Alloptropes and Solids, TBR

[leathersofa]

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I have two questions on alltotropes and

The first is the following:
How can the difference in melting point between two allotropes BEST be explained?
A. The two have different polarities.
B. The two have different molecular weights.
C. The two have different interatomic bonding.
D. The two have different electronegativities.

Answer: B (colored white if you wanna give a try) <--

I do not understand why it's not C, and TBR said how since all of the atoms have the same electronegativity, allotropes are nonpolar (eliminating D). I don't understand how allotropes are nonpolar just because of electronegativity...please explain this question, thank you!!

The second question is the following:
This question has a phase diagram in which each material has TWO solid phases..
How can the two different solids be explained for the compounds?
A . The two solids have different lattice structures.
B. The two solids have different connectivity of atoms.
C . The two solids have different molecular masses.
D. The two solids have different chiral centers.

The answer is A <--

I get why the answer isn't c but why can't it be B or D? how can 2 solids with different lattice structures have the same boiling and melting points?

thank you!!

 

[sps27]

Pardon my ignorance, but what is an allotrope? Also, what chapter and book in TBR is this from? I have not read that chapter at all......

 

[leathersofa]

This is from Gen Chem Book 2 and Section 7 (Phases and Phase Changes). They have a passage on allotropes in this section (page 114)

 

[SSerenity]

I think the reason is because the answer choices say: different interATOMIC bonding. Had it said different interMOLECULAR bonding, it would be correct. The molecules that make up the compound are still the same, so the atomic connectivity must be the same.

 

[leathersofa]

I think the reason is because the answer choices say: different interATOMIC bonding. Had it said different interMOLECULAR bonding, it would be correct. The molecules that make up the compound are still the same, so the atomic connectivity must be the same.

What is interatomic bonding then..?

even in isomers, the molecules are the same but it is the atomic connectivity that varies. so why can't that be teh case with this compound?

 

[monkeyvokes]

The second question is the following:
This question has a phase diagram in which each material has TWO solid phases..
How can the two different solids be explained for the compounds?
A . The two solids have different lattice structures.
B. The two solids have different connectivity of atoms.
C . The two solids have different molecular masses.
D. The two solids have different chiral centers.

The answer is A <--

I get why the answer isn't c but why can't it be B or D? how can 2 solids with different lattice structures have the same boiling and melting points?

thank you!!

Maybe B is too general and they, more specifically, have different lattice structures. They may or may not have different chiral centers?

 

[monkeyvokes]

For the first question, I think molecular weight is the best option given for different physical properties.
Also, different allotropes would be nonpolar because you are bonding the same atom with itself. No dipole is going to occur.

 

[leathersofa]

For the first question, I think molecular weight is the best option given for different physical properties.
Also, different allotropes would be nonpolar because you are bonding the same atom with itself. No dipole is going to occur.

but why is the answer for the first question B?

I now understand why it can't be A or D because allotropes are nonpolar, but why not D? What does interatomic bonding mean exactly?

 

[monkeyvokes]

but why is the answer for the first question B?

I now understand why it can't be A or D because allotropes are nonpolar, but why not D? What does interatomic bonding mean exactly?

Which letter are you wondering about?
I think interatomic and intramolecular bonding is the same thing, hopefully someone else can clarify if it is not.

 

[leathersofa]

whoops, sorry i meant C..
couldn't they have different interatomic bonding because diamond and graphite are bonded differently?

 

[monkeyvokes]

should all be nonpolar covalent

 

[leathersofa]

should all be nonpolar covalent

Hmm okay..just to make sure I understand what you're saying, you're saying that though for example diamond and graphite are allotropes of carbon and even though diamond has sp3 bonding and graphite bonds in sheets, their interatomic bonding is nonpolar covalent and as a result, they have the same interatomic bonding and intramolecular forces.

Is this what you mean? Thank you!

 

[monkeyvokes]

Hmm okay..just to make sure I understand what you're saying, you're saying that though for example diamond and graphite are allotropes of carbon and even though diamond has sp3 bonding and graphite bonds in sheets, their interatomic bonding is nonpolar covalent and as a result, they have the same interatomic bonding and intramolecular forces.

Is this what you mean? Thank you!

That is what it sounds like to me. Hopefully if any if that is inaccurate then someone will comment.
thanks

 

[leathersofa]

could someone explain their reasoning for the second question? thanks

 

[monkeyvokes]

could someone explain their reasoning for the second question? thanks

different lattice structures have different three dimensional arrangements e.g. graphene vs diamond - two different types of solids from the same element

 

[leathersofa]

different lattice structures have different three dimensional arrangements e.g. graphene vs diamond - two different types of solids from the same element

But though the phase diagram has two solid phases, both solids have the same melting and boiling point in the phase diagram. this is not the case with diamond and graphite

i was thinking it was B (different connectivity of atoms) because the two solids could be isomers like solid ethanol isomers or something..does this reason make sense?

thank you so much, monkeyvokes for all your help!

 

[monkeyvokes]

But though the phase diagram has two solid phases, both solids have the same melting and boiling point in the phase diagram. this is not the case with diamond and graphite

i was thinking it was B (different connectivity of atoms) because the two solids could be isomers like solid ethanol isomers or something..does this reason make sense?

thank you so much, monkeyvokes for all your help!

yeah that could make sense. This may be one of those instances in which you need to choose the best possible answer, rather than the only correct answer.

 

[DocAH]

That is what it sounds like to me. Hopefully if any if that is inaccurate then someone will comment.
thanks

The answer key actually says, "While it is true that allotropes have different interatomic bonding, bonds are not broken during a phase change so a difference in melting point can not be explained this way. Choice C is eliminated."

Though, I thought bonds are broken during a phase change? I thought that bonds are not broken when heat is added and there is a constant increase in temperature (and thereby kinetic energy). But bonds are broken when the phase changes from s --> l (in this case)?

@BerkReviewTeach can explain? This is from the phases & phase changes section