EK 1001 Organic Chemistry #264

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heylollipop

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This question is under the "Alkanes" section.

In cold weather, diesel fuel, which is composed of 14 carbon alkanes, freezes. Which of the following would work best as an additive to diesel fuel to prevent freezing?

A. H2O
B. ethanol
C. gasoline (composed of pentanes through octanes)
D. mineral oil (composed of 16 to 18 carbon alkanes)

As I understand it, we want to an additive that will prevent freezing, meaning decreasing the freezing point, or stay in the liquid form. The answer is C. I got that water and ethanol would H-bonds, raising the freezing point. But why would a shorter carbon chain lower the freezing point, as apposed to a longer chain? Someone please enlighten me. Thank you in advance!

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Here is how I understand it: A lot of physical properties of solutions have to do with the concentration of solutes. Freezing point for instance would be decreased by increasing the concentration of solutes in the liquid phase. Because like dissolves like, we would expect diesel to be able to dissolve other non-polar alkanes quite readily. This leads us to answers C or D. Because smaller molecular weight alkanes have lower melting point and therefore lower freezing point, we would prefer to dissolve a smaller alkane chain into our diesel. That leads us to choice C. H-bonding is a good thought, but there are no hydrogen acceptors in an alkane chain.

Hope this makes sense.
 
Here is how I understand it: A lot of physical properties of solutions have to do with the concentration of solutes. Freezing point for instance would be decreased by increasing the concentration of solutes in the liquid phase. Because like dissolves like, we would expect diesel to be able to dissolve other non-polar alkanes quite readily. This leads us to answers C or D. Because smaller molecular weight alkanes have lower melting point and therefore lower freezing point, we would prefer to dissolve a smaller alkane chain into our diesel. That leads us to choice C. H-bonding is a good thought, but there are no hydrogen acceptors in an alkane chain.

Hope this makes sense.
(edit) I get where I went wrong now. I incorrectly reasoned that 14+ chain hydrocarbons would exist in liquid form, but the truth is, they would exist in solid form since their freezing/melting point is at a temperature above smaller chain hydrocarbons. To form a liquid mixture/prevent freezing, you'd need to combine it with smaller chain hydrocarbons because those exist in liquid form (because of lower freezing/melting point temperature).

@chill3, Thank you for your explanation. Question though, ...I thought the whole freezing point depression thing generally applies to ions added to a solution, or is that the case for any mixture of solutes? Any idea?
 
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Wow!

The "mish-mosh" of supplied solutions to the original question are more confusing than the question itself.

The question does have to do with freezing point depression. When a contaminant dissolves in a pure substance (i.e. ethylene glycol [anti-freeze] in pure water), the freezing point of the pure substance, the solvent, is depressed (i.e. won't freeze at 0 degrees Celsius).

The hydrogen bonding that is possible in choices A and B, H2O and ethanol, would effect their respective boiling points, but h-bonding has little to do with this question, and diesel fuel is hydrophobic, so those two substances wouldn't dissolve, so they wouldn't affect the freezing point depression. A and B are out!

That leaves you with choices C and D. Now you are looking at dispersion forces determining the possibility of diesel remaining a liquid at low temperatures (i.e the great MW of diesel fuel, a 14 carbon hydrocarbon). Although the question stem does not indicate the exact temperature, it does say "it's cold," so we need something to depress the freezing point of diesel fuel.

Mineral Oil is a 16-18 carbon long hydrocarbon, so it will not dissolve into diesel, so choice D is out!

Choice C, gasoline, has everything for which you are looking.

1) lesser MW than diesel fuel, so it will dissolve
2)Able to dissolve because both liquids are non-polar
3) 1&2 allow for freezing point depression

So the best answer is choice C.

Note: Just think of the freezing point depression equation --> dT=kf *molality*i. Just because the i, the van't hoff factor, is considered due to neither the solute nor solvent in question being ionic, it doesn't mean that freezing point depression isn't the topic at hand.

I hope this helps!
 
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Here is how I understand it: A lot of physical properties of solutions have to do with the concentration of solutes. Freezing point for instance would be decreased by increasing the concentration of solutes in the liquid phase. Because like dissolves like, we would expect diesel to be able to dissolve other non-polar alkanes quite readily. This leads us to answers C or D. Because smaller molecular weight alkanes have lower melting point and therefore lower freezing point, we would prefer to dissolve a smaller alkane chain into our diesel. That leads us to choice C. H-bonding is a good thought, but there are no hydrogen acceptors in an alkane chain.

Hope this makes sense.

The bolded, underlined, and crossed out phrase from your quote is wrong. Lower melting point does not necessarily mean lower freezing point.
 
The bolded, underlined, and crossed out phrase from your quote is wrong. Lower melting point does not necessarily mean lower freezing point.
When in reference to the question it can be assumed they are talking about n-alkanes that are straight chain. So his statement is true. Smaller n-alkanes have lower melting and therefore lower freezing points. Or please clarify your reasoning for saying that is not true. Ty.
 
When in reference to the question it can be assumed they are talking about n-alkanes that are straight chain. So his statement is true. Smaller n-alkanes have lower melting and therefore lower freezing points. Or please clarify your reasoning for saying that is not true. Ty.

I will quote the portion I underlined again.
"Because smaller molecular weight alkanes have lower melting point and therefore lower freezing point"

His quote was about MW and freezing point. He never alluded to straight chain vs branching, etc., so my point stands that, as I will quote from my post "Lower melting point does not necessarily mean lower freezing point." As in not taking MW as a rule of thumb.

You may have assumed that point from the question itself, but it's a stretch to assume that was what the responder meant based on his quote.

Furthermore, "smaller or bigger" are relative terms in this case. A 18-carbon straight chain alkane is "big" due to its MW. A t-butyl, a 4-carbon group, is "big" due to volume.

Moreover, as I said, when comparing straight chain vs. bulky groups, a straight chain has a higher boiling point, and depending on packing may also have a lower freezing point, so the lower MP = lower FP is not helpful. In your quoted scenario, the lower melting point may lead to a HIGHER freezing point. (i.e. freezing at a temperature greater than the freezing point of water). A "LOWER" freezing point means a GREATER freezing point depression occurred.
 
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I will quote the portion I underlined again.
"Because smaller molecular weight alkanes have lower melting point and therefore lower freezing point"

His quote was about MW and freezing point. He never alluded to straight chain vs branching, etc., so my point stands that, as I will quote from my post "Lower melting point does not necessarily mean lower freezing point." As in not taking MW as a rule of thumb.

You may have assumed that point from the question itself, but it's a stretch to assume that was what the responder meant based on his quote.

Furthermore, "smaller or bigger" are relative terms in this case. A 18-carbon straight chain alkane is "big" due to its MW. A t-butyl, a 4-carbon group, is "big" due to volume.

Moreover, as I said, when comparing straight chain vs. bulky groups, a straight chain has a higher boiling point, and depending on packing may also have a lower freezing point, so the lower MP = lower FP is not helpful. In your quoted scenario, the lower melting point may lead to a HIGHER freezing point. (i.e. freezing at a temperature greater than the freezing point of water). A "LOWER" freezing point means a GREATER freezing point depression occurred.
Fair enough. Ty for response, but the answer in the back is in reference to alkane length saying...
"Answer C is the only answer that would be less likely to form a solid. Answers A and B hydrogen bond and mineral oil is longer than diesel fuel. The idea is that an additive with a lower melting point needs to be used."

(lower MP being a shorter alkane for this question specifically)

Also,,, I would ask why a 16-18 carbon alkane would not dissolve in a 14C alkane but that isn't very relevant now.
Mineral Oil is a 16-18 carbon long hydrocarbon, so it will not dissolve into diesel, so choice D is out!
 
Fair enough. Ty for response, but the answer in the back is in reference to alkane length saying...
"Answer C is the only answer that would be less likely to form a solid. Answers A and B hydrogen bond and mineral oil is longer than diesel fuel. The idea is that an additive with a lower melting point needs to be used."

(lower MP being a shorter alkane for this question specifically)

Also,,, I would ask why a 16-18 carbon alkane would not dissolve in a 14C alkane but that isn't very relevant now.

My reply in which I crossed out one of the phrases was in regard to not using longer necessary means lower.

I don't use EK science problems. I look through the forums for extra practice, so I don't have their official answer, which wasn't great in itself.

It is easier to draw an inference when the answer key is in front of the reader.
 
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