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Cracking

Worksheet Overview

QUESTION 1 of 10

Most of the time, small hydrocarbon molecules are more useful than long ones. Unfortunately, a lot of crude oil contains very long molecules. What can we do about this?

Cracking is the name for processes which breaks long hydrocarbon molecules into shorter ones. We don't add or remove any atoms; we just break the molecule into shorter pieces. 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. This means that 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), which a saturated compound cannot.

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.

bromine

catalyst

compressing

cracking

fracking

heating

reactant

steam

vapour

Look at this molecule.

What is its name?

How can we tell alkanes and alkenes apart?

Decane is an alkane with ten carbon atoms. Pentane is an alkane with five carbon atoms. Could we crack decane to produce two pentane molecules?

Yes

No

There isn't enough information to be sure

Why are alkenes useful? Pick a reason and an explanation.

Yes

No

There isn't enough information to be sure

If we cracked hexane, C6H14, and one of the products was ethene, C2H4, what would the other product be? Pick a name and a formula.

Yes

No

There isn't enough information to be sure

Pentadecene is an alkene with fifteen carbon atoms. What is its chemical formula?

C15H15

C15H30

C15H32

What is the description of a compound with one or more C=C or C≡C bonds?

If we cracked octane, C8H18, and one of the products was propane, C3H8, what would the other product be? Pick a name and a formula.

Which of these statements apply to alkanes or alkenes? Tick all the correct boxes.

 AlkanesAlkenes
Is a hydrocarbon
Is saturated
Is unsaturated
Turns bromine water colourless
Can be produced by cracking
  • Question 1

Fill in the gaps in this paragraph. Use words from this list, but not all of them.

bromine

catalyst

compressing

cracking

fracking

heating

reactant

steam

vapour

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

Look at this molecule.

What is its name?

CORRECT ANSWER
ethene
EDDIE SAYS
It's got two carbons, so it is eth-something. It has a double bond, so it is an alkene, so its name is ethene.
  • Question 3

How can we tell alkanes and alkenes apart?

CORRECT ANSWER
EDDIE SAYS
Alkenes are unsaturated, so they can react with bromine water. Alkanes are saturated, so they do nothing. If we see the colour change, the hydrocarbon must be an alkene.
  • Question 4

Decane is an alkane with ten carbon atoms. Pentane is an alkane with five carbon atoms. Could we crack decane to produce two pentane molecules?

CORRECT ANSWER
No
EDDIE SAYS
When we crack an long alkane, we can't just produce alkanes; there has to be one or more alkene molecules as well, One way to see this is to think about the hydrogen atoms. The other way is to remember that breaking the C-C backbone produces some C=C double bonds.
  • Question 5

Why are alkenes useful? Pick a reason and an explanation.

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 6

If we cracked hexane, C6H14, and one of the products was ethene, C2H4, what would the other product be? Pick a name and a formula.

CORRECT ANSWER
EDDIE SAYS
Think about the numbers first. Hexane is six carbons, 14 hydrogens. The ethene needs two carbons, four hydrogens, so there are four carbons, 10 hydrogens left. The alkane with four carbons will have (2 x 4) + 2 = 10 hydrogens, which is what we want, and that is butane.
  • Question 7

Pentadecene is an alkene with fifteen carbon atoms. What is its chemical formula?

CORRECT ANSWER
C15H30
EDDIE SAYS
Alkenes have the general formula CnH2n, so there are twice as many hydrogen atoms as carbon atoms.
  • Question 8

What is the description of a compound with one or more C=C or C≡C bonds?

CORRECT ANSWER
unsaturated
EDDIE SAYS
We call them unsaturated because it\'s possible to add something else to them (by breaking the double or triple bond); the compound isn\'t completely full yet.
  • Question 9

If we cracked octane, C8H18, and one of the products was propane, C3H8, what would the other product be? Pick a name and a formula.

CORRECT ANSWER
EDDIE SAYS
This is the same idea as before. Octane is 8 carbons, 18 hydrogens. The propane needs 3 carbons, 8 hydrogens, so there are five carbons, 10 hydrogens left. The alkene with five carbons will have (2 x 5) = 10 hydrogens, which is what we want, and that is pentene.
  • Question 10

Which of these statements apply to alkanes or alkenes? Tick all the correct boxes.

CORRECT ANSWER
 AlkanesAlkenes
Is a hydrocarbon
Is saturated
Is unsaturated
Turns bromine water colourless
Can be produced by cracking
EDDIE SAYS
This grid summarises the key similarities and differences for alkanes and alkenes; check the introduction if you\'re not sure about any of these points.
---- OR ----

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