# Dimension of Entropy

• December 7th, 2005, 05:02 PM
Ultrashogun
Dimension of Entropy
The dimension of Entropy S is Joule/Kelvin, so the entropic term is ST with the dimension of an energy. How can I Imagine this energy? If I take a chemical reaction with deltaG = deltaH - deltaTS, what exactly is the entropy?

Is it the heat which is splitting up inter- and intramolecular bonds?
• December 7th, 2005, 05:13 PM
The P-manator
My guess is that it's all the enrgy released and needed during the reaction.
• December 7th, 2005, 05:15 PM
Ultrashogun
The energy that is released or put into the reaction is the entalpy, H.
• December 7th, 2005, 05:18 PM
John Galt
I've resolved not to think about entropy as that only speeds the ultimate demise of intelligence in the Universe. This site might help, but I'm not smart enough to be sure.
http://<font color="blue">http://www...py.html</font>
• December 7th, 2005, 06:46 PM
Scifor Refugee
The “energy” term in entropy is the amount of energy that could be extracted from the system without changing its enthalpy. The classic example is doing work with gas expansion; imagine you have a divided container with a lot of compressed gas on one side and a vacuum on the other. You could harness this system to do work by using the expansion of the gas into the low-pressure part of the chamber to drive some sort of mechanism. However, the enthalpy of the gas after it expands will be exactly the same as it was when it was compressed. The energy that you are able to extract from such a system would be equal to S*T.

To look at it from a more chemical point of view, entropy and enthalpy compete with each other to determine whether or not a reaction will be spontaneous. The entropy term is in units of energy so that we can quantitatively compare a change in entropy with a change in enthalpy to see which will have a larger effect.
• December 7th, 2005, 06:50 PM
Ultrashogun
Quote:

Originally Posted by Scifor Refugee
The “energy” term in entropy is the amount of energy that could be extracted from the system without changing its enthalpy. The classic example is doing work with gas expansion; imagine you have a divided container with a lot of compressed gas on one side and a vacuum on the other. You could harness this system to do work by using the expansion of the gas into the low-pressure part of the chamber to drive some sort of mechanism. However, the enthalpy of the gas after it expands will be exactly the same as it was when it was compressed. The energy that you are able to extract from such a system would be equal to S*T.

To look at it from a more chemical point of view, entropy and enthalpy compete with each other to determine whether or not a reaction will be spontaneous. The entropy term is in units of energy so that we can quantitatively compare a change in entropy with a change in enthalpy to see which will have a larger effect.

Shouldnt the energy be Q/T, not S/T, as S/T would have dimension [Joule/ Kelvin Kelvin]? And the expansion you are talking about would have to be adiabatic no?
• December 7th, 2005, 07:08 PM
Scifor Refugee
Quote:

Originally Posted by Ultrashogun
Shouldnt the energy be Q/T, not S/T, as S/T would have dimension [Joule/ Kelvin Kelvin]?

Yes, I saw that mistake and tried to change it right after I saw it, but it looks like you were too quick for me.
Quote:

And the expansion you are talking about would have to be adiabatic no?
Yes, if there is no change in the enthalpy of the gas then by definition it would be adiabatic.
• December 26th, 2005, 11:50 PM
Laboratory Mike
Technically, entropy is in units of energy, but since the amount of energy present is heavily affected by tempearture, it is most commonly presented as a function of temperature: S(T)=T*S. S is also related to presure and volume changes, and there's quite a few equations involved (residual properties come to mind). In my field entropy tends to be related to "irreversibility" in chemical and mechanical processes, and is treated as "lost energy."

I'm sure that physical chemists have a more in-depth understanding of entropy than I would, but "lost energy" is a good way to visualize entropy.

And for Scifor, adiabatic compression is isentropic, meaning there is no change in entropy. Isenthalpic changes, such as throttling, do not cause a change in enthalpy. Also, when you do work by gas expansion, it's the change in enthalpy that determines the amount of work that can be done. Entropy is, as noted above, a measure of how much energy is lost in a process (like your expanding gas process) due to friction, heat exchanges, etc.