Full question:
First of all, bear in mind that this question is incomplete. Here is the full question:
Which substance has the lowest standard molar entropy at 25°C?
a) N₂O₄(g)
b) He(g)
c) NO₂(g)
d) NO(g)
Answer:
He has the lowest standard molar entropy (126.1 J/K.mol)
Explanation:
Standard entropy is the absolute entropy of a substance at 1 atm and 25°C.
Helium is a monatomic gas, which cannot execute rotational or vibrational motions, in contrast with the other gases of the list. We need to keep in mind that that the spinning motion of an atom about its own axis does not constitute a rotational motion because it does not displace the position of the nucleus.
The lowest standard molar entropy at 25°c for this gases (in J/K.mol) is:
N₂O₄(g) : 304.3
He(g) : 126.1
NO₂(g): 240.46
NO(g): 210.6
The standard molar entropy is directly proportional to molar mass hence He(g) has the lowest standard molar entropy.
The entropy of a substance refers to the degree of disorderliness of a substance. Usually, gases are more disorderly than liquids and liquids are more disorderly than gases. This means that a gas has the highest degree of disorderliness while a solid has the least degree of disorderliness.
Having said that, we can see that among the options, the substance that has the lowest standard molar entropy at 25°c is He(g) because He(g) has the lowest molar mass.
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Missing parts;
Which substance has the lowest standard molar entropy at 25°C? N2O4(g) He(g) NO2(g) NO(g).
