Sunday, February 3, 2008

JEE Revision - Entropy


Entropy (S) is a measure of the amount of disorder in a substance Gases with their rapid random motion are high in entropy, and solids with their ordered crystalline lattice are low in entropy.

The change in entropy (∆S) is determined just like a heat of formation problem, only use entropy values instead.

∆Srxn = (the sum of ∆Sproducts ) - (the sum of the ∆Sreactants )

Note that the units for entropy are given in J / mol K. ∆S is in J / K, since the moles cancel out.

K stands for Kelvins, the temperature unit on the absolute scale. Also called the Kelvin scale, it is named for Lord Kelvin, who developed it, so the units should be capitalized.

Entropy is temperature affected. It is large at high temperatures, and small at low temperatures. Enthalpy is not temperature affected.

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