Molar enthalpy from Data booklet

[98.40_for_sure]

For the attached question, part's c) and d)
Why can't you use the delta H given in the data booklet?
Since they said "how much energy... standard conditions...per mole of glucose"
Can't you say E = delta H (2816 from data book) x 1

[Mao]

Because that'll just be silly. I'd imagine this exercise will attempt to compare databook values with experimental (calculated) values.

[98.40_for_sure]

Uhh... explain why it is 'silly'? seems to make sense to me at this ungodly hour

[akira88]

Uhh... explain why it is 'silly'? seems to make sense to me at this ungodly hour

I think it's "silly" because it's basically just finding the information in the data booklet and copying it down as an answer. Lol I'm up doing chemistry too, booo. Maybe since I'm in this thread I'll ask something too:
N204(g) <---> 2NO2 (g)
N204 is a colourless gas and NO2 is a brown gas. A mixture of N204 and NO2 is in a syringe. The plunger is then pushed in so the volume is halved. Is the brown colour more intense or less intense than before the volume was halved?

Well simple question really: Decrease in volume= increase in pressure, therefore the equilibrium position shifts to the direction with the least particles (the left, towards the N204). So I said the colour would be less intense. But nooo the answers have to go really in depth- the colour apparently is more intense, because as you increase the pressure, the concentration of the gases increase, therefore an increase in NO2 means a more intense brown colour.
O.O
In VCAA exams do we really have to be this picky? Or is it not picky at all? Or maybe it's just my fatigue lol. zZZzz

[98.40_for_sure]

You said... the colour is less intense? It's more intense though isn't it? Since the system only 'partially' opposes the change by shifting to the left. The overall colour change is more intense.

So your answer is completely wrong lol

[akira88]

You said... the colour is less intense? It's more intense though isn't it? Since the system only 'partially' opposes the change by shifting to the left. The overall colour change is more intense.

So your answer is completely wrong lol

Yeah I realised after I posted it up and couldn't be bothered getting rid of it haha.
Sleepiness ftw...

[98.40_for_sure]

Coffee + V + berocca + black powder = all night study

[andy456]

For the attached question, part's c) and d)
Why can't you use the delta H given in the data booklet?
Since they said "how much energy... standard conditions...per mole of glucose"
Can't you say E = delta H (2816 from data book) x 1

This was a bitch of a question.... (well for me)

we know that 1 mole of glucose produces 1 mole of electrons (from part a. half equation)
n(electrons)= 24
Q = n(electrons) x F
   = 24 x 96500
   =2316000

E= VIt
It=Q

E=VQ
E = 1.22 (from part b.) x 2316000
   = 2825520J
   =2825.52kJ/mole

I thinks thats how you do it....

[sajib_mostofa]

For the attached question, part's c) and d)
Why can't you use the delta H given in the data booklet?
Since they said "how much energy... standard conditions...per mole of glucose"
Can't you say E = delta H (2816 from data book) x 1

This was a bitch of a question.... (well for me)

we know that 1 mole of glucose produces 1 mole of electrons (from part a. half equation)
n(electrons)= 24
Q = n(electrons) x F
   = 24 x 96500
   =2316000

E= VIt
It=Q

E=VQ
E = 1.22 (from part b.) x 2316000
   = 2825520J
   =2825.52kJ/mole

I thinks thats how you do it....

I recall a question similiar question like that where you had to combine the two formula's to get E=QV. Might have been the 2008 VCAA exam.

[98.40_for_sure]

Yeah i know that's how you're meant to do it. But i don't see why you can't just use the E = delta H x no. of mol formula straight off since you know n=1, and delta H is given in data book for glucose...

[toshibaj]

Yeah i know that's how you're meant to do it. But i don't see why you can't just use the E = delta H x no. of mol formula straight off since you know n=1, and delta H is given in data book for glucose...

Obviously the people who wrote the exam overlooked the fact that molar enthalpy of glucose is in the data book.

If this were a VCAA exam it'd probably be okay to do it the way you did it (it would be like the 6/7 double bonds question from unit 3) but the examiners didn't want to test your ability to identify something in the data book in this case.

So the best thing to take from this is to learn from it, just in case a similar question comes up with a substance that isn't in the data book in the exam.

[matt123]

Yeah i know that's how you're meant to do it. But i don't see why you can't just use the E = delta H x no. of mol formula straight off since you know n=1, and delta H is given in data book for glucose...

tbh

i did that paper yesterday and i was tempted to do the same as you.
however

i think its trying to say ... find the energy per mol of glucose IN THIS REACTION ...
I.e . show how you would work it out , if we had 1 mol of glucose.
so they want you to just do the workings to PROVE that that we actualy get 2830 or watever amount of energy from this combustion reaction.

[Greggler]

its a dodgily worded question. i just popped down the number straight from the datat book, but i knew it couldnt be right considering it if like 3 marks, but then theres only two lines for working. in conclusion, stav is gay

[stonecold]

its a dodgily worded question. i just popped down the number straight from the datat book, but i knew it couldnt be right considering it if like 3 marks, but then theres only two lines for working. in conclusion, stav is gay

That bloody shits me up the walls.  they give you like half a page for a 2 mark question, and then for a 4 marker, you get 2 lines. go figure...

[stonecold]

Uhh... explain why it is 'silly'? seems to make sense to me at this ungodly hour

I think it's "silly" because it's basically just finding the information in the data booklet and copying it down as an answer. Lol I'm up doing chemistry too, booo. Maybe since I'm in this thread I'll ask something too:
N204(g) <---> 2NO2 (g)
N204 is a colourless gas and NO2 is a brown gas. A mixture of N204 and NO2 is in a syringe. The plunger is then pushed in so the volume is halved. Is the brown colour more intense or less intense than before the volume was halved?

Well simple question really: Decrease in volume= increase in pressure, therefore the equilibrium position shifts to the direction with the least particles (the left, towards the N204). So I said the colour would be less intense. But nooo the answers have to go really in depth- the colour apparently is more intense, because as you increase the pressure, the concentration of the gases increase, therefore an increase in NO2 means a more intense brown colour.
O.O
In VCAA exams do we really have to be this picky? Or is it not picky at all? Or maybe it's just my fatigue lol. zZZzz

We did this experiment in class, and yeah, that is exactly what happens.  Initially, it darkens because the concentration of both the colourless and brown gases increase, then, as the system moves to restore equilibrium, conc N2O4 increases, conc NO2 decreases, and then it will get lighter.  However it will not get as light as it used to be because the increase in concentration due to the decrease in volume is only partially offset.