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Chemical Equilibrium
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 (cont.)
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.
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.
Types of chemical reactions
Nonreversible chemical reactions: the
reaction proceeds until all of the reactants
are used up and the products are formed
A+B
C+D
• Nonreversible reactions often produce
– a gas that escapes
– Insoluble solid product (precipitate)
• Here, there is likely a limiting reactant
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)
What is equilibrium?
• Chemical reactions often reach a balancing
point, or equilibrium.
•When the rate of
the forward and the
reverse reactions
are the same
•http://ull.chemistry.uakron.edu/genobc/Chapter_08/
A saturated solution in equilibrium
• Start with a beaker of solid
RADIOACTIVE NaI at the bottom
• Pour a saturated solution of nonradioactive NaI into the beaker
• This solution is already saturated—it already
holds the max amount of NaI possible
• Wait several hours
• Solid is filtered out and the solution is
tested. It is radioactive! What does
this tell us?
•http://dbhs.wvusd.k12.ca.us/webdocs/Equilibrium/Dynamic-Equilibrium.html
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.
• The reaction reaches equilibrium in
figure d.
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.
Equilibrium in chemical reactions
• Being in equilibrium does not imply
that 50% of the amount present is
reactant and 50% is product
• There may be more total moles of
product or more total moles of reactant
A+B
C + D
The longer arrow implies that there is more reactant
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.
The Equilibrium Constant Expression
Consider the reaction: aA + bB  cC + dD
Equilibrium Expressions (cont.)
• The equilibrium constant, Keq, is the
numerical value of the ratio of product
concentrations to reactant concentrations,
with each concentration raised to the
power equal to its coefficient in the
balanced equation.
• 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
Z + 2B
2C + X
(all substances are gases at the same conditions)
Concentrations are as follows: Z= 0.75M,
B = 1.25M, C = 1.1M, X = 1.0M
•Keq =
[Z]a ·[B]b
[C]c
·[X]d
•[1.1]2 ·[1.0] =
•1.0
2
[0.75] ·[1.25]
•What does the Keq value of 1.0 represent?
•Relative amounts of product and reactant are equal since
the equilbrium constant is equal to 1.0!
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.
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).
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 Assessment
A reaction is in equilibrium when:
A. there are more products than reactants
B. the amount of products equals
the reactants
D
C
0%
A
D. the rate of the forward and
reverse reactions are equal
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. the rate of the forward reaction
is greater than the reverse
reaction
Section 17.1 Assessment
The value of the equilibrium constant is
constant for a given ____.
A. temperature
B. pressure
D
A
0%
C
D. density
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. volume
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.
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.
Henri Le Chatelier (1850-1936) was
interested in the relationship between science
and industry. He wanted to find a way to
maximize the yield that can be obtained from
any chemical reaction.
The Industrial Revolution was taking off and
this led to the need for the production of all
kinds of chemicals.
• Intro to LeChats
Le Châtelier’s Principle (cont.)
• Principle #1 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.
Le Châtelier’s Principle (cont.)
• The equilibrium shifts to the right to
produce more products.
• Stress is relieved by shifting to the left,
converting products to reactants.
Le Châtelier’s Principle (cont.)
• Principle#2 Increasing pressure shifts the
system to the left, 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.
• 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.
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.
Le Châtelier’s Principle (cont.)
• Principle #3 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.
Ex.
2SO2 + O2----->2SO3 + heat
In this exothermic rxn heat is a product.
Le Châtelier’s Principle (cont.)
• Any change in temperature results in a
change in Keq.
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 Assessment
Which does NOT result in a shift of the
equilibrium to the right?
A. removing products
B. adding reactants
C. increasing concentration of reactants A. A
D
C
A
0%
B. B
C. C
0%
0%
0%
D. D
B
D. adding products
Section 17.2 Assessment
Any change in ____ results in a change
in Keq.
A. temperature
B. pressure
D
A
0%
C
D. concentration
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. volume
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 (cont.)
solubility product constant
common ion
common ion effect
Equilibrium constant expressions can
be used to calculate concentrations
and solubilities.
Calculating Equilibrium Concentrations
• Equilibrium constants can be used to
calculate unknown concentrations of
products when other concentrations are
known.
• 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.
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.
The Solubility Product Constant (cont.)
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.
The Solubility Product Constant (cont.)
• Use the initial concentrations of ions in
solution in the solubility product constant
expression to calculate Qsp.
• If Qsp < Ksp the solution is unsaturated and no
precipitate will form.
• If Qsp = Ksp the solution is saturated and no
change will occur.
The Solubility Product Constant (cont.)
• 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.
The Solubility Product Constant (cont.)
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.
Common Ion Effect
Equilibrium constants provide many practical
applications.
BaSO4 is an insoluble salt that has medical
applications. Doctors use it as an X-ray
contrast medium (Ba absorbs more radiation
than most atoms).
Ba ions are poisonous and in order to make
sure they are not digested, a soluble salt like
Na2SO4 is given with the Ba.
As SO4 ions increase the Ba ions must
decrease-this is the common ion effect.
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 Assessment
The presence of a common ion ____ the
solubility of the dissolved substance.
A. decreases
B. increases
D
A
0%
C
D. speeds up
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. does not change
Section 17.3 Assessment
If Qsp > Ksp
A. the solution is unsaturated and
no precipitate will form
D
D. a common ion must be present
A
0%
C
C. a precipitate will form, reducing
the concentrations of the ions in
the solution
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. the solution is saturated and
no precipitate will form
Chemistry Online
Study Guide
Chapter Assessment
Standardized Test Practice
Image Bank
Concepts in Motion
Section 17.1 A State of
Dynamic Balance
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
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
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.
A change in ____ alters both the
equilibrium position and the equilibrium
constant.
A. pressure
A
0%
D
D. density
C
C. volume
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. temperature
A(n) ____ equilibrium is when all reactant
and products are in the same physical
state.
A. endothermic
A
0%
D
D. homogeneous
C
C. heterogeneous
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. exothermic
Which is NOT an example of a
homogeneous equilibria?
A. H2(g) +I2(g) ↔ 2HI(g)
B. 2H2(g) +O2(g) ↔ 2H2OI(g)
D
A
0%
C
D. C2H5OH(l) ↔ C2H5OH(g)
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. HCl(aq) + NaOH(aq) ↔
NaCl(aq) + H2O
Barium sulfate is toxic to humans, so why
can it be ingested for use with
gastrointestinal x-rays?
A. It is minimally poisonous.
A
0%
D
D. Your body readily absorbs it.
C
C. It has low solubility.
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. It has high solubility.
The state in which the forward and
reverse chemical reactions take place at
the same rate is called ____.
A. Le Châtelier’s Principle
A
0%
D
D. chemical equilibrium
C
C. Charles’s Law
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. Henry’s Law
Which does NOT describe a system that
has reached chemical equilibrium?
A. No new product is formed by the
forward reactions.
D. All the reactants have been used up.
D
A
0%
C
C. The concentration of products
is equal to the concentration
of reactants.
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. The forward and reverse
reactions occur at equal rates.
Adding product to a chemical equilibrium:
A. does nothing
B. creates a stress and shifts
the equilibrium to the right
D
A
0%
C
D. causes more product to form
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. creates a stress and shifts
the equilibrium to the left
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.
A
0%
D
D. Decrease the C2H6.
C
C. Increase the C2H4.
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. Decrease the heat.
Which does NOT result in a shift of the
equilibrium to the right?
A. removing products
B. adding reactants
D
A
0%
C
D. decreasing the concentration
of reactants
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. increasing concentration
of reactants
A(n) ____ equilibrium is when all reactant
and products are present in more than
one physical state.
A. endothermic
A
0%
D
D. homogeneous
C
C. heterogeneous
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. exothermic