Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 13

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CHAPTE R 14? Entropy and Free Energy

Section 14.3 Review

Entropy Changes in a System
14.3.1 Determine the sign of ?S for each of the following processes.

(a) Freezing ethanol

(b) Evaporating water

(c) Mixing carbon tetrachloride and benzene

(d) Heating water

(e) Condensing bromine vapor

14.3.2 Determine the sign of ?S for each of the following reactions.

(a) 2O3(g)

(b) 4Fe(s) + 3O2(g)

(c) 2H2O2(aq)

(d) 2Li(s) + 2H2O(l)

(e) 2NH3(g)

3O2(g)
2Fe2O3(s)
2H2O(l) + O2(g)
2LiOH(aq) + H2(g)

N2(g) + 3H2(g)

14.3.3 Consider the gas-phase reaction of A2 (blue) and B2 (orange) to form AB3. Write the
correct balanced equation and determine the sign of ?S for the reaction.

14.4 ? Entropy Changes in the Universe
Recall that the system typically is the part of the universe we are investigating (e.g., the reactants
and products in a chemical reaction). The surroundings are everything else [9 Section 10.1].
Together, the system and surroundings make up the universe. We have seen that the dispersal or
spreading out of a system’s energy corresponds to an increase in the system’s entropy. Moreover,
an increase in the system’s entropy is one of the factors that determines whether or not a process
is spontaneous. However, correctly predicting the spontaneity of a process requires us to consider
entropy changes in both the system and the surroundings.
Consider the following processes:
• An ice cube spontaneously melts in a room where the temperature is 25°C. In this case, the
motional energy of the air molecules at 25°C is transferred to the ice cube (at 0°C), causing
the ice to melt. There is no temperature change during a phase change. However, because
the molecules are more mobile and there are many more different possible arrangements
in liquid water than there are in ice, there is an increase in the entropy of the system.
In this case, because the process of melting is endothermic, heat is transferred from the
surroundings to the system and the temperature of the surroundings decreases. The slight
decrease in temperature causes a small decrease in molecular motion and a decrease in the
entropy of the surroundings.
?Ssys is positive.
?Ssurr is negative.
• A cup of hot water spontaneously cools to room temperature as the motional energy of the
water molecules spreads out to the cooler surrounding air. Although the loss of energy from

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