in a closed
flask is represented
by C2H5OH(l)
C2H5OH(g).
SECTION
17.1
A State of Dynamic Balance
Equilibrium Constants
• For a given reaction at a given temperature, Keq
will always be the same regardless of the initial
concentrations of reactants and products.
SECTION
17.1
Section Check
A reaction is in equilibrium when:
A. there are more products than reactants
B. the amount of products equalsthe reactants
C. the rate of the forward reaction is greater
than the reverse reaction
D. the rate of the forward and reverse reactions
are equal
SECTION
17.1
Section Check
The value of the equilibrium constant is
constant for a given ____.
A. temperature
B. pressure
C. volume
D. density
SECTION
17.2
Factors Affecting Chemical Equilibrium
• Describe how various
factors affect chemical
equilibrium.
• Explain how Le
Châtelier’s principle
applies to equilibrium
systems.
reaction rate: the change
in concentration of a
reactant or product per
unit time, generally
calculated and expressed
in moles per liter per
second.
Le Châtelier’s principle
When changes are made to a system at
equilibrium, the system shifts to a new
equilibrium position.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle
• Le Châtelier’s Principle was proposed in
1888 and states that if stress is applied to a
system at equilibrium, the system shifts in
the direction that relieves the stress.
• Stress is any kind of change in a system that
upsets the equilibrium.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle (cont.)
• Adjusting the concentrations of either the
reactants or the products puts stress on a
system in equilibrium.
• Adding reactants increases the number of
effective collisions between molecules and
upsets the equilibrium.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle (cont.)
• The equilibrium shifts to the right to
produce more products.
• The addition or removal of a reactant or
product shifts the equilibrium in the direction
that relieves the stress.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle (cont.)
• Increasing pressure shifts the system to
the right, and more products are formed.
• Changing the volume (and pressure) of an
equilibrium system shifts the equilibrium only
if the number of moles of gaseous reactants
is different from the moles of gaseous
products.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle (cont.)
• If the number of moles is the same on both sides
of the balanced equation, changes in pressure
and volume have no effect on the equilibrium.
• When the volume of the reaction vessel is
decreased, the equilibrium position shifts
towards whichever side has fewer total moles of
gases.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle (cont.)
The reaction
between CO and H2
is at equilibrium.
Lowering the piston
decreases the volume
and increases the
pressure.
As a result, more molecules
of the products form. Their
formation relieves the stress
on the system.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle (cont.)
• Changes in temperature alter the
equilibrium position and the equilibrium
constant.
• If heat is added to an equilibrium system, the
equilibrium shifts in the direction in which the
heat is used up.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle (cont.)
• Any change in temperature results in a
change in Keq.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Le Châtelier’s Principle (cont.)
• A catalyzed reaction reaches equilibrium
more quickly, but with no change in the
amount of product formed.
SECTION
17.2
Section Check
Which does NOT result in a shift of
the equilibrium to the right?
A. removing products
B. adding reactants
C. increasing concentration of reactants
D. adding products
SECTION
17.2
Section Check
Any change in ____ results in a change
in Keq.
A. temperature
B. pressure
C. volume
D. concentration
SECTION
17.3
Using Equilibrium Constants
• Determine equilibrium
concentrations of
reactants and products.
• Calculate the solubility
of a compound from its
solubility product
constant.
• Explain the common ion
effect.
solubility: the
maximum amount of
solute that will dissolve
in a given amount of
solvent at a specific
temperature and
pressure
SECTION
17.3
Using Equilibrium Constants
solubility product constant
common ion
common ion effect
Equilibrium constant expressions can
be used to calculate concentrations
and solubilities.
SECTION
17.3
Using Equilibrium Constants
Calculating Equilibrium Concentrations
• Equilibrium constants can be used to calculate
unknown concentrations of products when other
concentrations are known.
or
• The equilibrium concentration for CH4 is
27.7mol/L
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant
• Some ionic compounds dissolve readily in
water, and some barely dissolve at all.
• The equilibrium constant expression for the
dissolving of a sparingly soluble compound is
called the solubility product constant, Ksp.
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant (cont.)
• The solubility product constant expression
is the product of the concentrations of the
dissolved ions, each raised to the power
equal to the coefficient of the ion in the
chemical equation.
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant (cont.)
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant (cont.)
• The values in the table can be used to
determine the solubility of a sparingly soluble
compound.
• Ksp can be used to predict whether a
precipitate will form when any two ionic
solutions are mixed.
• Will a precipitate form in the above doublereplacement reaction?
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant (cont.)
• In the above reaction, a precipitate is likely to form
only if either product, KCl or Fe4(Fe(CN)6)3 has
low solubility.
–KCl has a Ksp= 21.7
–Fe4(Fe(CN)6)3 has a Ksp = 3.3 x 10-41
• So precipitate from KCl would be unlikely but for
iron(III)ferrocyanide there is a possibility if the
concentrations of its ions are large enough.
• How large is large enough?
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant (cont.)
• First, you must calculate the concentrations of
the ions.
• The table shows the
concentration of the ions of
reactants and products in the
original solutions and in the
mixture immediately after equal
volumes of the two solutions
were mixed.
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant (cont.)
• Now you can use the data in the table to make a
trial to see if the concentrations of Fe3+ and
Fe(CN)64- in the mixed solution exceed the value
of Ksp when substituted into the solubility
product constant expression.
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant (cont.)
• When you make the substitution,
it will not necessarily give the
solubility product constant.
Instead, it provides a number
called the ion product (Qsp).
Qsp is a trial value that can be
compared with Ksp.
SECTION
17.3
Using Equilibrium Constants
The Solubility Product Constant (cont.)
• Now you can compare Qsp and Ksp.
– Qsp = 7.8 x 10-10 and Ksp = 3.3 x 10-41
• The outcome can have one of three outcomes:
– If Qsp < Ksp the solution is unsaturated and no
precipitate will form.
– If Qsp = Ksp the solution is saturated and no
change will occur.
– If Qsp > Ksp a precipitate will form, reducing the
concentrations of the ions in the solution until the
product of their concentrations in the Ksp
expression equals the numerical value of Ksp.
SECTION
17.3
Using Equilibrium Constants
The Common Ion Effect
• Why is PbCrO4 less
soluble in aqueous
solution of K2CrO4 than
in pure water?
• The K2CrO4 solution
contains CrO42– ions
before any PbCrO4
dissolves.
SECTION
17.3
Using Equilibrium Constants
The Common Ion Effect (cont.)
• A common ion is an ion that is common to
two or more ionic compounds.
• The lowering of the solubility of a substance
because of the presence of a common ion is
called the common ion effect.
SECTION
17.3
Section Check
The presence of a common ion ____ the
solubility of the dissolved substance.
A. decreases
B. increases
C. does not change
D. speeds up
SECTION
17.3
Section Check
If Qsp > Ksp ____.
A. the solution is unsaturated and no precipitate
will form
B. the solution is saturated and no precipitate
will form
C. a precipitate will form, reducing the
concentrations of the ions in the solution
D. a common ion must be present
CHAPTER
Chemical Equilibrium
17
Resources
Chemistry Online
Study Guide
Chapter Assessment
Standardized Test Practice
SECTION
17.1
A State of Dynamic Balance
Study Guide
Key Concepts
• A reaction is at equilibrium when the rate of the forward
reaction equals the rate of the reverse reaction.
• The equilibrium constant expression is a ratio of the molar
concentrations of the products to the molar concentrations
of the reactants with each concentration raised to a power
equal to its coefficient in the balanced chemical equation.
• The value of the equilibrium constant expression, Keq, is
constant for a given temperature.
SECTION
17.2
Factors Affecting Chemical Equilibrium
Study Guide
Key Concepts
• Le Châtelier’s principle describes how an equilibrium
system shifts in response to a stress or a disturbance.
• When an equilibrium shifts in response to a change in
concentration or volume, the equilibrium position
changes but Keq remains constant. A change in
temperature, however, alters both the equilibrium
position and the value of Keq.
SECTION
17.3
Using Equilibrium Constants
Study Guide
Key Concepts
• Equilibrium concentrations and solubilities can be
calculated using equilibrium constant expressions.
• Ksp describes the equilibrium between a sparingly
soluble ionic compound and its ions in solution.
• If the ion product, Qsp, exceeds the Ksp when two
solutions are mixed, a precipitate will form.
• The presence of a common ion in a solution lowers the
solubility of a dissolved substance.
CHAPTER
17
Chemical Equilibrium
Chapter Assessment
A change in ____ alters both the
equilibrium position and the equilibrium
constant.
A. pressure
B. temperature
C. volume
D. density
CHAPTER
17
Chemical Equilibrium
Chapter Assessment
A(n) ____ equilibrium is when all reactants
and products are in the same physical
state.
A. endothermic
B. exothermic
C. heterogeneous
D. homogeneous
CHAPTER
17
Chemical Equilibrium
Chapter Assessment
Which is NOT an example of a
homogeneous equilibria?
A. H2(g) +I2(g) ↔ 2HI(g)
B. 2H2(g) +O2(g) ↔ 2H2OI(g)
C. HCl(aq) + NaOH(aq) ↔
NaCl(aq) + H2O
D. C2H5OH(l) ↔ C2H5OH(g)
CHAPTER
17
Chemical Equilibrium
Chapter Assessment
Barium sulfate is toxic to humans, so why
can it be ingested for use with
gastrointestinal x-rays?
A. It is minimally poisonous.
B. It has high solubility.
C. It has low solubility.
D. Your body readily absorbs it.
CHAPTER
17
Chemical Equilibrium
Chapter Assessment
The state in which the forward and
reverse chemical reactions take place at
the same rate is called ____.
A. Le Châtelier’s Principle
B. Henry’s Law
C. Charles’s Law
D. chemical equilibrium
CHAPTER
17
Chemical Equilibrium
Standardized Test Practice
Which does NOT describe a system that has
reached chemical equilibrium?
A. No new product is formed by the forward
reactions.
B. The forward and reverse reactions occur at
equal rates.
C. The concentration of products is equal to the
concentration of reactants.
D. All the reactants have been used up.
CHAPTER
17
Chemical Equilibrium
Standardized Test Practice
A(n) ____ equilibrium is when reactants
and products are present in more than
one physical state.
A. endothermic
B. exothermic
C. heterogeneous
D. homogeneous
CHAPTER
17
Chemical Equilibrium
Standardized Test Practice
Adding product to a chemical equilibrium ____.
A. does nothing
B. creates a stress and shifts the equilibrium to
the right
C. creates a stress and shifts the equilibrium to
the left
D. causes more product to form
CHAPTER
17
Chemical Equilibrium
Standardized Test Practice
Ethylene (C2H4) reacts with hydrogen to
form ethane (C2H6). C2H4(g) + H2(g) ↔
C2H6(g) + heat. How could you increase
the amount of hydrogen in the system?
A. Increase the heat.
B. Decrease the heat.
C. Increase the C2H4.
D. Decrease the C2H6.
CHAPTER
17
Chemical Equilibrium
Standardized Test Practice
Which does NOT result in a shift of the
equilibrium to the right?
A. removing products
B. adding reactants
C. increasing concentration of reactants
D. decreasing the concentration of reactants
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