End Show Slide 1 of 28 © Copyright Pearson Prentice Hall > Entropy and Free Energy Free Energy and Spontaneous Reactions A spontaneous reaction occurs.

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Presentation transcript:

End Show Slide 1 of 28 © Copyright Pearson Prentice Hall > Entropy and Free Energy Free Energy and Spontaneous Reactions A spontaneous reaction occurs naturally and favors the formation of products at the specified conditions. 18.4

End Show Slide 2 of 28 © Copyright Pearson Prentice Hall > Entropy and Free Energy Free Energy and Spontaneous Reactions 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

End Show Slide 3 of 28 © Copyright Pearson Prentice Hall > Entropy and Free Energy 18.4 Free Energy and Spontaneous Reactions Spontaneous reactions produce substantial amounts of products at equilibrium and release free energy. Gibb’s free energy/free energy is energy that is available to do work and is represented by the symbols ΔG; it is calculated from the ΔH (change in enthalpy; heat of reaction), temperature (T in kelvin) and ΔS (change in entropy) for the reaction. What part does entropy play in chemical reactions?

End Show Slide 4 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. An increase in entropy favors the spontaneous chemical reaction; a decrease favors the nonspontaneous reaction. 18.4

End Show Slide 5 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

End Show Slide 6 of 28 © Copyright Pearson Prentice Hall > Entropy and Free Energy Entropy Entropy increases when a substance is divided into parts. 18.4

End Show Slide 7 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

End Show Slide 8 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

End Show © Copyright Pearson Prentice Hall Entropy and Free Energy > Slide 9 of 28 Enthalpy, Entropy, and Free Energy What two factors determine the spontaneity of a reaction? 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

End Show Slide 10 of 28 © Copyright Pearson Prentice Hall > Entropy and Free Energy Enthalpy, Entropy, and Free Energy 18.4 The numerical value of ΔG is negative in spontaneous processes because the system loses free energy.

End Show © Copyright Pearson Prentice Hall Slide 11 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 12 of 28 End Show 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 13 of 28 End Show 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 14 of 28 End Show 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 15 of 28 End Show 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 16 of 28 End Show 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.