VCE Chemistry Question Thread

[geminii]

Hey everyone! How come heavier molecules tend to travel further up the stationary phase (paper) in chromatography, despite the influence of gravity??

[TFAnime123]

Hey guys,
I need help with this question and an explanation to the answer.

Which one of the following has the lowest boiling point?"
A) 2,3-dimethylbutane
B) Hexane
C) 2,3-dimethylpentane
D) 3-methylpentane

[peterpiper]

Hey guys,
I need help with this question and an explanation to the answer.

Which one of the following has the lowest boiling point?"
A) 2,3-dimethylbutane
B) Hexane
C) 2,3-dimethylpentane
D) 3-methylpentane

Is it A? Because my thought process was that the shorter the chain the less surface area on which dispersion forces act intermolecularly. And the more branched it is the less able they are able to pack together.

[KANYEWEST]

hello,

just wondering if anyone has access to:
 EDROLO : UNIT 4 AOS 1 - HOW CAN THE DIVERSITY OF CARBON COMPOUNDS BE EXPLAINED AND CATEGORISED? - TOPIC EXAM 2

by teacher just had a child so instead we have a sub, and the topic 2 exam is locked (the sub has no access to edrolo) whereas the topic 1 exam i have already completed!

thank you, have a good night

[sonnyangel]

Hey everyone! How come heavier molecules tend to travel further up the stationary phase (paper) in chromatography, despite the influence of gravity??

Not sure if this answers your question but molecules with a strong affinity to water tend to travel further up the stationary phase, it's not so dependent on the size of the molecule

[VanillaRice]

Is it A? Because my thought process was that the shorter the chain the less surface area on which dispersion forces act intermolecularly. And the more branched it is the less able they are able to pack together.

I'd agree with this 

Not sure if this answers your question but molecules with a strong affinity to water tend to travel further up the stationary phase, it's not so dependent on the size of the molecule

It's worth noting that the mobile phase is not necessarily always water - there are also nonpolar (as well as non-water polar) solvents which can be the mobile phase. The compounds which move the furthest up the plate are usually the ones that have the highest affinity for the mobile phase.

[mb_xo]

Hey I'm a bit confused with  "the comparison of fossil fuels (coal, crude oil, petroleum gas, coal seam gas) and biofuels (biogas, bioethanol,
biodiesel) with reference to energy content.

Could someone please give me an explanation of energy contents of these like which releases most, little etc and in order thankyou...

[tinagranger]

MC question: One faraday of electric current is equivalent to -
ans = 6.02 x 10²³ electrons per second.

Someone please explain?

[sonnyangel]

MC question: One faraday of electric current is equivalent to -
ans = 6.02 x 10²³ electrons per second.

Someone please explain?

Q(electric charge)=n(electrons) * Faraday's constant (96500)
Therefore, one faraday of electric current means that Q=96500,
----> 96500=n(electrons) * 96500
----> n(electrons)=1
n(electrons) is in mols, hence the answer is 6.02 x 10^23 electrons per second

Hope that helps!

[peanut]

Which has a higher solubility in water, ethanol or ethanal? Why?

[VanillaRice]

Which has a higher solubility in water, ethanol or ethanal? Why?

Ethanol - the electronegativity difference between the O and H of the alcohol group is relatively large, and the alcohol group will also allow the molecule to form hydrogen bonds with water. While there is still a dipole moment between the C and O of the aldehyde (C=O) the delta charged regions are not as great (so no hydrogen bonding). Therefore, the intermolecular bonds between ethanal and water are not as great as those of ethanol and water (which makes ethanol more water-soluble).

Hope this helps 

[peanut]

Ethanol - the electronegativity difference between the O and H of the alcohol group is relatively large, and the alcohol group will also allow the molecule to form hydrogen bonds with water. While there is still a dipole moment between the C and O of the aldehyde (C=O) the delta charged regions are not as great (so no hydrogen bonding). Therefore, the intermolecular bonds between ethanal and water are not as great as those of ethanol and water (which makes ethanol more water-soluble).

Hope this helps 

Thanks, however, my textbook seems to state that aldehydes DO form hydrogen bonds with water. Could this be a "weaker" hydrogen bond with water (than ethanol and water)?

[sweetiepi]

Thanks, however, my textbook seems to state that aldehydes DO form hydrogen bonds with water. Could this be a "weaker" hydrogen bond with water (than ethanol and water)?

I believe that aldehydes do form weaker hydrogen bonds with water.

I found this diagram that says that ethanal is less soluble in water.

(Not sure how to explain this though )

[VanillaRice]

Thanks, however, my textbook seems to state that aldehydes DO form hydrogen bonds with water. Could this be a "weaker" hydrogen bond with water (than ethanol and water)?

My bad! insanipi is right, the there is hydrogen bonding possible C=O --- H-O, which is weaker than that for ethanol and water (since the dipole is not as big).

[Bri MT]

Thanks, however, my textbook seems to state that aldehydes DO form hydrogen bonds with water. Could this be a "weaker" hydrogen bond with water (than ethanol and water)?

Given that aldehydes have no hydrogen directly bonded to a oxygen fluorine or neon I thought that aldehydes are highly polar but don't experience hydrogen bonding. Could someone explain why they think hydrogen bonding is present?