Can Entropy Be Reduced in a Spontaneous Thermodynamic Process?

  • Context: Graduate 
  • Thread starter Thread starter jornrune
  • Start date Start date
  • Tags Tags
    Expansion
Click For Summary
SUMMARY

The discussion centers on the thermodynamic implications of mixing two bodies of water vapor at different pressures and temperatures. When a valve is opened between a small chamber containing H2O gas at 0.5 Bar and 200°C and an infinitely large reservoir at 1.0 Bar and 240°C, the system reaches equilibrium at 1.0 Bar, 220°C, and 2900 kJ/kg. The participant questions whether this spontaneous process results in a reduction of entropy, which appears to contradict the second law of thermodynamics. The conclusion drawn is that while enthalpy and temperature stabilize, the entropy reduction must be reconciled with thermodynamic principles.

PREREQUISITES
  • Understanding of the second law of thermodynamics
  • Familiarity with thermodynamic properties of gases
  • Knowledge of adiabatic processes
  • Basic principles of equilibrium in thermodynamic systems
NEXT STEPS
  • Study the implications of the second law of thermodynamics in spontaneous processes
  • Explore adiabatic process calculations in thermodynamics
  • Learn about entropy changes in mixing processes
  • Investigate the properties of water vapor using steam tables
USEFUL FOR

Students and professionals in thermodynamics, chemical engineers, and anyone interested in the principles of energy transfer and entropy in spontaneous processes.

jornrune
Messages
4
Reaction score
0
Hi! I have a practical problem that causes me to wonder. It seems like I have to choose from gaining energy from nothing, or to get a reduction in entropy from a spontaneous process.

Imagine a small enclosed chamber filled with dry H2O gas. The properties are:
0.5 Bar
8.0 kJ/(kg*K)
200C
2850kJ/kg

An infinitely large reservoir of H2O gas has the following properties:
1.0 Bar
8.0kJ/(kg*K)
240C
2950kJ/kg

Here is a reference chart:
http://www.steamtablesonline.com/images/steam tables p-h diagram (large).png

A valve between the large reservoir and the small chamber is opened and the two gasses are mixed until they reach equilibrium inside the chamber and the pressure is stabilized at 1 bar. Am I correct in assuming that no work has been done and that the properties will be as follows?:
1.0 Bar
7.9kJ/(kg*K)
220C
2900kJ/kg

This seems to be correct as far as temperature and pressure goes. It also seems to add up with the enthalpy as the process is thought to be adiabatic. However, the entropy is lowered, and the process seems to be a so called spontaneous process. The question is if this goes along with the second law of thermodynamics. (It can be read here if you need it refreshed: http://en.wikipedia.org/wiki/Second_law_of_thermodynamics)

Also, I see no room for different results. The pressure is given by the infinite source. This means we can not leave the 1 bar line. There is no way we can get any condensation, so we have the following options:
1: The entropy is actually reduced
2: Enthalpy is lost or gained, and so is temperature

...or am I overlooking something here?

Thanks for any response!
 
Science news on Phys.org
If you open up the valve and wait for equilibrium, the gas inside your small chamber will be identical to the one in the large reservoir.
 

Similar threads

Replies
2
Views
4K
  • · Replies 11 ·
Replies
11
Views
3K
  • · Replies 5 ·
Replies
5
Views
22K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 4 ·
Replies
4
Views
6K