Molar heat of combustion vs. Molecular weight. (1 Viewer)

[nadz001]

lol ok i thought it was 1400kl/mol n for b i wrote somthin to do with the more molecular weight the more higher the boiling point is therefore it will release high energy when break the bond dnt nooo now after readin all our answers:{

 

[Fish Sauce]

the graph can NOT go through the origin if the origin is zero...

and b is incomplete combustion

we did a whole assessment task worth 25% on this shit

Impure alkanols is also a valid reason. I can't think of any besides those two.

 

[Steth0scope]

yea soz and that too

 

[Shrikar]

Did anyone actually draw a line from the origin that passed through the points, then continued on?

I actually just joined the dots.

 

[Forbidden.]

Impure alkanols is also a valid reason. I can't think of any besides those two.

Yeah maybe there was too much carbon-14 isotopes in the structure of the alcohol if I were ever to have thought of impurities.

 

[Looking Glass]

Fuck offcourse, incomplete combustion. How the fuck did I miss that? "Experiment not conducted at SLC" - fuck, what the fuck man.

 

[tmde1st]

Silly question:

did you remember to plot the molecular weights according to the substances molecular weight and not a fixed point in the x-axis??

well i got 1360

Sweet, i got 1360 as well, i think, or something that was very close to it. The molecular weights increased by 14 each time, so you could still use the x axis, with increments of 14.

 

[LankanG]

yeh i got 1360kj / mol

didnt they label x-axis as the molecular weight axis for us ??
confusing stuff...but yeh my reason was the incomplete combustion of the alkanol may have occured due to the insufficient supply of oxygen

 

[Shrikar]

I don't even remember doing that question.

 

[NJMORTIMER]

I don't even remember doing that question.

This competition is over! You sir win the HSC!
On an unrelated note, I got around the 1300 mark as well. Did anyone else accidentaly put the units on the graph as gmol^-1 instead of using the molecular mass units, dang I hope I don't lose too many marks for that...

 

[aja152]

i did a line of best fit and my answer to the propanol was 1370.
i didnt write the units,, would i matter? (hope not -_-)

my reason was incomplete combustion, insufficient supply of oxygen^^

 

[NJMORTIMER]

I don't think it would matter as long as you wrote the numbers along the x axis as *something* x 6.022x10^-23
But I didn't even do that, I just put the wrong units down

*Edit* It turns our the units for molecular mass are 'u'
Did anyone put this down as the units for the x axis? Did we even cover this in the course? What did you all put down for the units?

 

[Shrikar]

This competition is over! You sir win the HSC!
On an unrelated note, I got around the 1300 mark as well. Did anyone else accidentaly put the units on the graph as gmol^-1 instead of using the molecular mass units, dang I hope I don't lose too many marks for that...

I was talking about the question which asked why the value was less than the theoretical value, not what the value of the molar heat of combustion of 1-propanol.

 

[ravdawg]

I don't think it would matter as long as you wrote the numbers along the x axis as *something* x 6.022x10^-23
But I didn't even do that, I just put the wrong units down

*Edit* It turns our the units for molecular mass are 'u'
Did anyone put this down as the units for the x axis? Did we even cover this in the course? What did you all put down for the units?

The units are g/mol...

 

[yoakim]

I honestly see no reason for using a line of best fit. It is an essential scientific skill, but I don't think the skill is always appropriate in this context? I don't see the use of a line of best fit when comparing alkanes, alkanols and alkanoic acids in relation to their molecular masses.

I drew three distinct lines, each corresponding to each type, all starting from 0. I don't see how that is incorrect or why people who did the same method as me should lose marks.

Anywho, my predicted value for propanol was 1300 KJ/mol, which is caused by a reason such as limited availability of oxygen - incomplete combustion.

 

[Originality]

i thought the "line best fit" only applies to calibration curves. Didnt thought it was necessary for anything else.

 

[yoakim]

Mmmmm! I forgot that the Molecular weights can never be zero.

 

[pires_w]

oh god! i had no idea for this question

but i plotted same 200 for vertical and 10 for horizontal. i had a line of best fit.
estimated combustion was 1360 kj/mol
and i had a really stupid answer for the last part >< "cuz the propanol had impurities in it" bah!! i couldn't think of anything!!

i put 1360 as well =]

my another ans was... incomplete combustion=]

 

[jah_lu]

line of best fit is used for several reasons
for one, many graphs dont start from 0 so it would be pointless to force it through the origin (think planck's constant exp in physics)
the lobf (or curve) is meant to provide a relationship between the data you got
it also makes your analysis more valid because the lobf excludes outliers

 

[Forbidden.]

line of best fit is used for several reasons
for one, many graphs dont start from 0 so it would be pointless to force it through the origin (think planck's constant exp in physics)
the lobf (or curve) is meant to provide a relationship between the data you got
it also makes your analysis more valid because the lobf excludes outliers

If I started further up the y-axis, one of the values would lie out even further.