Textbook Problem 4H.1

[Marcus Lagman 2A]

Hello!

Question: Which substance in each of the following pairs has the higher molar entropy at 298 K: (a) HBr(g) or HF(g); (b) NH3(g) or Ne(g); (c) I2(s) or I2(l); (d) 1.0 mol Ar(g) at 1.00 atm or 1.0 mol Ar(g) at 2.00 atm?

I do not recall if Dr. Lavelle discussed in lecture how to find which substance has the higher molar entropy. If someone could perhaps refresh me on the topic, it would be appreciated!

[Varsha Ravi 3E]

I think this problem is about entropy generally rather than molar entropy, so for each pair of substances you try to figure out which one has the higher entropy. For example, the last question asks about which of two gases has higher a higher entropy at two different pressures.

[Alexandra Ahlschlager 1L]

Some general guidelines for how to determine what has higher entropy is by looking at the states of the molecules. So liquids will have higher entropy than solids, and gases - which are very disordered - will have higher entropy than liquids. You can also look at degeneracy, so a compound with a lot of different possible orientations will have a higher entropy, which follows looking at the equation S = kb*lnW. Increasing volume will also increase entropy since there are basically more possible locations that the molecules can be in, and you can also use volume’s relationship to pressure to help you reason out part d. Hope this helps a little!

[Massimo_Capozza_1G]

So the larger the molecule, the more complex it is and in turn the higher the enthalpy?

[Angelica Soriano 3L]

So the larger the molecule, the more complex it is and in turn the higher the enthalpy?

Yes, generally, that is the case for entropy! (assuming that "enthalpy" was a typo)