The Richard Stockton College of New Jersey
Chemistry Program, School of Natural Sciences and Mathematics
PO Box 195, Pomoma, NJ
CHEM 3410: Physical Chemistry I — Fall 2008
Homework 3
Due in Class: September 17, 2008
1. A chemical reaction occurs isothermally a 300 K in a gas mixture that b ehaves ideally, and the
total amount of gas increases by 0.27 moles. If ∆U= 9.4 kJ what is ∆H?
2. The molar heat capacity, Cp,m of SO2gas is described by the follwoing equation over the range
300–1700 K:
CP,m =R3.093 + 6.967 ×10−3T
K−45.81 ×10−7T2
K2+ 1.035 ×10−9T3
K3
where Tis the absolute temperature in Kelvin and the ratios Tn
Knensure that Cp,m has the correct
units. Assuming ideal gas behavior, calculate q,w, ∆U, and ∆Hif 1 mole of SO2gas is heated
from 75◦C to 1350◦C at a constant pressure of 1 atm. Does the sign you calculate for wmake
sense?
HINT: It is useful to remember the definition of enthalpy and that you are dealing with an ideal
gas, much like Problem 1.
3. Ammonia gas (NH3) in a pressure cylinder is quickly brought to a temperature of 1200K by rapidly
compressing it to 10 atm. At the end of the compression the piston is fixed so that the system
remains at constant volume.
At 1200K NH3will start to decompose according to the reaction:
2NH3−> N2+ 3H2
Assume that no NH3has decomposed during the rapid compression.
(a) What is the pressure inside the system when all the NH3has decomposed at a constant
temperature of 1200K?
(b) Compute the heat one needs to supply/extract from the cylinder to keep the temperature at
1200K during the decomposition of the ammonia in the cylinder?
(c) If the decomposition reaction occurred adiabatically, what would be the temperature of the
system after complete decomposition
DATA:
∆H for the decomposition reaction at 1200 K: 87 kJ/mol (per mole of N2formed)
Cp,N2= 33 J/mol-k
Cp,H2= 33 J/mol-k
Cp,NH3= 36 J/mol-k
Assume that all gasses behave ideally.
Some properties of ideal gasses:
Cp−Cv=R
∂U
∂V T
= 0