Example 9.5
Predict the sign of
ΔS
o for each of the following processes:
a) H
(g) + I 2
(g) 2
→
2HI(g)
There are 2 moles of gas on each side of the
equation, so the number of moles of gas
does not change. Thus, we predict that
ΔS
o ~ 0.
Tabulated value:
SΔ
o = +21J
-1.K
b) 3H
(g) + N 2
(g) 2
→
2NH
(g) 3
SΔ
o < 0 because the number of moles of gas decreases (4 mol
→
2 mol).
Tabulated value:
SΔ
o = -199 J
.K
-1^
c) Ag(s) + NaCl(s)
→
AgCl(s) + Na(s)
SΔ
o ~ 0 because there are no gases involved.
All reactants and products are in the same
state, so we cannot predict the sign, but we
can predict that the magnitude is small.
Tabulated value:
SΔ
o = +33 J
-1.K
. Note that this is comparable to Part a where there was
no change in the number of moles of gas but small compared to Part b where there was a change in the number of moles of gases.
d) H
O(l) 2
→
H
O(g) 2
SΔ
o > 0 because a gas is produced.
Tabulated value:
ΔS
o = +118 J
.K
-1.
Example 9.6
Indicate which process in each pair increases the entropy of the system more:
a) adding 10 J of heat to neon at 300 K or adding 50 J of heat to neon at 300 K
The heat is added at the same
temperature, so the greater entropy change occurs when
the greater amount of heat is
added. Consequently, adding 50 J results in a greater
entropy change than adding 10 J at 300 K.
b) adding 10 J of heat to neon at 300 K or adding 10 J of heat to neon at 800K
The amount of heat added is the same, so the greater entropy change occurs when the heat is added at the lower tem
perature. Consequently, adding 10 J results in a greater
entropy change at 300 K.
c) adding 50 J of heat to neon at 800 K or adding 10 J of heat to neon at 300 K
ΔS = q/T. (50/800) > (10/300), so adding 50 J at
800 K causes a larger entropy change.
d) melting 1 g of neon at its melting point or evaporating 1 g of neon at its boiling point
ΔS
fusion
= S
liquid
- S
solid
~ 0
because no gases are involved.
ΔS
evaporation
= S
gas
- S
liquid
>> 0
because gas is produced.
Chapter 9 Reaction Energetics
© by
North
Carolina
State
University