TrueTears question thread

[TrueTears]

cool thanks polky!

[TrueTears]

Also there are a few Q on the bottom on page 3, if anyone could lend a help it would be greatly appreciated

[TrueTears]

Fractional distillation has been described as a series of simple distillation. Explain

[TrueTears]

Also this question:

Methane undergoes a series of substitution reactions with bromine to form a number of components with the following boiling points ( all measured in deg Celsius): Bromomethane (3.6 deg), dibromomethane (97 deg), tribromomethane (150 deg), and tetrabromomethane (190 deg). Describe how you would obtain pure samples of each compound from the mixture using fractional distillation.

I know how fractional distillation works, but just a bit stomped on this Q. When it says "undergoes a series of reactions" does that mean all the 4 products, ie Bromomethane, dibromomethane etc etc are all mixed in the one distillation flask? Or is it just 1 product in the distillation flask and then have bromine react again to form the next kind of product?

Any help would be really helpful XD

[iamdan08]

Also this question:

Methane undergoes a series of substitution reactions with bromine to form a number of components with the following boiling points ( all measured in deg Celsius): Bromomethane (3.6 deg), dibromomethane (97 deg), tribromomethane (150 deg), and tetrabromomethane (190 deg). Describe how you would obtain pure samples of each compound from the mixture using fractional distillation.

I know how fractional distillation works, but just a bit stomped on this Q. When it says "undergoes a series of reactions" does that mean all the 4 products, ie Bromomethane, dibromomethane etc etc are all mixed in the one distillation flask? Or is it just 1 product in the distillation flask and then have bromine react again to form the next kind of product?

Any help would be really helpful XD

Hmmm...the wording of the question is a bit ambiguous. I think what the question is asking, is that if you underwent the series of reactions and the distillation flask contained all 4 products (perhaps because there was not 100% yield in some of the steps, so there will be several species at the end), how would you obtain the pure samples.  So i think the question is really just asking how you would obtain the pure samples using fractional distilation, since in most reactions, yield is not 100% and reactions contain both reactants and products at the end (you learn more about this in unit 4). Hope that helps a little.

[iamdan08]

Fractional distillation has been described as a series of simple distillation. Explain

I'm pretty sure you don't need to know about simple distillations (i could be wrong though) but essentially, a fractional distillation is used when a simple distillation is not good enough to fully separate the species. Fractional distillation is more effective because instead of repeating simple distillations to eventually separate the species, fractional distillation does this in one go, continually evaporating and condensing as the species  travels up the fractionating column.  So in a way, fractional distillation can be described as a series of simple distillations.

[TrueTears]

thanks iamdan.

so for this Q

Methane undergoes a series of substitution reactions with bromine to form a number of components with the following boiling points ( all measured in deg Celsius): Bromomethane (3.6 deg), dibromomethane (97 deg), tribromomethane (150 deg), and tetrabromomethane (190 deg). Describe how you would obtain pure samples of each compound from the mixture using fractional distillation.

how would you get pure samples of each compound using fractional distillation?

[TrueTears]

Currently hydrogen is produced by steam reforming of alkanes such as methane:



why is hydrogen produced by this method not a renewable source of energy?

[iamdan08]

how would you get pure samples of each compound using fractional distillation?

If you notice, the boiling temperatures of each of these compounds are different. In fractional distillation, you heat the sample and it evaporates, the components rising up the fractionating column, condensing at different heights according to their boiling temperature. Compounds with a low boiling temperature rise and condense at the top, while compounds with higher boiling temperatures condense lower down. This effectively separates the components and you can obtain pure samples of each. The reason the higher boiling temperature compounds condense down the bottom is because they are heavier.

[TrueTears]

ahhh awesome, so pretty much all i have to say for that question is

the components rising up the fractionating column, condensing at different heights according to their boiling temperature. Compounds with a low boiling temperature rise and condense at the top, while compounds with higher boiling temperatures condense lower down. This effectively separates the components and you can obtain pure samples of each.

[iamdan08]

Yeah that looks right. Just mention that the reason they separate at different heights is because the compounds with the higher boiling temperatures are  usually heavier. Looking at the diagram here might make it a bit clearer:

http://www.energyinst.org.uk/education/coryton/images/column.gif

[TrueTears]

ohh thats much better diagram haha, the one in my book only has 1 exit at the top, so i was confused how you get the other samples which condensed lower. But now i get it, thanks again XD

[TrueTears]

also just a bit unclear on Q 33 part b)

[TrueTears]

Atomic absorption spectroscopy and UV-visible spectroscopy both involve absorption of light. Both can be used to determine the amount of copper in a solution.
a) What species absorbs the light when copper nitrate is analysed by i) UV-visible spectroscopy ii) AAS?
b) Which technique would be simpliest for analysis of approximately 0.5M of copper nitrate solution? Explain.
c) How is the light of the required wavelength selected in i) UV-visible spectroscopy ii) ASS.

[TrueTears]

also Q 5... not quite sure about the table. If anyone could lend a hand that would really be helpful XD