slide 1 of 28 chemistry 18.4. © copyright pearson prentice hall slide 2 of 28 18.4 entropy and free...
TRANSCRIPT
© 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 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.
© Copyright Pearson Prentice Hall
Entropy and Free Energy >
Slide 3 of 28
Free Energy and SpontaneousReactions
Free Energy and Spontaneous Reactions
What are two characteristics of spontaneous reactions?
18.4
Slide 4 of 28
© Copyright Pearson Prentice Hall
>Entropy and Free Energy Free Energy and SpontaneousReactions
A spontaneous reaction occurs naturally and favors the formation of products at the specified conditions.
18.4
Slide 5 of 28
© Copyright Pearson Prentice Hall
>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.
18.4
Slide 6 of 28
© Copyright Pearson Prentice Hall
>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.
© Copyright Pearson Prentice Hall
Entropy and Free Energy >
Slide 7 of 28
Entropy
Entropy
What part does entropy play in chemical reactions?
18.4
Slide 8 of 28
© Copyright Pearson Prentice Hall
>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.
18.4
© Copyright Pearson Prentice Hall
Entropy and Free Energy >
Slide 9 of 28
Entropy
An increase in entropy favors the spontaneous chemical reaction; a decrease favors the nonspontaneous reaction.
18.4
Slide 10 of 28
© Copyright Pearson Prentice Hall
>Entropy and Free Energy Entropy
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.
18.4
Slide 11 of 28
© Copyright Pearson Prentice Hall
>Entropy and Free Energy Entropy
Entropy increases when a substance is divided into parts.
18.4
Slide 12 of 28
© Copyright Pearson Prentice Hall
>Entropy and Free Energy 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.
18.4
Slide 13 of 28
© Copyright Pearson Prentice Hall
>Entropy and Free Energy Entropy
Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.
18.4
© Copyright Pearson Prentice Hall
Entropy and Free Energy >
Slide 14 of 28
Enthalpy, Entropy, and Free Energy
Enthalpy, Entropy, and Free Energy
What two factors determine the spontaneity of a reaction?
18.4
© Copyright Pearson Prentice Hall
Entropy and Free Energy >
Slide 15 of 28
Enthalpy, Entropy, and Free Energy
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.
18.4
Slide 16 of 28
© Copyright Pearson Prentice Hall
>Entropy and Free Energy Enthalpy, Entropy, and Free Energy18.4
© Copyright Pearson Prentice Hall
Entropy and Free Energy >
Slide 17 of 28
Enthalpy, Entropy, and Free Energy
Simulation 25
Simulate how enthalpy and entropy changes combine to determine the spontaneity of a reaction.
© Copyright Pearson Prentice Hall
Entropy and Free Energy >
Slide 18 of 28
Gibbs Free-Energy
Gibbs Free-Energy
Is the Gibbs free-energy change positive or negative in a spontaneous process?
18.4
Slide 19 of 28
© Copyright Pearson Prentice Hall
>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.
18.4
© Copyright Pearson Prentice Hall
Slide 22 of 28
Section Quiz
-or-Continue to: Launch:
Assess students’ understanding of the concepts in Section
18.4 Section Quiz.
18.4
© Copyright Pearson Prentice Hall
Slide 23 of 28
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.
© Copyright Pearson Prentice Hall
Slide 24 of 28
18.4 Section Quiz.
2. Free energy is always available from reactions that are
a. endothermic.
b. nonspontaneous.
c. at equilibrium.
d. spontaneous.
© Copyright Pearson Prentice Hall
Slide 25 of 28
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
© Copyright Pearson Prentice Hall
Slide 26 of 28
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.
© Copyright Pearson Prentice Hall
Slide 27 of 28
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.
© Copyright Pearson Prentice Hall
Slide 28 of 28
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