Knowing intrinsic properties of elements/compounds for GC

[zhngbrg]

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On DAT Destroyer GC #114, it asks to rank the melting points of MgO, fructose, NaCl, and HF. I originally put that HF would have a higher melting point than fructose since it would exhibit hydrogen bonding, but the answer said that fructose has a higher melting point.

Although I understand that it would make sense for fructose to have a higher melting point since it is a very large molecule compared to HF, I guess my question is on the actual DAT, at what point in size difference should you say that London dispersion forces "outweigh" the effects of hydrogen bonding? This seems like kind of a difficult and ambiguous distinction to make.

Also, for problem #105 in GC Destroyer, it asks:
"An unknown metal reacted with HCl to give a dark granular solid. Which metal most likely reacted?"

a. Zn
b. Pt
c. Ag
d. Au
e. S

I guessed Zn since it's the least shiny (lol), which was correct, but I don't really understand a better way to come to that conclusion. The answer just says that Pt, Ag, and Au are rather unreactive. Can anyone give a more indepth explanation of how to approach these types of problems?

 

[tommyngu]

Melting points are based on the strength of the intermolecular forces of the molecule. Fructose would actually have more hydrogen bonding than HF because of all of the OH groups so fructose has a higher melting point than HF.
Is the answer MgO < HF < fructose < NaCl ?
My thinking is that MgO is polar, but has no H-bonding.
HF is also polar, but has H-bonding
fructose has even more H-bonding
NaCl is ionic which is stronger than H-bonding.

As for the second question, I don't know... At first, I was thinking it was a solubility problem and so I would choose Ag since Cl- is insoluble with Ag2+, but Ag and HCl are actually not reactive...

Correction: MgO is actually ionic so the correct order should be: HF < fructose < NaCl < MgO

 

[Crithu]

Also, for problem #105 in GC Destroyer, it asks:
"An unknown metal reacted with HCl to give a dark granular solid. Which metal most likely reacted?"

a. Zn
b. Pt
c. Ag
d. Au
e. S

I guessed Zn since it's the least shiny (lol), which was correct, but I don't really understand a better way to come to that conclusion. The answer just says that Pt, Ag, and Au are rather unreactive. Can anyone give a more indepth explanation of how to approach these types of problems?

This is my guess:
The question says an unknown metal reacts, so we can already rule out choice e, Sulfur which is a non-metal.
If you look at a reactivity series, Silver (Ag), Gold (Au), and Platinum (Pt) are all less reactive than Hydrogen, so they would not undergo a single replacement reaction (in most cases).

 

[zhngbrg]

Melting points are based on the strength of the intermolecular forces of the molecule. Fructose would actually have more hydrogen bonding than HF because of all of the OH groups so fructose has a higher melting point than HF.

Oh right, that's embarrassing -_-

This is my guess:
The question says an unknown metal reacts, so we can already rule out choice e, Sulfur which is a non-metal.
If you look at a reactivity series, Silver (Ag), Gold (Au), and Platinum (Pt) are all less reactive than Hydrogen, so they would not undergo a single replacement reaction (in most cases).

Hmm ok, I've never seen a reactivity series in either my Gen Chem classes or when reviewing with Chad's. Would it be worth memorizing for the DAT or is that overkill?

 

[Crithu]

Oh right, that's embarrassing -_-



Hmm ok, I've never seen a reactivity series in either my Gen Chem classes or when reviewing with Chad's. Would it be worth memorizing for the DAT or is that overkill?

I memorized it for my gen chem class, but I don't know if it would really be a good use of your time. Someone might have a better insight, as I haven't taken my DAT yet, but if this is the only question I wouldn't bother.

If you do decide to memorize it, there are several mnemonics that can help.

 

[tommyngu]

I don't really think we're required to know the reactivity series... it's something you would look up in a chart. There really isn't any trend to it. Does anyone have any other explanations to the second question without the use of reactivity series?

 

[FancyFloss]

Hmm ok, I've never seen a reactivity series in either my Gen Chem classes or when reviewing with Chad's. Would it be worth memorizing for the DAT or is that overkill?

Took the 2014 DAT, you won't encounter ranking or the reactivity series. It will ask which of the molecules has the highest boiling point but it won't ask you to rank them. A good way to study is to rank them though.