In: Chemistry
Select the substance with the larger standard state molar entropy from each of the following pairs and justify your answer (2 pts each):
(a) Equal volumes of CO (g) at 2.0 bar pressure or 0.2 bar pressure
(b) 100% by volume chloroform (CHCl3) or 5% by volume chloroform
(c) Sodium iodide(aq) or sodium iodate(aq)
(d) Pb(s) or P(s)
(e) Ethanol(l) or methanol(l)
* "Standard State Molar Entropy" is the entropy of 1 mol of a substance at a standard temperature of 298 K; the units of S° are J/(mol·K).
a) Since it is calculated or a particular substance of 1 mole at a particular temperature, the Standard molar entropy(S°) is mainly dependent on Temperature and no. of moles of that substance, in the first case the molar entropy of the pair would be the same.
* Standard molar entropy of substances are calculated and tabulated (can be found in text-books and internet) and few of the selected substance's molar entropy are shown in the attached image :
From the table given a few trends can be observed such as :
Larger complex molecules have more standard molar enthalpy values than that ofsmaller or simpler molecules., as there are more possible arrangements of atoms in space for larger, more complex molecules, increasing the number of possible microstates.
The atoms/molecules can undergo several types of molecular motion, including translation, rotation, and vibration motions. The greater the molecular motion of a system, the greater the number of possible microstates and the higher the entropy.
b) Since 100% by volume of chloroform has higher amount of CHCL3 molecules than 5% , the former has more standard molar entropy.
c) From the images below:
:
we can conclude that sodium iodate { NaIO3} has more standard molar entropy than Sodium iodide {NaI} cause of molecule complexity which allows more rotation and more entropy.
d) Phosporous standard molar entropy is 41.1 J K-1mol-1.,
and lead standard molar entropy is 64.8 J K-1mol-1.,
where Cp is the constant pressure , temp dependent molar heat capacity and S0 is standard molar entropy,
Since Lead atoms are larger than Phosporous ,has larger vol per atom and weak interatomic forces .
Which leads to more Cp/T value than Phosporous and hence the standard molar entropy for Pb is > than P
e)Soft crystalline substances and those with larger atoms tend to have higher entropies because of increased molecular motion and disorder. Similarly, the absolute entropy of a substance tends to increase with increasing molecular complexity because the number of available microstates increases with molecular complexity. For example, compare the S° values for CH3OH(l) and CH3CH2OH(l). Finally, substances with strong hydrogen bonds have lower values of S°, which reflects a more ordered structure.
S° of Ethanol > Methanol.