### thermo equation explanation

Posted:

**Tue Feb 05, 2019 9:57 pm**What are you calculating with this equation: delt S (entropy) = q/T and why does T need to be in K?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=77&t=41455

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Posted: **Tue Feb 05, 2019 9:57 pm**

What are you calculating with this equation: delt S (entropy) = q/T and why does T need to be in K?

Posted: **Tue Feb 05, 2019 10:36 pm**

This equation is calculating the change in the entropy of a system

Temperature is in Kelvin and Heat is in joules because the units for entropy is (J K^-1)

Temperature is in Kelvin and Heat is in joules because the units for entropy is (J K^-1)

Posted: **Tue Feb 05, 2019 11:29 pm**

What is the difference between the equation mentioned above and the equation S=k(b) ln w?

Posted: **Tue Feb 05, 2019 11:57 pm**

The equation ΔS=q/T is applicable to an isothermal, reversible reaction.

The other equation ΔS=k_{B}lnW is used to calculate residual entropy, meaning the system is cooled to nearly 0K so molecules are barely moving. We are calculating only the entropy of molecules, not a reaction with heat. A good way to tell these apart is to ask yourself whether you need to calculate degeneracy.

The other equation ΔS=k

Posted: **Wed Feb 06, 2019 7:42 am**

These equations both calculate the entropy, or disorder, of the system. There are lots of equations to calculate entropy depending on the conditions of the system. The first equation is used for a reversible, isothermal process when the heat and temperature is given. Therefore, this is the equation that is used to calculate the entropy when given heat capacity charts. The second equation is used for residual entropy and is calculated at absolute zero (0 K). It tells the possible positions of the system and involves degeneracy. Basically you choose which equation to use based on the conditions and which values you are given.