For each pair of substances, choose the one that you expect to have the higher standard molar entropy (S°) at 25 °C. Explain your choices. a. CO(g); CO₂(g) b. CH₃OH(l); CH₃OH(g) c. Ar(g); CO₂(g) d. CH₄(g); SiH₄(g) e. NO₂(g); CH₃CH₂CH₃(g) f. NaBr(s); NaBr(aq)

Key Concepts

Standard Molar Entropy (S°)

Standard molar entropy (S°) is a measure of the randomness or disorder of a substance at a standard temperature of 25 °C and 1 atm pressure. It reflects the number of accessible microstates for a given amount of substance, with higher values indicating greater disorder. Entropy is influenced by factors such as molecular complexity, phase of matter, and temperature.

Molecular Complexity

Molecular complexity refers to the number of atoms and the structural arrangement within a molecule. More complex molecules, which have larger numbers of atoms and more degrees of freedom (such as rotational and vibrational modes), typically exhibit higher entropy. In comparing substances, the one with a more intricate structure is likely to have a higher standard molar entropy.

Gas Phase Behavior

In the gas phase, the behavior of molecules is characterized by greater freedom of movement compared to solids and liquids. Gases have higher entropy due to the increased distance between molecules and the ability to occupy a larger volume. When comparing gases, factors such as molecular size and shape can significantly affect their standard molar entropy, with larger or more branched molecules generally having higher entropy.