Chemistry Homework

[andymac1991]

Im not too sure about this so can someone please help me. The question is:

Write a paragraph on how the typical length of carbon chain in each fraction affects the fractions use, using the words viscosity, flammability and evaporation

[brachiopod]

I am presuming you are talking about the fractional distillation of crude oil?

One way of thinking about it is the longer the carbon chain, the more they get tangled up with eachother, thus they require more energy to move. This results in a higher viscosity and higher boiling point the longer the C-C chain length in the molecule.

Imagine a plate of peas and a plate of spaghetti. Shake the plate of peas from side to side and up and down, and they roll and jump about and pass one another without much problem. Try to make the spaghetti move and you require a hefty shake of the plate to even get one gloopy movement.

The peas are short chain molecules (eg methane, ethane) which are gases at room temperature (ie have a low boiling point).

Now imagine something like macaroni, longer than peas, but shorter than spaghetti. They move if you shake the plate about, but not as much as peas. These would be your lighter fractions. They are volatile liquids at room temperature. (NOTE - Volatile does NOT mean explosive, but means it evaporates easily)

[Cont.]

[brachiopod]

[Cont.]

Right now the bit to think about...

Viscosity - this is how 'runny' a liquid is. If the molecules can move past eachother freely or easily, the liquid will have a low viscosity. Now think about your peas, macaroni and spaghetti and ask yourself which will have the higher viscosity.

Volatility - (= How volatile a liquid is or How easy it evaporates) Give the underside of your plate of peas, macaroni, spaghetti etc. a sharp smack. If the 'molecules' jump in the air, then they are relatively volatile ie they evaporate easily (they have emough energy to escape from the surface of the liquid). Which of your three foods is the most volatile?

Flammability - in order to 'burn', the molecule must react with oxygen. In order to do this, the O2 oxygen molecules must be able to get in close to the hydrocarbon molecule. (We will ignore activation energies for the purposes of this question). Chuck a handful of... errr... sweetcorn (oxygen !!) onto each of your plates and shake. The one that mixes most evenly with the sweetcorn will have the greater flammability. Which do you think that will be?

Final Note - Liquid petrol is not actually flammable - it is the vapour that is. Can you explain this with your sweetcorn model? !!!!

[charliebaps]

brachiopod - you're great. not my question but I learnt something - wish I'd had you as one of my teachers when I was at school!!

[brachiopod]

*blush*

[aka estie]

Brachiopod - don't you just hate it when you take all that time and effort to type out an answer, and you don't even get thanked for it?

[brachiopod]

*sigh* yes, estie, I know what you mean. It's what you expect from 14 year olds I suppose. (I am assuming 1991 is andymac's year of birth)

I shall just have to lurk around Science sulking.

[andymac1991]

Sorry Brachiopod. Thanks a lot for the info, I understand it now - Sorry I didnt thank you, I havent been on this website for a few days now

[brachiopod]

No probs, andy, hope it helped.