Standard Entropies and the Third Law of Thermodynamics

If A// TAS is negative for the reaction, you would predict that it is spontaneous left to right, as written. However, if AH — TAS is positive, you would predict that the reaction is nonspontaneous in the direction written but spontaneous in the opposite direction. If AH — TAS is zero, the reaction is at equilibrium. 

To determine expaimentally the entropy of a substance, you first measure the heat absorbed by the substance by warming it at various temperatures. That is, you find the heat capacity at diffraent temperatures. You then calculate the entropy as we will describe. This determination of the entropy is based on the third law of thermodynamics. 

The process just described is essentially the numerical evaluation of an integral, which you can obtain as follows The heat absorbed for temperature change dT is Cp T)dT, and the entropy change is Cp(T)dT/T,The standard entropy at temperature Tis 

The third law of thermodynamics states that a substance that is perfectly crystalline at OK has an entropy of zero. This seems reasonable. A perfectly crystalline substance at 0 K should have perfect order. When the temperature is raised, however, the substance increases in entropy as it absorbs heat and energy disperses through it. 

Standard entropy of methyl chloride, CH3CI, at various temperatures (approximate schematic graph] 

---