Transcript Ch18 Lesson18_5

18.5 Free Energy and Entropy >
Chapter 18
Reaction Rates and Equilibrium
18.1 Rates of Reaction
18.2 The Progress of Chemical
Reactions
18.3 Reversible Reactions
and Equilibrium
18.4 Solubility Equilibrium
18.5 Free Energy and Entropy
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18.5 Free Energy and Entropy >
CHEMISTRY
& YOU
How can a fire start on its own?
Sometimes a fire can
occur without an
external source of
ignition, such as a
match or an electrical
spark. Spontaneous
combustion is the
term used to
describe these fires.
2
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Free Energy and Spontaneous
Reactions
What are two characteristics of
spontaneous reactions?
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Some of the energy released in a
chemical reaction can be harnessed to do
work, such as pushing the pistons in an
internal-combustion engine.
• The energy that is available to do work is
called free energy.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Spontaneous Versus Nonspontaneous
Reactions
You can write a balanced equation for a
chemical reaction, but the reaction may not
actually take place.
CO2(g)  C(s) + O2(g)
• Experience tells you that this reaction does
not tend to occur.
• Carbon and oxygen react to form carbon
dioxide, not the reverse.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Spontaneous Versus Nonspontaneous
Reactions
The world of balanced chemical equations is
really divided into two groups.
• One group contains equations representing
reactions that actually occur.
• The other contains equations representing
reactions that do not tend to occur, or at
least not efficiently.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Spontaneous Versus Nonspontaneous
Reactions
A spontaneous reaction occurs naturally
and favors the formation of products at
the stated conditions.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Spontaneous Versus Nonspontaneous
Reactions
Spontaneous
reactions produce
large amounts of
products and
release free
energy.
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Fireworks displays
are the result of highly
favored spontaneous
reactions.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Spontaneous Versus Nonspontaneous
Reactions
A chemical reaction that does not favor the
formation of products at the stated
conditions is called a nonspontaneous
reaction.
• Such reactions produce little, if any, product.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Reversible Reactions
In nearly all reversible reactions, one
reaction is favored over the other.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Reversible Reactions
Consider the decomposition of carbonic
acid in water.
H2CO3(aq)
<1%
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CO2(g) + H2O(l)
>99%
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Reversible Reactions
Consider the decomposition of carbonic
acid in water.
H2CO3(aq)
<1%
CO2(g) + H2O(l)
>99%
• The forward reaction is spontaneous and
releases free energy.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Reversible Reactions
Consider the decomposition of carbonic
acid in water.
H2CO3(aq)
<1%
CO2(g) + H2O(l)
>99%
• The forward reaction is spontaneous and
releases free energy.
• The combination of carbon dioxide and water
to form carbonic acid is a nonspontaneous
reaction.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Reversible Reactions
When solutions of cadmium nitrate and sodium
sulfide are mixed, the products are aqueous
sodium nitrate and solid yellow cadmium sulfide.
Cd(NO3)2(aq) + Na2S(aq)
CdS(s) + 2NaNO3(aq)
• A precipitate of cadmium
sulfide forms
spontaneously.
• The reverse reaction is
nonspontaneous.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
The Rate of Spontaneous Reactions
The terms spontaneous and
nonspontaneous do not refer to the rate
of a reaction.
• Some spontaneous reactions are so slow
that they appear to be nonspontaneous.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Changing the conditions of a chemical
reaction can affect whether a reaction will
occur.
• A reaction that is nonspontaneous in one
set of conditions may be spontaneous in
other conditions.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Photosynthesis is a multistep reaction that
takes place in plant leaves.
• Outside of plants, carbon dioxide and water do not
normally combine to produce sugar and oxygen.
• This complex process could not happen without the
energy supplied by sunlight and plant pigments such
as chlorophyll.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Coupled Reactions
Sometimes a nonspontaneous reaction
can be made to occur if it is coupled to a
spontaneous reaction.
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18.5 Free Energy and Entropy > Free Energy and
Spontaneous Reactions
Coupled Reactions
Sometimes a nonspontaneous reaction
can be made to occur if it is coupled to a
spontaneous reaction.
• One reaction releases energy that is used
by the other reaction.
• Coupled reactions are common in the
complex biological processes that take
place in living organisms.
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18.5 Free Energy and Entropy >
CHEMISTRY
& YOU
Decomposition reactions that occur inside
a pile of oily rags or a damp stack of hay
cause heat to build up. If the heat cannot
escape, the temperature within the pile or
stack will rise. How can a rise in
temperature cause a fire to start on its
own?
20
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18.5 Free Energy and Entropy >
CHEMISTRY
& YOU
Decomposition reactions that occur inside
a pile of oily rags or a damp stack of hay
cause heat to build up. If the heat cannot
escape, the temperature within the pile or
stack will rise. How can a rise in
temperature cause a fire to start on its
own?
The combustion reaction is a nonspontaneous
reaction that can be made to occur when it is
coupled to the spontaneous decomposition
reaction. The decomposition reaction releases
energy that is used by the combustion reaction.
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18.5 Free Energy and Entropy >
Which of the following is ALWAYS true
of spontaneous reactions?
A. They produce heat and are not reversible at
the stated conditions.
B. They release free energy and favor the
formation of products at the stated conditions.
C. They are coupled with a nonspontaneous
reaction and are easily reversible at the stated
conditions.
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18.5 Free Energy and Entropy >
Which of the following is ALWAYS true
of spontaneous reactions?
A. They produce heat and are not reversible at the
stated conditions.
B. They release free energy and favor the
formation of products at the stated
conditions.
C. They are coupled with a nonspontaneous reaction
and are easily reversible at the stated conditions.
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18.5 Free Energy and Entropy > Entropy
Entropy
What part does entropy play in a
reaction?
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18.5 Free Energy and Entropy > Entropy
You might expect that only exothermic
reactions are spontaneous. Some
processes, however, are spontaneous
even though they absorb heat.
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18.5 Free Energy and Entropy > Entropy
You might expect that only exothermic
reactions are spontaneous. Some
processes, however, are spontaneous
even though they absorb heat.
• Consider what happens as ice melts.
• As it changes from a solid to a liquid, 1 mol
of ice at 25oC absorbs 6.0 kJ of heat from
its surroundings.
• If you consider only enthalpy changes, it is
difficult to explain why the ice melts.
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18.5 Free Energy and Entropy > Entropy
Some factor other than the enthalpy
change must help determine whether a
physical or chemical process is
spontaneous.
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18.5 Free Energy and Entropy > Entropy
Some factor other than the enthalpy
change must help determine whether a
physical or chemical process is
spontaneous.
• The other factor is related to order.
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18.5 Free Energy and Entropy > Entropy
Entropy is a measure of the disorder
of a system.
• The law of disorder states that the natural
tendency is for systems to move in the
direction of increasing disorder or
randomness.
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18.5 Free Energy and Entropy > Entropy
A dog walker with several dogs could represent
relative order and disorder.
This
situation
represents
disorder.
All of the dogs are on
leashes and are
strolling orderly along
the path.
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The dogs are no longer
wearing leashes and
are running freely.
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18.5 Free Energy and Entropy > Entropy
Entropy can affect the direction of a
reaction.
Reactions in which entropy
increases as reactants form
products tend to be favored.
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18.5 Free Energy and Entropy > Entropy
For a given substance, the entropy of the gas is
greater than the entropy of the liquid or the solid.
Thus, entropy increases in reactions in which
solid reactants form liquid or gaseous products.
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18.5 Free Energy and Entropy > Entropy
Entropy increases when a substance is divided
into parts.
• For instance, entropy increases when an
ionic compound dissolves in water.
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18.5 Free Energy and Entropy > Entropy
Entropy tends to increase in chemical reactions
in which the total number of product molecules
is greater than the total number of reactant
molecules.
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18.5 Free Energy and Entropy > Entropy
Entropy tends to increase when the
temperature increases. As the temperature
rises, the molecules move faster and faster,
which increases the disorder.
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18.5 Free Energy and Entropy >
Which of the following would have an
increase in the entropy of the reaction
system?
A. 2NH4NO3(s)  2N2(g) + 4H2O(l) +O2(g)
B. 2H2(g) + O2(g)  2H2O(l)
C. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
D. 2Fe(s) + O2(g) + 2H2O(l)  2Fe(OH)2(s)
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18.5 Free Energy and Entropy >
Which of the following would have an
increase in the entropy of the reaction
system?
A. 2NH4NO3(s)  2N2(g) + 4H2O(l) +O2(g)
B. 2H2(g) + O2(g)  2H2O(l)
C. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
D. 2Fe(s) + O2(g) + 2H2O(l)  2Fe(OH)2(s)
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18.5 Free Energy and Entropy > Enthalpy and Entropy
Enthalpy and Entropy
What two factors determine
whether a reaction is
spontaneous?
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18.5 Free Energy and Entropy > Enthalpy and Entropy
The size and direction of enthalpy
changes and entropy changes
together determine whether a
reaction is spontaneous.
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18.5 Free Energy and Entropy > Enthalpy and Entropy
Consider an exothermic reaction in which
entropy increases.
• The reaction will be spontaneous because
both factors are favorable.
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18.5 Free Energy and Entropy > Enthalpy and Entropy
A reaction can be spontaneous if:
• A decrease in entropy is offset by a large
release of heat.
• An increase in enthalpy is offset by an
increase in entropy.
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18.5 Free Energy and Entropy > Enthalpy and Entropy
The table below summarizes the effect of enthalpy
and entropy changes on the spontaneity of reactions.
How Enthalpy Changes and Entropy Changes
Affect Reaction Spontaneity
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Enthalpy change
Entropy change
Is the reaction spontaneous?
Decreases
(exothermic)
Increases (more disorder in
Yes
products than in reactants)
Only if unfavorable enthalpy
change is offset by
favorable entropy change
Increases
(endothermic)
Increases
Decreases
(exothermic)
Only if unfavorable entropy
Decreases (less disorder in
change is offset by
products than in reactants)
favorable enthalpy change
Increases
(endothermic)
Decreases
No
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18.5 Free Energy and Entropy >
Would the following exothermic
reaction be spontaneous? Explain
why or why not.
2KClO3(s)
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2KCl(s) +3O2(g)
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18.5 Free Energy and Entropy >
Would the following exothermic
reaction be spontaneous? Explain
why or why not.
2KClO3(s)
2KCl(s) +3O2(g)
Two molecules of solid are transformed into
2 molecules of solid and 3 molecules of
gas, so entropy is increased in the
reaction. A reaction that is exothermic with
an increase in entropy will be spontaneous.
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18.5 Free Energy and Entropy > Free Energy Change
Free Energy Change
How is the value of ΔG related to
the spontaneity of a reaction?
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18.5 Free Energy and Entropy > Free Energy Change
Free energy is often expressed as Gibbs
free energy.
• This term is named for Josiah Gibbs, the
scientist who defined this thermodynamic
property.
• The symbol for Gibbs free energy is G.
• Free energy can either be released or
absorbed during a physical or chemical
process.
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18.5 Free Energy and Entropy > Free Energy Change
The equation below is used to calculate
the change in Gibbs free energy (ΔG).
• ΔS is the change in entropy.
• ΔH is the change in enthalpy.
• T is the temperature in Kelvins.
ΔG = ΔH – TΔS
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18.5 Free Energy and Entropy > Free Energy Change
When the value of ΔG is negative,
the process is spontaneous. When
the value is positive, the process
is nonspontaneous.
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18.5 Free Energy and Entropy >
The entropy change for the following
reaction at 298 K is 3.0 J/mol·K, and the
enthalpy change is –394 kJ/mol.
C(s) + O2(g)
CO2(g)
Calculate the Gibbs free energy change
and determine whether the reaction will
occur spontaneously.
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18.5 Free Energy and Entropy >
The entropy change for the following
reaction at 298 K is 3.0 J/mol·K, and the
enthalpy change is –394 kJ/mol.
C(s) + O2(g)
CO2(g)
Calculate the Gibbs free energy change
and determine whether the reaction will
occur spontaneously.
ΔG = –394 kJ/mol – (298 K  0.0030 kJ/mol·K)
ΔG = –395 kJ/mol
50
The reaction is spontaneous.
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18.5 Free Energy and Entropy > Key Concepts
Spontaneous reactions produce large
amounts of products and release free energy.
Reactions in which entropy increases as
reactants form products tend to be favored.
The size and direction of enthalpy changes
and entropy changes together determine
whether a reaction is spontaneous.
When the value of ΔG is negative, a process
is spontaneous. When the value is positive, a
process is nonspontaneous.
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18.5 Free Energy and Entropy > Key Equation
ΔG = ΔH – TΔS
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18.5 Free Energy and Entropy > Glossary Terms
• free energy: the energy available to do
work
• spontaneous reaction: a reaction that
favors the formation of products at the
specified conditions; spontaneity
depends on enthalpy and entropy
changes
• nonspontaneous reaction: a reaction
that does not favor the formation of
products at the specified conditions
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18.5 Free Energy and Entropy > Glossary Terms
• entropy: a measure of the disorder of a
system; systems tend to go from a state
of order (low entropy) to a state of
maximum disorder (high entropy)
• law of disorder: it is a natural tendency
of systems to move in the direction of
maximum chaos or disorder
54
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18.5 Free Energy and Entropy >
BIG IDEA
Chemical Reactions, Matter, and Energy
Changes in enthalpy and entropy can be
used to explain why some reactions occur
naturally and others do not.
55
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18.5 Free Energy and Entropy >
END OF 18.5
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