Alkane's boiling point graph

When I plot the boiling points of the Alkane, there is a curve. I know it is because of the amount of Van de Walls forces that makes their boiling point go up as the molecules get bigger. But the increase is 2n+2 of H and 1 of C everytime so the graph should be a straight line. Why is it a curve? (i am sure the boiling points and my graph is correct)

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This is simple chemistry, it

This is simple chemistry, it does not call for much debate. Have you also noticed that branched chain alkanes have lower melting and boiling points than staight chain alkanes of the same molecular mass,It would have been expected that since the molecular mass is the same, the melting points and boiling points of both branch and straight chain alkanes will be the same, but this is not so, the straight chain alkanes have higher boiling and melting points than their branched counterparts(with the same molecular mass) due to the presence of greater dipole moment( dipole-dipole interactions) among the carbon atoms than the corresponding branched alkanes because of the decrease in length of the chain. Branching weakens this dipole moment(Vaan der Waals forces), this explain clearly the reason for your graph being curve and not straight, the weakening of the dipole interaction is not a linear relationship, and so, you should not expect a straight line graph.

The longer the chains get,

The longer the chains get, the more tangled up they can be, so the energy required to untangle them (disperse them) is greater, thus a higher temperature is needed.-Kummetz Corp LLC

though i have already heard

though i have already heard about alkane but it's actual or real application i haven't known yet. i mean i have almost no idea on it's purpose, as well, how important if it's loss in the surface of the earth.

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A nice, non-technical answer, which I think was valid enough for me to use at school:

The longer the chains get, the more tangled up they can be, so the energy required to untangle them (disperse them) is greater, thus a higher temperature is needed.

That is in addition to Van der Waals

Thank You for helping me :), but i think you misunderstood my question. I was asking why is the graph a curve instead of a straight line. Thx.

The graph is a curve because

The graph is a curve because the alkane chanes get coiled as you raise the temperature.

Scott, try plotting the graph aginst N** 2/3 and you will get a straightline.

I will explain this further. I worked on this for my doctorate. Langmuir had predicted this long back.

MJ

[quote="scottyiu"]Thank You for helping me :), but i think you misunderstood my question. I was asking why is the graph a curve instead of a straight line. Thx.[/quote]
Assuming it is an upwards curve, then my answer was answering your question wrt the curvature.

Van der Waals would give a straight line.
The additional tangling up would further increase the boiling point, in a non-uniform manne r(the longer chains giving more and more variations of ways of tangling), hence giving a curved line.

If it curves down though, I have no idea.

Thank you for answering. The curve is going down and i just found out why. It is because as the molecules get longer, it folds and some of the Vaan der Waals forces is acting on itself. For more info, go to http://www.csam.montclair.edu/~burch/bppaper.pdf Thx

It's actually interesting, though.

When I did A level chemistry, I was taught that alkane VdW forces were practically non-existant.

How times (and teachers, I would imagine) change!

Cool, i am learning A level stuff at GCSE level :)

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