CK-12-Chemistry Intermediate

20.2. Spontaneous Reactions and Free Energy http://www.ck12.org

Lesson Review Questions

Reviewing Concepts

1. What is true about the relative amounts of reactants and products at the end of a spontaneous reaction?


2. Can a proposed reaction be spontaneous and yet still not be observed to occur? Explain.


3. The forward reaction is spontaneous for a particular reversible reaction. What can you conclude about the
   reverse reaction?


4. Explain how free energy is used to determine whether or not a reaction is spontaneous.


5. Under what conditions of enthalpy and entropy change is a reaction always spontaneous? Under what
   conditions is a reaction never spontaneous?


6. If the entropy change is unfavorable for a certain reaction, is the reaction more likely to be spontaneous at a
   high temperature or a low temperature?


7. If the enthalpy change is unfavorable, but the entropy change is favorable, would a high temperature or a low
   temperature be more likely to lead to a spontaneous reaction?

Problems

8. Based on the values of∆H,∆S, and T shown below, calculate∆G and predict whether the reaction will occur
   spontaneously.
   a.∆H =−245 kJ/mol;∆S =−361 J/K•mol; T = 325 K
   b.∆H = +87.6 kJ/mol;∆S =−112 J/K•mol; T = 295 K
   c.∆H =−95.5 kJ/mol;∆S = +21.9 J/K•mol; T= 15°C
   d.∆H = +104.9 kJ/mol;∆S = +177 J/K•mol; T = 246°C


9. Referring to problem number 8: Which reaction will be spontaneous at any temperature? Which will be
   nonspontaneous at any temperature?


10. One mole of mercury(II) oxide decomposes according to the following reaction: HgO(s)+ 90 .7 kJ→Hg(l)+
    1
    2 O^2 (g).
    a. Calculate∆G for the reaction at 25°C and predict whether the reaction is spontaneous or not.
    b. Calculate∆G at 800°C and predict whether the reaction is spontaneous at this temperature.
    c. Assuming the values for∆H and∆S do not vary with temperature, calculate the lowest Kelvin tempera-
    ture at which the reaction would be spontaneous.

Further Reading / Supplemental Links

• GibbsFreeEnergy, http://www.kentchemistry.com/links/Kinetics/Gibbs.htm


• Thermodynamics:GibbsFreeEnergy, http://www.chem.tamu.edu/class/majors/tutorialnotefiles/gibbs.htm

Points to Consider

When a reversible reaction is at equilibrium, the concentrations of the reactants and products are constant, so neither
the forward nor reverse reaction is spontaneous (favored).

• How can we determine the conditions under which a certain reaction will be at equilibrium?