MCQ 19, Fractional Distillation

[The]

I did B :] made sense to mee

[jimmy999]

I talked to my teacher about it and he believes B is the correct answer

[ilovevce]

Why can't C also be considered correct? When I did this (admittedly rushedly) I thought that the water the evaporates would rapidly condense on the packed glass beads in the column. Hence the change from midway through the column to the top would be very little. This mixture is just ethanol and water. Nobody is discussing C but I think A is truly the only one that can be discounted.

Think about it this way: as the water vapour moves up the column, it condenses, and then starts to trickle back down the beads. However, a fraction of this condensate is then vaporised again by the hot gases rising from below, and will continue moving up the colum again, where it will condense once more. Then as it trickles down a fraction will again be vaporised, and so on and so forth.
This practically results in a lower and lower concentration of water as you move towards the top, discounting option C.

[TrueTears]

I picked B because that's what Heinemann says. lol.

[monokekie]

D makes more sense to me now

[NE2000]

Why can't C also be considered correct? When I did this (admittedly rushedly) I thought that the water the evaporates would rapidly condense on the packed glass beads in the column. Hence the change from midway through the column to the top would be very little. This mixture is just ethanol and water. Nobody is discussing C but I think A is truly the only one that can be discounted.

Think about it this way: as the water vapour moves up the column, it condenses, and then starts to trickle back down the beads. However, a fraction of this condensate is then vaporised again by the hot gases rising from below, and will continue moving up the colum again, where it will condense once more. Then as it trickles down a fraction will again be vaporised, and so on and so forth.
This practically results in a lower and lower concentration of water as you move towards the top, discounting option C.

Actually now that I look at it, are you sure there are glass beads in that column? I know there are meant to be but it looks more like a thermometer..... :S

In any case, while B may be considered the best answer. What I'm saying is that if B and D are both accepted, then should C be accepted too as it is essentially an intermediate between the two?

[rhjc.1991]

I think D won't be accepted now.

Most likely, the examiners would assume that we have the Heinemann knowledge.

[NE2000]

And also just to ask a question that was nagging in the back of my mind on the other contentious MC: the hydrogen as a carrier gas one. Without meaning to beat on about something that's long over and can't be changed, wouldn't hydrogen react with the stationary phase (generally high boiling point liquid hydrocarbons like methyl esters) at such high temperatures. And would hydrogen blow up the flame ionization detector by any chance (that has oxygen)?

[Over9000]

Mate, who knows? The exam is over, just wait until the grades come out and inspect your paper, nobody knows for sure the right answers, nobody here is the chief VCAA examiner.

[monokekie]

my examiner teacher said its B.........

[kashmuq]

Ok so is it definite that B is one of if not the only answer? And also I don't understand why people are getting worked up about this 1 mark MC question when there were so many short answer questions that had much more of an impact XD

[Over9000]

every mark counts mate.

[kashmuq]

My teacher said he expects only around 10% of state to get this question right and if you go by what he said, this question won't have that much of an impact in the scheme of things - even though every mark counts...

[biggzee]

i said B since the beads are in the fractionating colomn specifically to recondense any vapour that isnt wanted there, so water would be recondensed more as the height incresed, so there would be more ethanol as a percentage

[/0]

I hope the examiners made a 'mistake' on the exam somewhere so VCAA will pressure them to make exam 2 easier.