Entropy Changes Based on Changes in Volume
Moderators: Chem_Mod, Chem_Admin
-
- Posts: 11
- Joined: Fri Sep 26, 2014 2:02 pm
Entropy Changes Based on Changes in Volume
In the equation for change in entropy (delta s)= nRln(V2/V1), when would you know whether or not there should be a negative sign in front of the "n"? Is it based on whether it's expansionary or not?
-
- Posts: 70
- Joined: Fri Sep 26, 2014 2:02 pm
Re: Entropy Changes Based on Changes in Volume
The equation for entropy is always written the same way (without a negative sign -- to see why there is no negative sign in the equation, look back in your course reader or review the lecture for how the equation is derived):

However, entropy can have a negative value, if the volume decreases (i.e.
), since the ratio of the volumes would be a fraction between 0 and 1 and the natural log of a number between 0 and 1 is always negative (and a negative number times n and R would give you a negative value of
).
And this makes logical sense, because if you decrease the volume, the system has fewer possible states and the entropy has therefore decreased.
However, entropy can have a negative value, if the volume decreases (i.e.
And this makes logical sense, because if you decrease the volume, the system has fewer possible states and the entropy has therefore decreased.
-
- Posts: 142
- Joined: Fri Sep 26, 2014 2:02 pm
Re: Entropy Changes Based on Changes in Volume
You will know if you need to use a negative sign depending on if you're measuring the delta S of the system or the surroundings. Look at Example 8.12 on page 312-313 for an example of where the book uses both the positive and negative equation.
Return to “Entropy Changes Due to Changes in Volume and Temperature”
Who is online
Users browsing this forum: No registered users and 1 guest