(continued)
3) Assuming no change in physical state, the entropy of a system
usually increases when the number of gaseous product particles
is greater than the number of gaseous reactant particles.
- The larger the number of gaseous particles, the more random
arrangements are available.
- In the following reaction, Change in S (system) is positive because two
gaseous particles react and three gaseous molecules are produced.
2SO3 (g) ----> 2SO2 (g) + O2 (g) Change in S (system) > 0
4) With some exceptions, you can predict the change in entropy
when a solid or a liquid dissolves to form a solution.
- The solute particles, which are separate and pure before dissolving,
become dispersed throughout the solvent.
- Therefore, dissolution usually increases the randomness and disorder
of the particles, and the entropy of the system increases.
- For the dissolving of sodium chloride in water, Change in S (system)
is positive.
NaCl (s) ----> Na (aq) + Cl (aq) Change in S (system) > 0
5) An increase in the temperature of a substance is always
accompanied by an increase in the random motion of its
particles.
- The kinetic energy of molecules increases with temperature.
- Increased kinetic energy means faster movement, more possible
arrangements, and increases disorder.
- Therefore, the entropy of any substance increases as its temperatire
increases, and Change in S (system) > 0
Entropy, the Universe, and Free Energy:
- The law of disorder states that the entropy of the universe
must increase as a result of a spontaneous reaction or process.
- Therefore, the following is true for any spontaneous process.
Change in S (universe) > 0
- Because the universe equals the system plus the surroundings,
any change in the entropy of the universe is the sum of
changes occuring in the system and surroundings.
Change in S (universe) = Change in S (system) + Change in S (surroundings)
- In nature, Change in S (universe) tends to be positive for reactions
and processes under the following conditions.
1) The reaction or process is exothermic, which means
?H (system) is negative.
- The heat released by an exothermic reaction raises the temperature
of the surroundings and thereby increases the entropy of
the surroundings.
- Change in S (surroundimgs) is positive
2) The entropy of the system increases, Change in S (system)
is positive.
- Thus, exothermic chemical reactions accompanied by an
increase in entropy are all spontaneous.