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Cracking

Worksheet Overview

QUESTION 1 of 10

A large part of crude oil is residue; very long hydrocarbon molecules. Some can be used for bitumen (used to make tar for roads), but there is more residue than we really want. How can we avoid wasting it?

Cracking is the name for a thermal decomposition processes; breaking long hydrocarbon molecules into shorter ones. All that we need to do is heat long-chain hydrocarbons, so that they turn into vapor. We then pass the vapor over a hot catalyst, to speed up the reaction. Alternatively, we can mix the hydrocarbon vapor with very hot steam. We can do this on a very large scale in a chemical works, but it also works as a small-scale experiment;

Molecules in a cracking reaction

Let's look at the molecules in this reaction close-up;

In this case, the long molecule of octadecane has been broken into three smaller pieces. The ethene is the one which collected as a gas in the experiment; it has such weak intermolecular forces that it is a gas at room temperature. One of the products is a smaller alkane (octane), but the other two are not. Look at the bond between the carbons in ethene, or the rightmost C=C bond in octene. They are double C=C bonds, not single C-C bonds. When the octadecane molecule cracked into three pieces, two of the C-C bonds in the backbone were broken. In effect, these broken bonds looped back into the molecule, so that each carbon still had four covalent bonds.

Ethene and octene are examples of another series of hydrocarbons, called the alkenes.

Alkenes

Alkenes are another homologous series of hydrocarbon molecules, like alkanes. The difference is that alkenes have a double C=C bond somewhere in the structure; in alkanes, all the bonds are single C-C bonds. Alkenes have isomers, because the double bond can be in different parts of the structure. Their general formula is CnH2n. The names follow the same pattern as for alkanes;

Number of carbon atoms Name Formula
2 C2H4 ethene
3 C3H6 propene
4 C4H8 butene
5 C5H10 pentene

Starting with pentene, the alkene names have the same prefixes used to name shapes; C6H12 is hexene ("hex" is six, as in hexagon) and so on. Scientists also talk about these compounds as saturated and unsaturated. A saturated compound only has single C-C bonds, while an unsaturated compound has one or more double C=C bonds, or even triple C≡C bonds. The point is that an unsaturated molecule can also bond to something else (by breaking up the double or triple bond), whereas a saturated compound cannot. The C=C bond in alkene molecules is its functional group; it's the characteristic part of the molecule where a chemical bond can be made.

This gives us a chemical test to tell saturated and unsaturated compounds apart. Bromine water is normally orange. If we add a saturated compound, it cannot react with the bromine, so nothing happens. If we add a unsaturated compound, the bromine in the water will bond with unsaturated compound where the double or triple bond was. The water will turn colourless, because it will no longer have bromine dissolved in it.

This means that bromine water will stay orange if we add an alkane, but become colourless if we add an alkene. 

Alkenes are useful, because they are the raw material for many plastics. We can break open one of the two bonds in the double bond, and that allows us to join alkene molecules together to make materials called polymers; there is another activity to help you learn about these.

Cracking happens when we heat long alkane molecules into a vapor, and pass the vapor over a catalyst, or mix it with steam. This breaks long alkane molecules (which aren't very useful) into smaller molecules. Some of these are useful small alkanes, and others are alkenes, which can be used to make plastics.

Fill in the gaps in this paragraph. Use words from this list, but not all of them. You may need to use some words more than once

bromine

catalyst

cracking

decomposition

fewer

hydrocarbon

longer

more

shorter

thermal

This table shows the approximate percentages of supply and demand for different fractions of crude oil;

Fraction Supply (%) Demand (%)
Refinery gas 3 5
Petrol 15 30
Kerosene 12 6
Diesel oil 20 24
Residue 50 35

Which fraction are we most likely to crack?

Pentacosane is C25H52; it's the paraffin wax used in candles. When we crack long hydrocarbons, there are many possible sets of products which we can make. For each set of products below, decide if it can result from the cracking of pentacosane.

C6H12, C10H20, C9H20

C6H14, C10H20, C9H18

C4H8, C4H8, C13H26

C3H8, C4H8, C13H26

Suppose we crack a molecule of hexane. Pick a pair of products which could result from this reaction.

methane and propene

propane and butene

ethane and butane

ethene and butane

Pentadecane is an alkane with fifteen carbon atoms. What is its chemical formula?

C15H15

C15H30

C15H32

Mark this paragraph about alkenes; tick each correct sentence.

Alkenes are saturated hydrocarbons.

A saturated hydrocarbon only has single C-C bonds.

An unsaturated hydrocarbon must have a C=C double bond.

The double C=C bond is the functional group for alkenes.

Bromine reacts at the functional group, which is why alkenes decolourise bromine water

Complete the gaps in this paragraph about the uses of hydrocarbons. Use some or all of these words (you may need to use some words more than once).

alkanes

alkenes

carbon

fuels

oxides

polymers

saturated

unsaturated

water

 

Alkenes are saturated hydrocarbons.

A saturated hydrocarbon only has single C-C bonds.

An unsaturated hydrocarbon must have a C=C double bond.

The double C=C bond is the functional group for alkenes.

Bromine reacts at the functional group, which is why alkenes decolourise bromine water

What is the name of the catalyst used in cracking experiments?

What is the specific safety precaution in the laboratory version of cracking?

Do not try to light the gas produced

Remove the delivery tube before removing the heat.

Do not touch the water

What would happen if we added C8H16 to bromine water?

The colour would change from colourless to orange

The bromine water would remain colourless

The bromine water would change from orange to colourless

The bromine water would remain orange

  • Question 1

Fill in the gaps in this paragraph. Use words from this list, but not all of them. You may need to use some words more than once

bromine

catalyst

cracking

decomposition

fewer

hydrocarbon

longer

more

shorter

thermal

CORRECT ANSWER
EDDIE SAYS
Cracking is a type of thermal decomposition; we need heat to break the bonds in the molecules. The catalyst is there to make the reaction go faster.
  • Question 2

This table shows the approximate percentages of supply and demand for different fractions of crude oil;

Fraction Supply (%) Demand (%)
Refinery gas 3 5
Petrol 15 30
Kerosene 12 6
Diesel oil 20 24
Residue 50 35

Which fraction are we most likely to crack?

CORRECT ANSWER
residue
EDDIE SAYS
We use cracking when there is more of a fraction than we need. In the table, that's true for kerosene and residue. We should do cracking for both, but it's more useful to do cracking for the residue, since it has the larger excess of supply over demand.
  • Question 3

Pentacosane is C25H52; it's the paraffin wax used in candles. When we crack long hydrocarbons, there are many possible sets of products which we can make. For each set of products below, decide if it can result from the cracking of pentacosane.

CORRECT ANSWER
C6H12, C10H20, C9H20
C6H14, C10H20, C9H18
C3H8, C4H8, C13H26
EDDIE SAYS
You need to make sure that both the carbons and the hydrogens balance across each reaction for it to work. That means that you have to have one alkane molecule on the products side, and all the rest alkenes; otherwise you run out of hydrogen atoms, or have carbon atoms left over.
  • Question 4

Suppose we crack a molecule of hexane. Pick a pair of products which could result from this reaction.

CORRECT ANSWER
ethene and butane
EDDIE SAYS
Before you start calculating, you need to translate the names into numbers of carbons. Meth = 1, Eth = 2, Prop = 3, But = 4, Hex = 6. Hexane has 6 carbons, so we need products which add up to six carbons. 2 + 3 and 3 + 4 don't work, but 2 + 4 does. When we crack an alkane, can only make one alkane, so that eliminates ethane + butane.
  • Question 5

Pentadecane is an alkane with fifteen carbon atoms. What is its chemical formula?

CORRECT ANSWER
C15H32
EDDIE SAYS
Alkanes have the general formula CnH2n+1.
  • Question 6

Mark this paragraph about alkenes; tick each correct sentence.

CORRECT ANSWER
A saturated hydrocarbon only has single C-C bonds.
The double C=C bond is the functional group for alkenes.
Bromine reacts at the functional group, which is why alkenes decolourise bromine water
EDDIE SAYS
Alkenes are unsaturated, because the double bond gives the possibility of adding something else to the molecule. Molecules with a C=C bond are unsaturated, but so are molecules with a C≡C triple bond.
  • Question 7

Complete the gaps in this paragraph about the uses of hydrocarbons. Use some or all of these words (you may need to use some words more than once).

alkanes

alkenes

carbon

fuels

oxides

polymers

saturated

unsaturated

water

 

CORRECT ANSWER
EDDIE SAYS
The key use of alkenes is that the unsaturated bond can be opened up to join them together to make polymers, which are the basis of plastics. There is more on that in another activity.
  • Question 8

What is the name of the catalyst used in cracking experiments?

CORRECT ANSWER
zeolite
EDDIE SAYS
Remember that different chemical reactions need different catalysts. Zeolites are quite common, we can get them in clay used for pots.
  • Question 9

What is the specific safety precaution in the laboratory version of cracking?

CORRECT ANSWER
Remove the delivery tube before removing the heat.
EDDIE SAYS
There are lots of other hazards and precautions in this experiment- mostly due to the risks of things being hot. The less obvious one is to prevent water being sucked into the hot test tube- it would turn to steam and shatter the tube.
  • Question 10

What would happen if we added C8H16 to bromine water?

CORRECT ANSWER
The bromine water would change from orange to colourless
EDDIE SAYS
Bromine water is orange. C8H16 is an alkene (it has twice as many hydrogens as carbons), so it will decolourise the bromine water.
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