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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
159
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 15
• Dissolving of a solid or liquid to form a solution When
solute particles become dispersed in a solvent, the disorder of
the particles and the entropy of the system usually increase.
• Change in temperature A temperature increase results in
increased disorder of the particles and an increase in entropy.
Practice Problems
13. Predict the sign of ⌬S system for each of these changes.
a. O 2 (g) 0 O 2 (aq)
b. C 6 H 6 (s) 0 C 6 H 6 (l)
c. C(s) ϩ CO 2 (g) 0 2CO(g)
d. C 12 H 22 O 11 (s) 0 C 12 H 22 O 11 (aq)
e. 2NO 2 (g) 0 N 2 O 4 (g)
Free energy For a reaction or process occurring at constant temperature and pressure, the energy that is available to do work is the
free energy (G). The change in free energy is related to the change
in enthalpy and the change in entropy by the following equation.
⌬G system ϭ ⌬H system Ϫ T⌬S system
In this equation, T is the Kelvin temperature. If ⌬G system is negative,
the reaction or process is spontaneous; if ⌬G system is positive, the
reaction or process is nonspontaneous.
Example Problem 15-6
Determining Reaction Spontaneity
For a chemical reaction, ⌬H system ϭ Ϫ81 kJ and ⌬S system
ϭ Ϫ215 J/K. Is the reaction spontaneous at 50°C?
Convert the temperature to kelvins.
T ϭ (50 ϩ 273) K ϭ 323 K
Find ⌬G system .
⌬G system ϭ ⌬H system Ϫ T⌬S system
⌬G system ϭ Ϫ81 000 J Ϫ (323 K)(Ϫ215 J/K)
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