TBR 6 physical sciences question

[Joker88]

---

Members do not see this ad.

I was having trouble understanding this question:

5. Which of the following gases would have a negative value for the a term?

A. C2H6(g)
B. NH4+(g)
C. H3COCH3(g)
D. H3CNH2(g)

B is the best answer. A negative a value is associated with a gas in which the molecules repel one another. The charged gas (NH4+) is the only gas whose particles do repel one another at the molecular level. The other three choices all exhibit van der Waals attraction, so choices A, C, and D are eliminated. Choices C and D also exhibit dipole interactions, further eliminating them. The best answer is B.


This is the general equation they gave in the passage:

​

 

[MT Headed]

First of all, do you recognize and understand the equation? This equation is the PVnRT of real gases, not ideal gases.

A real gas molecule actually takes up volume. An ideal gas doesn't. The (V-nb) is an attempt to account for the extra volume that a real gas has, on account of the very real molecular volume.

A real gas has attractive forces between the molecules. An ideal gas doesn't. Imagine the extreme case where the gas molecules are so attractive that they clump together and plop into the bottom of your balloon. Well, now there aren't any molecules beating on the sides of the balloon, so the pressure goes down. The (P+a(n/V)^2) term attempts to take this natural loss of pressure into account.

But what's this? The question is asking about a negative value for "a"? Hmm, that means that the gas molecules must actually repel each other, even more that a real gas would. If you examine the answer choices now, I'm sure you will realize that in answer (B) the gas molecules would be particularly asocial and would rather beat themselves against the side of the balloon.

 

[paul411]

Read up on real gases. Above poster's explanation is a good start.

 

[whiteshadodw]

a negative a value indicates repulsion. like charge repels like charge, so NH4+, an overall positive molecule, will repel against itself. positive vs positive.

but if the question asked for a positive 'a value', you'd look for something that is polar and attracts itself. H20 is a good example.

A is nonpolar, so i'd guess it has an "a value" of zero
B is charged
C and D are slightly polar and if anything they attract each other just a bit, D more so than C due to possible H bonding.