Boiling and Melting Point Clarifications Please

[BKLYN718]

Hey SDN,
I'm trying to just get some clarification on Melting point and Boiling Point properties between Ionic and Covalent/ Molecular bonds?

I had a question thats asked for which had the highest Melting Point. Between NaF(s), NaI(s), NaBr(s),NaCl(s)
answer: NaF
-I get the fact that they're all Ionic so bond between Na-F requires more energy to break due to the size of F versus the other halogens.

Then I saw a question that asked for which has the highest MP/BP: C-I, C-F, C-Br, C-Cl
answer: C-I
- I also understand as we go down the group the bond becomes more polarizable and easier to dissociate so I can see why it would increase MP and BP.
- I was also taught that going down groups 17 and 18 and the heavier the hydrocarbons would also increase MP and BP.

So I guess my question is
1) Is it alright to assume that molecular bonds, and halogens/ Noble gases increase BP and MP going down a group?
2) Is it okay to assume that strong intermolecular bonds such as Ionic increase BP and MP when going up Group 17

Apologies if this was a stupid question to post. Just needed some clarification.

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[dentalstudent2021]

1. NaF(s), NaI(s), NaBr(s),NaCl(s) answer: NaF...my reasoning is strictly based on electronegativity. F bonds are very hard to break because F is most electronegative atom.

2. highest MP/BP: C-I, C-F, C-Br, C-Cl...answer: C-I...The primary forces here are vander-waals and these forces becomes stronger as weight increases. Iodine is the heaviest of all the halogens listed, thus leads to higher MP/BP

I am not completely sure about the assumptions you have listed. never really learned it that way.

 

[gangazi]

*Patiently waits for the orgoman

 

[rak173]

I always get confused between BP and MP. Does a higher BP always mean a higher MP and a lowest BP is same as lowest MP?

 

[DAT Destroyer]

Hey SDN,
I'm trying to just get some clarification on Melting point and Boiling Point properties between Ionic and Covalent/ Molecular bonds?

I had a question thats asked for which had the highest Melting Point. Between NaF(s), NaI(s), NaBr(s),NaCl(s)
answer: NaF
-I get the fact that they're all Ionic so bond between Na-F requires more energy to break due to the size of F versus the other halogens.

Then I saw a question that asked for which has the highest MP/BP: C-I, C-F, C-Br, C-Cl
answer: C-I
- I also understand as we go down the group the bond becomes more polarizable and easier to dissociate so I can see why it would increase MP and BP.
- I was also taught that going down groups 17 and 18 and the heavier the hydrocarbons would also increase MP and BP.

So I guess my question is
1) Is it alright to assume that molecular bonds, and halogens/ Noble gases increase BP and MP going down a group?
2) Is it okay to assume that strong intermolecular bonds such as Ionic increase BP and MP when going up Group 17

Apologies if this was a stupid question to post. Just needed some clarification.

You are confusing two HUGE concepts......When dealing with ionic compounds,,,,,,The melting point depends on 2 factors,,,,,,charge and distance. The energy needed to break the crystal lattice is called the Lattice energy. Charge is the more important factor. Since charge is equal when dealing with the fluorides, chlorides, bromides, and iodides of sodium in this example, we must look at distance. Alas.... Fluorine is the smallest , hence the specie would have the greatest lattice energy and consequently the highest melting point. Now for the second part of your perspicacious inquiry......C-I, C-Br, C-Cl. and C-F does not exist in a crystal lattice the way ionic compounds do, thus the Van Der Waals Force is the MAIN factor. Indeed so, The C-I compound has the highest melting and boiling point. For species such as Noble Gases, Halogens, and hydrocarbons mass is generally the main criteria. Thus, Halogens and Noble gases have increasing melting and boiling points down a group. For Ionic compounds, you need to be more careful. For example, MgO would have a higher melting point than NaF. Why ? As I said before....CHARGE dominates over the size. The Brown and LeMay text does a wonderful job showing you examples of the discussed trends if you need more clarification. Thanks for such a delightful question to the SDN community.

Hope this helps.

Dr. Romano

 

[BKLYN718]

You are confusing two HUGE concepts......When dealing with ionic compounds,,,,,,The melting point depends on 2 factors,,,,,,charge and distance. The energy needed to break the crystal lattice is called the Lattice energy. Charge is the more important factor. Since charge is equal when dealing with the fluorides, chlorides, bromides, and iodides of sodium in this example, we must look at distance. Alas.... Fluorine is the smallest , hence the specie would have the greatest lattice energy and consequently the highest melting point. Now for the second part of your perspicacious inquiry......C-I, C-Br, C-Cl. and C-F does not exist in a crystal lattice the way ionic compounds do, thus the Van Der Waals Force is the MAIN factor. Indeed so, The C-I compound has the highest melting and boiling point. For species such as Noble Gases, Halogens, and hydrocarbons mass is generally the main criteria. Thus, Halogens and Noble gases have increasing melting and boiling points down a group. For Ionic compounds, you need to be more careful. For example, MgO would have a higher melting point than NaF. Why ? As I said before....CHARGE dominates over the size. The Brown and LeMay text does a wonderful job showing you examples of the discussed trends if you need more clarification. Thanks for such a delightful question to the SDN community.

Hope this helps.

Dr. Romano

Thank you so much for the clarification! You have always been a big help. Glad I was able to ask before my exam in a couple weeks

 

[gangazi]

You are confusing two HUGE concepts......When dealing with ionic compounds,,,,,,The melting point depends on 2 factors,,,,,,charge and distance. The energy needed to break the crystal lattice is called the Lattice energy. Charge is the more important factor. Since charge is equal when dealing with the fluorides, chlorides, bromides, and iodides of sodium in this example, we must look at distance. Alas.... Fluorine is the smallest , hence the specie would have the greatest lattice energy and consequently the highest melting point. Now for the second part of your perspicacious inquiry......C-I, C-Br, C-Cl. and C-F does not exist in a crystal lattice the way ionic compounds do, thus the Van Der Waals Force is the MAIN factor. Indeed so, The C-I compound has the highest melting and boiling point. For species such as Noble Gases, Halogens, and hydrocarbons mass is generally the main criteria. Thus, Halogens and Noble gases have increasing melting and boiling points down a group. For Ionic compounds, you need to be more careful. For example, MgO would have a higher melting point than NaF. Why ? As I said before....CHARGE dominates over the size. The Brown and LeMay text does a wonderful job showing you examples of the discussed trends if you need more clarification. Thanks for such a delightful question to the SDN community.

Hope this helps.

Dr. Romano

Does MgO have higher boiling point than NaCl as well?

 

[DAT Destroyer]

Does MgO have higher boiling point than NaCl as well?

Yes.....indeed. However, when dealing with ionic solids, we normally discuss melting points. 6500 degrees F vs 2500 degrees F.

Hope this helps