Chemistry 2720 Fall 1998 Test 1 Solutions

1. The reaction uses up a gas and forms a crystalline solid. The product is more ``ordered'' than the reactants so the entropy should decrease.
2. The reaction is

   

   The enthalpy of reaction at is

   

   To get the enthalpy of reaction at , consider the following thermodynamic cycle:

   , the enthalpy change at the standard temperature ( ). We need to calculate and :

   

   The difference between the two is about 3kJ/mol out of 2800 (about 0.1%). Metabolic studies are rarely this accurate so the difference is probably not significant.

3. The reaction is

   

   For this reaction,

   

   We want the heat at constant volume, i.e. .

   

   The reaction is exothermic, i.e. heat is produced.

4. We have the entropy of liquid water at . We imagine heating the water from to , then evaporating the water, and finally heating the steam to its final temperature.

   

5. The condensation of steam is spontaneous if .

   

   Since the change in entropy of the universe is positive for this process, it is spontaneous.

6. 1. First, use the density to determine the mass of ethanol produced per hour:

      

      The water is warmed by the condensation which releases

      

   2. Again, we need to calculate the mass of water from the volume which passes through the condenser in an hour:

      

      Now calculate the temperature rise:

      

      The final temperature is therefore .