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General Chemistry Principles, Patterns, and Applications, 2011

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elationship between ΔG <strong>and</strong> Kp, the equilibrium constant of a reaction involving gases, or K, the<br />

equilibrium constant expressed in terms of concentrations. If ΔG° < 0, then Kor Kp > 1, <strong>and</strong> products are<br />

favored over reactants. If ΔG° > 0, then K or Kp < 1, <strong>and</strong> reactants are favored over products. If ΔG° = 0,<br />

then K or Kp = 1, <strong>and</strong> the system is at equilibrium. We can use the measured equilibrium constant K at one<br />

temperature <strong>and</strong> ΔH° to estimate the equilibrium constant for a reaction at any other temperature.<br />

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

<br />

The change in free energy of a reaction can be expressed in terms of the st<strong>and</strong>ard freeenergy<br />

change <strong>and</strong> the equilibrium constant K or K p <strong>and</strong> indicates whether a reaction will<br />

occur spontaneously under a given set of conditions.<br />

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

Relationship between st<strong>and</strong>ard free-energy change <strong>and</strong> equilibrium constant<br />

Equation 18.38: ΔG° = −RT ln K<br />

Temperature dependence of equilibrium constant<br />

Equation 18.40<br />

Equation 18.41<br />

: ln K = -DH°RT + DS°R<br />

: ln K2K1= DH°R(1T1-1T 2)<br />

Calculation of K at second temperature<br />

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

1. Do you expect products or reactants to dominate at equilibrium in a reaction for which ΔG° is equal to<br />

a. 1.4 kJ/mol?<br />

b. 105 kJ/mol?<br />

c. −34 kJ/mol?<br />

2. The change in free energy enables us to determine whether a reaction will proceed spontaneously. How is<br />

this related to the extent to which a reaction proceeds?<br />

3. What happens to the change in free energy of the reaction N 2 (g) + 3F 2 (g) → 2NF 3 (g) if the pressure is<br />

increased while the temperature remains constant? if the temperature is increased at constant pressure?<br />

Why are these effects not so important for reactions that involve liquids <strong>and</strong> solids?<br />

4. Compare the expressions for the relationship between the change in free energy of a reaction <strong>and</strong> its<br />

equilibrium constant where the reactants are gases versus liquids. What are the differences between these<br />

expressions?<br />

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

Saylor.org<br />

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