TSR George
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in a fractionating column, temperatures are hotter at the bottom and cooler at the top. long-chain hydrocarbons with higher boiling points condense at the bottom, but short-chain hydrocarbons with lower boiling points condense at the top where the temp is cooler.

why don’t the short-chain hydrocarbons condense at the bottom if they have lower boiling points?
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bluebirchtree
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(I'm really not qualified to answer this question, so I apologise in advance if this confuses you further. This is a guess and you are much better off asking your science teacher.)

The long-chain hydrocarbons have stronger intermolecular bonds than short-chain hydrocarbons. This means that the short-chain hydrocarbons are more likely to evaporate into a gas at high temperatures because less energy is required to break their intermolecular bonds. They don't condense at the bottom because the long-chain hydrocarbons are in a liquid state and liquids are denser than gases.
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Pigster
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(Original post by bluebirchtree)
(I'm really not qualified to answer this question, so I apologise in advance if this confuses you further. This is a guess and you are much better off asking your science teacher.)

The long-chain hydrocarbons have stronger intermolecular bonds than short-chain hydrocarbons. This means that the short-chain hydrocarbons are more likely to evaporate into a gas at high temperatures because less energy is required to break their intermolecular bonds. They don't condense at the bottom because the long-chain hydrocarbons are in a liquid state and liquids are denser than gases.
Have you ever thought about becoming a science teacher?

I guess the OP is just confused over what boiling point means.

Short chain HCs indeed have low BPs. A low BP refers to the temperature that they need heating until they boil. A low BP means it doesn't take much heat until they boil. At low Ts, short chain HCs have enough energy to boil and hence remain as gases until T drops below their BP (i.e. at the top of the fractionating tower).

Long chain HCs have high BPs - they need high Ts before they will boil, i.e. they condense at high T's, i.e. the bottom of the tower.
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bluebirchtree
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(Original post by Pigster)
Have you ever thought about becoming a science teacher?

I guess the OP is just confused over what boiling point means.

Short chain HCs indeed have low BPs. A low BP refers to the temperature that they need heating until they boil. A low BP means it doesn't take much heat until they boil. At low Ts, short chain HCs have enough energy to boil and hence remain as gases until T drops below their BP (i.e. at the top of the fractionating tower).

Long chain HCs have high BPs - they need high Ts before they will boil, i.e. they condense at high T's, i.e. the bottom of the tower.
listen to them OP not me my reply was a complete and utter guess
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Pigster
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At the start in FD, T is sooo high that everything is a gas. This mixture of really hot gases is squirted into the FD tower. Near the bottom, T is slightly cooler than the incoming stream of gas, so the chemicals with the highest BPs condense into liquids and therefore are separated from the rest of the gas mixture.

Remember, a high BP means that a high T is needed so that it stays as a gas.

"they require less heat to boil so why don’t they boil at the bottom". They do!. But, rather than thinking of them boiling at the bottom, you should think in terms of 'still being a gas' at the bottom. You are thinking along the right lines, short chains should do boil at the bottom. The only flaw in your logic is that they have already boiled.
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