CBE 40B, Winter 2025: Process Thermodynamics
Homework #1
(due: 1/21/25 at 9 am, due to holiday; upload a PDF copy of your answer sheet in Canvas)
Total: 30 pts
1. For each of the following cases, indicate which thermodynamic variables (from the list) can
be determined from the information given. Note: You do not need to provide the actual
values of the thermodynamic variables; just indicate which ones can be determined. (12
pts, 3 pts each/ 1 pt per variable)
Condition
a. Ethanol (solid:liquid) at 6 MPa
Thermodynamic variables of interest
b. 10 kg, 1:1 liquid: vapor (steam) water
system at pressure 1MPa, having a
defined overall specific volume of 1 x
10–3 m3/kg
c. Chlorobenzene vapor at room
temperature and defined specific
volume
d. Methane gas at 1 mol and room
temperature
̂ g, H, T
U
̂g
U, P, V
U, P, V
H, P, Ŝ
2. Textbook problems. For both problems, to get full points:
• Indicate what the system is, if not yet identified
• Show your complete solution and make sure to use the appropriate mass balance
equations
a. (Derived from Textbook Problem 2.1) As a result of a chemical spill, benzene is
evaporating at the rate of 1 gram per minute into a room that is 6 m × 6 m × 3 m in size
and has a ventilation rate of 10 m3/min. Assuming the air in the room initially has 0.005
g/m3 benzene before the measurement, compute the concentration of benzene in the
room after 30 minutes. The system of interest in this problem is the benzene in the
room; benzene spill is not part of the system. (8 pts)
Hint: Use the rate-of-change form of the mass balance to get an expression for the
concentration of benzene (in mass/volume, or 𝜌) at any given time.
b. (Problem 2.2) The insecticide DDT has a half-life in the human body of approximately
7 years. That is, in 7 years its concentration decreases to half its initial concentration.
Although DDT is no longer in general use in the United States, it was estimated that 25
years ago the average farm worker had a body DDT concentration of 22 ppm (parts
per million by weight). Estimate what the farm worker's present concentration would
be. (5 pts)
Hint: Consider this rate of concentration change equation (mass flowing out of the
system) as a rate-of-change form of the mass balance, where k is a constant:
3. (5 pts) Provide a simplified energy balance expression to describe the rate at which shaft
̇ 𝒔 ) must be applied to a constant volume tank comprised of multiple gas streams
work (𝑾
at steady state. Assume the system has negligible changes in kinetic and potential energy.