Fractional Distillation Flashcards
What is a fraction?
a mixture of hydrocarbons with similar boiling points and numbers of carbon atoms
What are alkanes?
most of the hydrocarbons from crude oil are alkanes
alkanes are a homologous series of compounds
How do hydrocarbons in different fractions differ from each other?
a) number of carbon/hydrogen atoms (size)
b) boiling points
c) ease of ignition
d) viscosity
e) most are alkanes
What is fractional distillation used for?
separating a mixture of liquids
separating crude oil into simpler/more useful mixtures/fractions
What process is used to separate crude oil?
fractional distillation
Why can crude oil be separated by fractional distillation?
because the different hydrocarbons have different boiling points
What happens during fractional distillation?
- crude oil is heated to evaporate it
- the vapours rise up a fractionating column
- the column is hotter at the bottom and colder at the top, it has a temperature gradient
- the fractions condense at different points because they have different boiling points
- fractions with larger hydrocarbons exit first
- smaller hydrocarbons exit nearer the top
- this is because larger hydrocarbons have higher boiling points than smaller ones
- the fractions condense back to a liquid at different points when the column becomes cooler than their boiling points
- the fractions are piped out the column
What happens to the gases fraction?
it does not condense and leaves the column at the top
What happens to the bitumen fraction?
it does not evaporate and leaves at the bottom
What order do the fractions leave in from top to bottom?
(petroleum) gases petrol kerosene diesel fuel oil bitumen
Why does petrol leave the column higher up than fuel oil?
it has a lower boiling point
so it takes longer before the column reaches a point that is cold enough for it to condense
What difference in the number of C and H atoms is there in the column?
there are more/the hydrocarbons are bigger at the bottom and smaller at the top
What difference in boiling point is there in the column?
the boiling point is lower at the top and higher at the bottom
What difference in ease of ignition is there in the column?
the most flammable fractions are at the top, and the least flammable are at the bottom
What difference in viscosity is there in the column?
the least viscous substances are at the top, and the most viscous are at the bottom
What does viscosity mean?
how runny/thick a substance is
Why does the boiling point increase as the number of carbon and hydrogen atoms increases?
- the strength of the intermolecular forces increases
- more energy is needed to overcome the forces
- this results in a higher boiling point
- the boiling point increases
Crude oil is a complex mixture of hydrocarbons.
Most belong to the alkane homologous series.
Diesel oil is a fuel produced from crude oil.
Explain how diesel oil is separated from crude oil.
(6 marks)
Crude oil is separated into different fractions by fractional distillation.
The crude oil is heated to around 350°C to evaporate it.
Some of the hydrocarbons stay in the liquid state and leave at the bottom of the column.
The other hydrocarbons rise inside the fractionating column in the gas state.
(state clearly which separation method is involved, and outline what happens to the crude oil after heating it)
The fractionating column has a temperature gradient inside it.
The column becomes cooler towards the top, so the hydrocarbons cool down as they rise.
They condense to a liquid when they reach a part of the column that is cool enough.
(the temperature gradient is important, as well as the hydrocarbons condensing)
The gases do not condense and they leave at the top. Diesel oil leaves through a pipe as a liquid fraction when it condenses.
(could also explain that smaller molecules have weaker intermolecular forces and lower boiling points)
What are gases used for?
domestic heating and cooking
What is petrol used for?
fuel for cars
What is kerosene used for?
fuel for aircraft
What is diesel oil used for?
fuel for some cars and trains
What is fuel oil used for?
fuel for large ships and some power stations
What is bitumen used for?
surfacing roads and roofs
Suggest two reasons to explain why bitumen is not suitable for use as a fuel
it is difficult to ignite/not very flammable
it has a high viscosity/is very viscous/does not flow easily
Describe how crude oil is separated into fractions by fractional distillation
- oil is evaporated and passed into a column
- the column is hotter at the bottom and cooler at the top
- the different hydrocarbons rise up the column, cool, and condense at different heights
