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Kinetics & Thermochemistry Lesson # 6

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Topic: Entropy
Inquiry Question: What are the characteristics of a spontaneous reaction?
I can… use the concept of entropy to predict the spontaneity of a reaction.
Think & Answer
1. What does the word spontaneous mean?
2. What are some example of things that happen
spontaneously? (Where the reverse process does not
happen spontaneously)
3. Whish is more likely to occur more spontaneously? An
exothermic reaction or an endothermic reaction?
Conditions and Equilibrium
• A system at equilibrium will remain so as
long as conditions remain the same.
• However, when conditions are changed,
equilibrium is disrupted.
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.
3
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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.
4
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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.
5
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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.
6
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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.
7
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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.
8
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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.
9
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18.5 Free Energy and Entropy > Entropy
Entropy
What part does entropy play in a
reaction?
10
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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.
11
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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.
12
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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.
13
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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.
14
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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.
15
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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.
16
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.
17
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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.
18
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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.
19
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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.
20
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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.
21
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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)
22
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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)
23
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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?
24
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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.
25
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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.
26
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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.
27
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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
28
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)
29
2KCl(s) +3O2(g)
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