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GCSE AQA C1 - Fractional Distillation

Silverrose

Could someone please explain the process of fractional distillation for AQA C1 please?
Thanks :smile:

Reply 1

CaptErin

Original post by Silverrose

Could someone please explain the process of fractional distillation for AQA C1 please?
Thanks :smile:

Hiya

Fractional distillation questions come up just about every year on the C1 paper, and they can't get very creative with it so it's usually the same points that you need to write:

- Crude oil is heated
- to evaporate hydrocarbons
- there is a temperature gradient in the chamber
- so fractions condense
- at different levels

That's from the June 2015 paper; only four of those five points needed to be stated for full (four) marks, and the key words are in bold; I'd try to memorise this because it is more than likely to come up again this year with an almost identical mark scheme.

Crude oil is a mixture, so it is made up of lots of different compounds - such as petrol and diesel - that are not bonded to each other, just mixed together. The individual compounds (two or more elements bonded together) that crude oil is made up of are all hydrocarbons, meaning they are only made of bonded hydrogen and carbon atoms. Different hydrocarbons have different boiling points because more energy (heat) is needed to break up the bonds between larger hydrocarbons.

Crude oil isn't much use as it is; it takes a lot of energy to heat so would be useless as a fuel, and is thick and gloopy. However, the individual hydrocarbons that it contains (also called 'fractions' :wink:

are very useful, so they are separated so they can be used - this is done via fractional distillation. Fractions being the individual hydrocarbons, distillation being the process of purifying a substance via heating and cooling.

First, the crude oil is heated in a furnace at the bottom of a fractional distillation chamber - a huge tower - until it evaporates (every part of it is a gas). The distillation chamber is said to have a temperature gradient; this just means that it is cooler at the top than it is at the bottom. Because each of the different fractions (compounds) has a different boiling point, they will condense (become a liquid again) when they reach the section of the tower that is cooled to their boiling point, and this will be a different section for each fraction.

In this way, the fractions with the highest boiling points will condense towards the bottom, and the fractions with the lowest boiling points will condense towards the top.

Because we know the point at which each fraction will condense, they can be piped away at this section - you don't need to know anything about how they are piped away. In this way, the condensed fractions can all be piped into different storage containers, and are thus separated.

At least, that's how I understand it. I'm not sure how much you need explaining so I just tried to include as much as possible, but I can try and expand upon it if that would be helpful? :smile:

I'd focus on just learning those key five points from the mark scheme, because it's about as complex as you need to know.

The only other information I'd add (because it's been somewhat included in a six-marker in the past, when it was implied that Butane was not evaporated from the crude oil) is that not all of the fractions evaporate; sometimes a fraction with a very high boiling point will be purposely left as a liquid and piped away straight away after the other fractions have evaporated because there is no point in evaporating it because it is already separated through the evaporation of other fractions.