C9- Hydrocarbons

Question 1

What is crude oil

Answer 1

Crude oil is a mixture of molecules called hydrocarbons

Question 2

How is crude oil formed

Answer 2

Crude oil is formed over millions of years from the skeletons of plankton buried in the mud.

-Crude oil is found in rocks and is a finite resource

Question 3

What are hydrocarbons

Answer 3

Hydrocarbons are molecules made out of hydrogen and carbon atoms only

Alkanes are a group of hydrocarbons

Question 4

What is the general formula for alkanes

Answer 4

The general formula for alkanes is CnH(2n+2)

You multiply the amount of carbon atoms by two and then add two to get the amount of hydrogen atoms

The first four alkanes are Methane,ethane,propane and butane

Question 5

How does the size if the hydrocarbon molecule affect its viscoxity

Answer 5

As the size of the hydrocarbon molecule increases, viscosity also increases. This means that long chain hydrocarbons are very viscous and move very slowly

Note:viscousity refers to the thickness of a fluid

Question 6

How does the size of the hydrocarbon molecule affect its flammability

Answer 6

As the size of the hydrocarbon increases its flammability decreases. This means that shorter chain hydrocarbons (e.g methane) are much more flammable than longer chain hydrocarbons

Question 7

How does the size of the hydrocarbon affect its boiling point

Answer 7

-Shorter chain hydrocarbons have lower boiling points than longer chain molecules. As the size of the hydrocarbon molecule increases so does the boiling point

Question 8

What happens when hydrocarbons are burnt

Answer 8

• When hydrocarbons are burnt they release energy

- The carbon and hydrogen atoms react with oxygen in combustion to release energy. The carbon and hydrogen are oxidised.

Question 9

What happens if the oxygen is unlimited in a combustion reaction

Answer 9

If the oxygen is unlimited in a combustion reaction then the reaction will produce carbon dioxide and water. This is called complete combustion

Hydrocarbon+oxygen—>carbon dioxide + water vapour

Question 10

What is fractional distillation and why is it used

Answer 10

• In order to make the hydrocarbons in crude oil useful you have to separate them using a technique called fractional distillation
• In fractional distillation the crude oil is split into different fractions. Each fraction contains hydrocarbons with a similar number of carbon atoms

Question 11

How does fractional distillation happen

Answer 11

1.Firstly the crude oil and heated to a very high temperature
2.This causes the crude oil to evaporate and turn into a has
3.The crude oil vapour then enters a fractional distillation column that is hotter at the bottom and colder at the top
4.The hydrocarbon vapours then rise up the fractional distillation column and condense when they reach their boiling point(they turn back into a liquid
5The liquid fractions are removed
6.The other hydrocarbon vapours continue to rise up the fractional distillation column until they reach their boiling points when they condense

Longer chain hydrocarbons condense into liquids early on whereas shorter chain hydrocarbons with low boiling points condense later on.

Question 12

What happens to very long chain hydrocarbons and very short chain hydrocarbons in a fractional distillation column

Answer 12

Very long chain hydrocarbons have very high boiling points and come out of the fractional distillation column at the bottom

Very short chain hydrocarbons have very low boiling points and so do not condense , they leave the column at the top as gasses

Question 13

What can different fractions of hydrocarbons be used for

Answer 13

Some hydrocarbon fractions can be used as fuel and others can be used as chemicals to make chemicals

Question 14

What happens during cracking

Answer 14

During cracking a longer chain hydrocarbon is broken down into a shorter chain hydrocarbon and an alkene.

Cracking is important as shorter chain hydrocarbons are in high demand as they are more flammable and can be used as fuels.

Question 15

What are the two different methods of cracking

Answer 15

• Catalytic cracking- This is where the hydrocarbon is heated to a very high temperature with a catalyst to break it down and vaporise it . The catalyst speeds the reaction up.
• Steam cracking-This is where the hydrocarbon is heated with steam at high temperatures. The hydrocarbon vapourises and mixes with the steam and is broken down into a shorter chain hydrocarbon and an alkene

Question 16

What are the key properties of alkenes

Answer 16

• Alkenes have a double covalent bond between two carbon atoms
• Alkenes are much more reactive than alkanes

Question 17

Why are alkenes useful

Answer 17

Alkenes are useful for making chemicals called polymers

They are also useful as a starting material for other chemicals

Question 18

How can you test for an alkene

Answer 18

You can test for alkenes by adding them to bromine water. If the solution turns colourless then an alkene is present

Question 19

What is the general formula for alkenes

Answer 19

The general formula for alkenes is CnH2n

The numebr of hydrogen atoms is the number of carbon atoms multiplied by two

(note:in a formula you put the hydrogen atoms on the end carbon atoms first )

Question 20

Why are alkenes described as unsaturated molecules

Answer 20

Alkenes are described as unsaturated molecules because they have two less hydrogen atoms than the alkane with the same number of carbon atoms

Question 21

Why do all alkenes react in a similar way

Answer 21

All alkenes react in a similar way because they all have a double covalent bond between the two carbon atoms