Slide 1 of 28 Chemistry 18.4. © Copyright Pearson Prentice Hall Slide 2 of 28 18.4 Entropy and Free Energy Inside a pile of oily rags or a stack of hay

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Chemistry 18.4

© Copyright Pearson Prentice Hall

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18.4 Entropy and Free Energy

Inside a pile of oily rags or a stack of hay that has not been thoroughly dried, decomposition causes heat to build up. When heat cannot escape, the temperature can become high enough to cause a fire. You will learn about the conditions that will produce a spontaneous chemical reaction.


Entropy and Free Energy >

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Free Energy and SpontaneousReactions

Free Energy and Spontaneous Reactions

What are two characteristics of spontaneous reactions?

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>Entropy and Free Energy Free Energy and SpontaneousReactions

A spontaneous reaction occurs naturally and favors the formation of products at the specified conditions.

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>Entropy and Free Energy Free Energy and SpontaneousReactions

A nonspontaneous reaction is a reaction that does not favor the formation of products at the specified conditions.

Photosynthesis is a nonspontaneous reaction that requires an input of energy.

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>Entropy and Free Energy18.4 Free Energy and SpontaneousReactions

Spontaneous reactions produce substantial amounts of products at equilibrium and release free energy.

Free energy is energy that is available to do work.



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Entropy

Entropy

What part does entropy play in chemical reactions?

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>Entropy and Free Energy Entropy

Entropy is a measure of the disorder of a system.

• Physical and chemical systems attain the lowest possible energy.

• The law of disorder states that the natural tendency is for systems to move in the direction of maximum disorder or randomness.

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Entropy

An increase in entropy favors the spontaneous chemical reaction; a decrease favors the nonspontaneous reaction.

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For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid.

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Entropy increases when a substance is divided into parts.

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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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Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

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Enthalpy, Entropy, and Free Energy


What two factors determine the spontaneity of a reaction?

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The size and direction of enthalpy changes and entropy changes together determine whether a reaction is spontaneous; that is, whether it favors products and releases free energy.

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>Entropy and Free Energy Enthalpy, Entropy, and Free Energy18.4



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Simulation 25

Simulate how enthalpy and entropy changes combine to determine the spontaneity of a reaction.



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Gibbs Free-Energy

Gibbs Free-Energy

Is the Gibbs free-energy change positive or negative in a spontaneous process?

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>Entropy and Free Energy Gibbs Free-Energy

The Gibbs free-energy change is the maximum amount of energy that can be coupled to another process to do useful work.

The numerical value of ΔG is negative in spontaneous processes because the system loses free energy.

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>Entropy and Free Energy Gibbs Free-Energy18.4

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>Entropy and Free Energy Gibbs Free-Energy

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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

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18.4 Section Quiz.

1. Free energy from a reaction is the amount of energy that is

a. absorbed by an entropy decrease.

b. equal to the enthalpy change.

c. wasted as heat.

d. available to do work.


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18.4 Section Quiz.

2. Free energy is always available from reactions that are

a. endothermic.

b. nonspontaneous.

c. at equilibrium.

d. spontaneous.


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18.4 Section Quiz.

3. Choose the correct words for the spaces: Spontaneous reactions produce ________ and substantial amounts of _________ at equilibrium.

a. free energy, products

b. no free energy, reactants

c. free energy, reactants

d. no free energy, products


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18.4 Section Quiz.

4. Which of the following involves a decrease in entropy?

a. Natural gas burns.

b. A liquid freezes.

c. Dry ice sublimes.

d. Water evaporates.


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18.4 Section Quiz.

5. A reaction is spontaneous if

a. enthalpy decreases and entropy increases.

b. enthalpy increases and entropy increases.

c. enthalpy decreases and entropy decreases.

d. enthalpy increases and entropy decreases.


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18.4 Section Quiz.

6. Choose the correct words for the spaces: Gibbs free-energy change is the _________ amount of energy that can be ___________ another process to do useful work.

a. maximum, coupled to

b. maximum, duplicated by

c. spontaneous, coupled to

d. minimum, duplicated by

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Slide 1 of 28 Chemistry 18.4. © Copyright Pearson Prentice Hall Slide 2 of 28 18.4 Entropy and Free Energy Inside a pile of oily rags or a stack of hay