Unformatted text preview: HW6_Chapter 21
212:
Q. The molar heat capacity of H2O(l) has an approximately constant value of
from 0 to 100 . Calculate
if two moles of H2O(l) are heated from 10
constant pressure. to 90 at A. W e use Equation 21.9 to write 217:
Q. The results of Problems 214 and 215 must be connected in the following way. Show that
the two processes can be represented by the diagram where paths A and B represent the processes in Problem 215 and 214, respectively. Now,
path A is equivalent to the sum of paths B and C. Show that
is given by and that the result given in Problem 216 follows.
A. In Problem 214, ethane is heated at constant volume, so (assuming ideal behavior) Likewise, in Problem 215, the system is kept at constant pressure and so These values correspond to those shown in the diagram. Now, path C represents an isothermal
process. Because we are assuming ideal behavior,
, which means that
. Then we can write as (Equation 20.22) Note that path A is equivalent to the sums of paths B and C, so
and
, we can write . Because and the result given in Problem 216 follows. 2112:
Q. W hy is ? A.
because gases are essentially completely unordered; the molecules of a gas
travel more or less randomly within the gas’s container. Liquids, however, are much more
cohesive and structured, and solids are very structured. The difference between the entropy of a
liquid and that of a solid is less than the difference between the entropy of a liquid and that of a
gas. 2127:
Q. Show for an ideal gas that A. W e express the partition function of an ideal gas as (Equation 17.38) and so We substitute into Equation 21.19 to write We use Stirling’s approximation and divide both sides of the equation by n to find 2140:
Q. In each case below, predict which molecule of the pair has the greater molar entropy under
the same conditions (assume gaseous species). a. CO CO2 b. CH3CH2CH3 c. CH3CH2CH2CH2CH3
A. a. CO2 (more atoms)
b. CH3CH2CH3 (more flexibility)
c. CH3CH2CH2CH2CH3 (more flexibility) 2142:
Q. Arrange the following reactions according to increasing values of
references).
a. S(s) + O2(g) SO2(g) (do not consult any b. H2(g) + O2(g) H2O2(l)
c. CO(g) + 3 H2(g) CH4(g) + H2O(l)
d. C(s) + H2O(g) CO(g) + H2(g)
A. Recall that molar entropies of solids and liquids are much smaller than those of gases, so
we can ignore the contributions of the solids and liquids to
when we order these reactions.
Considering only the gaseous products and reactants, we can find
for each reaction to be
a. b. c. The correct ordering of the reactions is therefore d.
. ...
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 Fall '08
 Bersuker
 Physical chemistry, Mole, pH, ideal behavior, approximately constant value

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