NORTH CAROLINA STATE UNIVERSITY

Department of Chemistry NAME________________________________

 

CH 431  Take-home Quiz 3

Physical Chemistry I      October 9, 2001

         Due date: October 16, 2001

 

R  =  8.314 =  1.987 =  0.08206

 

1.      A hiker caught in a rainstorm absorbs 1 liter of water in his/her clothing.  It is windy so that this volume is quickly evaporated at 20oC (the heat of vaporization of water is 2447 kJ/kg at this temperature).

  1. If the hiker is the “system” calculate q, the heat transferred. (3 points)

 

 

 

 

 

 

  1. If all this heat were removed from the hiker what drop in body temperature would the hiker experience (ignore the metabolism of the hiker)?  The hiker weighs 60 kg and has a heat capacity equal to that of water.  (3 points)

 

 

 

 

 

 

 

 

 

 

  1. How many grams of sucrose would the hiker have to metabolize (quickly) to replace the heat of evaporating one liter of water to maintain his/her original body temperature?  You can use the heat of reaction at 25oC; the reaction is:

                              Sucrose (s) + oxygen (g) ® carbon dioxide (g) + water (l)

      (4 points)

 

 

 

 

 

 

 

 

 

2.      a. An engineer is designing a solar heating unit for a house.  A bed of granite rocks

with a surface area of 10 m2 and volume 10 m3 will be heated by direct sunlight.  The specific heat of the rocks is 4 Jg-1K-1 and the average density is 4000 kg/m3.  Assume that the effective radiant power of the sun is 0.1 kW/m2 and that there are two hours of effective heating per day.  Assuming a morning temperature of 298 K, perfect heat transfer, and no losses, what final temperature will be achieved by the rocks at the end of the day? (3 points)

 

 

 

 

 

b. Assuming perfect heat exchange between the rocks and pumped air, what volume of

air (Cp = 29.0 Jmol-1K-1) can be heated by 10 K? (2 points)

 

 

 

 

 

3.      a. How much solar energy is required to heat 1 mole of circulation water from 300 K

to 310 K? (2 points)

 

 

 

b.  A flow of air at 275 K enters a heat exchanger at 22.5 L/sec.  The hot water (310 K) supplied from a flat plate solar panel enters the inner tubing of the heat exchanger at 0.018 L/sec.  Assuming that the heat exchanger has no losses and that the water and air reach equilibrium as they exit calculate the temperature of the heated air.  (2 points)

 

 

 

 

c.       Treating the heat exchange process as a constant pressure expansion of the

atmosphere and compression of liquid water, calculate the volume change of 1 L of air and liquid water, respectively. (1 point)

 

 

 

 

d.   Calculate the entropy change in both the water and air separately and the total entropy change for the process.