NORTH CAROLINA STATE UNIVERSITY

Department of Chemistry                        

Take-home quiz 4

 

1.      Calculate the absolute entropy of xenon at 400 K.

Solution:

Use the Sakur-Tetrode equation (20.45) in McQuarrie and Simon.

Following example 20-6 the calculations can be decomposed into two parts:

is not changed

 

However, the term


is different both because of the temperature difference (400 K vs 273 K) and the mass difference (Xe vs. Ar).

 

2.      Calculate the change in entropy of 2.00 moles of H2O (l) [Cp = 75.3 J/mol-K] heated from 10oC to 90oC.


 

 

3.      Calculate the values of DS molar the expansion and heating of one mole of N2 (g) from 20 L and 273 K to 300 L and 400 K.  You may use Cp = 29.4 J/mol-K for N2.

 

 


 

4.      In arid climates a structure can be cooled based only on the evaporation of water.  A "swamp cooler" consists of a straw filter and a fan.  As water is poured onto the filter it evaporates and cools air that is blown through the filter by the fan.  What volume of air can be cooled from 313 K to 298 K using a the heat exchanged from evaporation of 1 L of water deposited onto the heat exchange element (straw) of an ideal adiabatic swamp cooler?  You assume that air is an ideal diatomic gas.

 

Solution:


 

5.      Calculate the enthalpy change for the reaction:

                                        C(s, graphite) + 2 H2 (g) + ½ O2 (g) ® CH3OH (l)

         given the following mean bond enthalpies.

 

Bond

X-Y

Mean Enthalpy

B(X-Y) kJ/mol

H-H

436

C-H

412

C-O

360

O-H

463

O=O

497

C(s)

717

 

 

Solution:

 


The enthalpy of "formation" of CH3OH can be estimated from the summed enthalpies of the bonds.

 


Inserting this value and the values from the table we find:


 

6.      Compare the standard enthalpy of hydrogenation of ethylene DrH = -137 kJ/mol to that of benzene (DrH = -205 kJ/mol).  Keeping in mind that it takes three H2 molecules to hydrogenate benzene and only one to hydrogenate ethylene, write down an equation for each process and compare the enthalpies of hydrogenation for the same stoichiometry of H2.  Why aren’t the enthalpies of hydrogenation of benzene and ethylene the same?

 

Benzene is aromatic.

 

7.      Calculate the entropy of the system and surroundings as well as the total entropy change for the compression of 0.1 moles an ideal gas from 0.2 L to 0.05 L.  The external pressure is 164 atm and the compression occurs in a single step.

 

Solution:

 


We calculate DSsys along a reversible path.