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### Third Law Clarification/ Practice Midterm Question 3E

Posted: **Wed Feb 14, 2018 1:07 am**

by **Alexia Joseph 2B**

Why is it false that molecules at exactly 0 degrees Kelvin have no entropy? In theory, isn't the third law of thermodynamics that the entropy of a perfect crystal is zero when the temperature is equal to absolute zero (0 K)?

### Re: Third Law Clarification/ Practice Midterm Question 3E

Posted: **Wed Feb 14, 2018 9:04 am**

by **Sarah Rutzick 1L**

Its true that the entropy of a perfect crystal is 0 at T=0 K. This is because perfect crystals only have one micro state, so using Boltzmann's equation and plugging in 1 for W, you get zero. However, most substances are not perfect crystals, so for these substances entropy is not 0 at T=0 K.

### Re: Third Law Clarification/ Practice Midterm Question 3E

Posted: **Wed Feb 14, 2018 9:51 am**

by **Chem_Mod**

To clarify further, since the statement did not specify a perfectly ordered material, the statement is false. Otherwise it would have been true. Important concept is that the material must be at 0K AND perfectly ordered.