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General Chemistry Principles, Patterns, and Applications, 2011

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free energy of formation<br />

(DG f ),<br />

is the change in free energy that occurs when 1 mol of a substance<br />

in its st<strong>and</strong>ard state is formed from the component elements in their st<strong>and</strong>ard states. Tabulated values of<br />

st<strong>and</strong>ard free energies of formation are used to calculate ΔG° for a reaction.<br />

K E Y T A K E A W A Y<br />

<br />

The change in Gibbs free energy, which is based solely on changes in state functions, is<br />

the criterion for predicting the spontaneity of a reaction.<br />

K E Y E QU A T I ON S<br />

Free-energy change<br />

Equation 18.23: ΔG = ΔH − TΔS<br />

St<strong>and</strong>ard free-energy change<br />

Equation 18.26: ΔG° = ΔH° − TΔS°<br />

C O N C E PTUAL P R OBLEMS<br />

1. How does each example illustrate the fact that no process is 100% efficient?<br />

a. burning a log to stay warm<br />

b. the respiration of glucose to provide energy<br />

c. burning a c<strong>and</strong>le to provide light<br />

2. Neither the change in enthalpy nor the change in entropy is, by itself, sufficient to determine whether a<br />

reaction will occur spontaneously. Why?<br />

3. If a system is at equilibrium, what must be the relationship between ΔH <strong>and</strong> ΔS?<br />

m 2AB<br />

A2B2 is exothermic<br />

4. The equilibriu in the forward direction. Which has the higher<br />

entropy—the products or the reactants? Why? Which is favored at high temperatures?<br />

5. Is ΔG a state function that describes a system or its surroundings? Do its components—ΔH <strong>and</strong> ΔS—describe<br />

a system or its surroundings?<br />

6. How can you use ΔG to determine the temperature of a phase transition, such as the boiling point of a liquid<br />

or the melting point of a solid?<br />

7. Occasionally, an inventor claims to have invented a “perpetual motion” machine, which requires no<br />

additional input of energy once the machine has been put into motion. Using your knowledge of<br />

thermodynamics, how would you respond to such a claim? Justify your arguments.<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

1674

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