Chemistry 152 Discussion Section Problems:
Oct. 11, 2011
1) Suppose 1.00 kJ of heat is transferred to 2.00 mol of argon, which initially is at
T=298K and P=1atm. What will the final temperature be if the heat is transferred
at: a) constant volume; b) constant pressure. Calculate ΔE and ΔH for each case.
Assume argon can be treated as an ideal monatomic gas.
Solution: Ideal monatomic gas, so Cv=3R/2=12.47 JK-1mol-1 and Cp=5R/2=20.79 JK-1
mol-1.
a) At constant volume
b) At constant pressure
2) Human beings do work during expansion and contraction of their lungs in
breathing. During each exhalation from the lungs an adult human pushes out
about 0.5L of gas against a constant atmospheric pressure of 1.00 atm. An adult
exhales about 15,000 times during a 24 hour period.
a) Calculate the amount of work done by an adult in breathing during a 24 hour
period.
b) Suppose all the work done by an adult in breathing in a 24 hour period is used to
lift a 900 kg (about 1 ton) weight. How high would this weight be lifted? Note the
acceleration due to gravity is g=9.8ms-2.
Solution: This one is a little tricky and has a surprising result.
a) the work is being done by the adult so
To work the rest of the problem we just need the amount of work done…which is:
–w=750kJ/day…i.e. drop the negative sign.
b) To calculate the height lifted…
3) Hydrogen H2 behaves as an ideal diatomic gas at temperatures above 200K and at
pressures below 50 atm. Suppose 6.00 moles of hydrogen are contained initially
in a 100L vessel at a pressure of 2 atm. The gas is cooled at constant pressure
from its initial volume to a final volume of 75L. calculate the following quantities
a) the final temperature at a volume of 75L
b) the work done on the gas
c) the internal energy change ΔE for the gas
d) the enthalpy change ΔH of the gas
e) heat absorbed by the gas
Solution:
a)
b)
c)
d)
e) Two ways…
or
4) A piece of zinc at T=293K that weighs 60.0 grams is dropped into 200.0 grams of
water at 373K. Assume the heat capacity of zinc is
and the
heat capacity of water is
. Note both heat capacites are in
units of joules per degree Kelvin per gram. Calculate the final temperature
reached by the zinc and the water.
Solution: