CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 20. Entropy and Free Energy

2H 2 (g) + O 2 (g)→2H 2 O(l)

∆S° = 2(69.9) - [2(131.0) + 1(205.0)] = -327 J/K•mol

The entropy change for this reaction is highly negative because three gaseous molecules are being converted into two
liquid molecules. According to the drive toward higher entropy, the formation of water from hydrogen and oxygen is
an unfavorable reaction. However, the reaction is also highly exothermic, and the drive toward a decrease in energy
allows the reaction to occur.

Lesson Summary

• The tendency for a reaction or process to result in a lowering of the energy of the system is one of the primary
  driving forces in chemistry. The majority of naturally occurring reactions are exothermic because the products
  are lower in energy.


• Entropy is a measure of the disorder of a system. The tendency for a reaction or process to result in an increase
  in the entropy of the system is another fundamental driving force. The entropy change for a reaction can often
  be predicted based on the physical states of the substances involved and on the total numbers of reactant and
  product molecules.


• The standard entropy of a reaction can be calculated from the standard entropies of the reactants and products.
  A positive entropy change is more favorable than a negative entropy change, but changes in both enthalpy and
  entropy need to be considered to determine whether a reaction will occur spontaneously.

Lesson Review Questions

Reviewing Concepts

1. In a certain reaction, the energy of the reactants is less than the energy of the products. Is the reaction
   endothermic or exothermic? Do you have enough information to say whether it is favorable or unfavorable?


2. What are the two driving forces for all chemical reactions and physical processes?


3. What is entropy and how does it relate to the spontaneity of a reaction?


4. How does an increase in temperature affect the entropy of a system?


5. Which system has the greater entropy in each of the following?
   a. solid sodium chloride or a sodium chloride solution
   b. bromine liquid or bromine vapor
   c. 25 g of water at 80°C or 25 g of water at 50°C
   d. liquid mercury or solid mercury


6. How does the entropy of a system change for each of the following processes?
   a. A solid melts.
   b. A liquid freezes.
   c. A liquid boils.
   d. A vapor condenses to a liquid.
   e. Sugar dissolves in water.
   f. A solid sublimes.


7. According to the table above (Table20.1), which allotrope of carbon, graphite, or diamond, contains more
   disorder within its structure?


8. Xenon gas has a larger absolute entropy than neon gas when both are at the same temperature and pressure.
   Why is this true?