Third+Law


 * Third Law of Thermodynamics**

The third law of thermodynamics defines the lowest possible energy state as a theoretical position called //absolute zero//. At this temperature, 0 K, the entropy of a perfect crystal is zero (S = 0).

Once materials are cooled to near absolute zero, some interesting things happen. First, some isotopes of helium remain in the liquid state, and none of the atoms can escape the liquid, creating a perfect vacuum and a vapor pressure of zero. Solids will shrink to their smallest possible size (minimum thermal expansion). Gaseous atoms start to behave in strangely quantum ways, and form Bose-Einstein condensates.

Entropy changes can be predicted in two cases: where there is a change in state (solids are more ordered than liquids, which are more ordered than gases), and when temperature is involved.

Temperature is proportional to the amount of molecular or atomic motion in a substance. Recall that at absolute zero, there is no entropy, so this is interpreted as having no random motion. As temperature rises, so does motion. Molecules move more, and "wiggle" more, and hence have a greater entropy.

Example questions:

Which has a greater measure of entropy?

A) helium gas at 25 C, or helium gas at 30 C?

B) liquid water or solid ice?

C) HCl (g) at STP or HCl (g) at 1 ATM and 25 C?

D) Na (s) + Cl2 (g) --> 2 NaCl (s)

Entropy can also be calculated. This is done the same way as calculating enthalpy (dS = S(products) - S(reactants)).

Try the first page problems 1 - 15 on this [|worksheet]. Estimate whether entropy will increase (+S) or decrease (-S). Then calculate dS for the odds.