# 21.8: Spectroscopic Entropies sometimes disgree with Calorimetric Entropies


The entropy of gases can be experimentally measured using calorimetry ($$S^\circ_{\text{exp}}$$) or calculated using spectroscopic methods ($$S^\circ_{\text{calc}}$$). For most molecules, the experiment and calculated values are in a good agreement, however, this is not true for all molecules. The discrepancy is referred to as residual entropy:

$\bar{S}_{\text{calc}}-\bar{S}_{\text{exp}} \nonumber$

Residual entropy arises from a material that can have many different states a 0 K. The third law of thermodynamics states that at zero kelvin, a substance will have an entropy of zero. In substances, such as glass, ice, and carbon monoxide, the substance can exist in many different configurations; it is not a perfect crystal, but must still have zero entropy according to third law. The material has residual entropy.

21.8: Spectroscopic Entropies sometimes disgree with Calorimetric Entropies is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.