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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 break 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;

If we start with liquid octadecane, and are able to make a gas, that proves that we have broken the large molecules into smaller pieces, because only small hydrocarbon molecules are gases at room temperature.

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. One of them is a smaller alkane (octane), but the other two are a bit different. 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. Ethene and octene are examples of another series of hydrocarbons, called the alkenes.

Alkenes

Alkenes are another 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

We can test whether a chemical is an alkane or an alkene by adding the chemical to bromine water, which is orange. Alkenes will turn the water colourless, but alkanes will do nothing, so the water stays orange.

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.

What is the name of the process that breaks large hydrocarbon molecules into smaller ones?

What conditions are needed to make cracking work?

High temperatures

High pressures

Catalyst or steam

Catalyst and steam

Think about this reaction some more;

How many atoms of each element are there on the products side of this reaction? Type in your answers as numbers, not words.

 

High temperatures

High pressures

Catalyst or steam

Catalyst and steam

Look at this molecule.

What can we say about this structure? Pick a half-sentence from each section.

High temperatures

High pressures

Catalyst or steam

Catalyst and steam

Look at this molecule.

What is its name?

pentane

pentene

decane

decene

Suppose we have a gas, and want to tell if it is ethane or ethene. What experiment can we do to check?

Add the gas to bromine water

Add the gas to limewater

Do a squeaky pop test

Mix the gas with Universal Indicator

Suppose we have a gas, and want to tell if it is ethane or ethene. We bubble the gas through bromine water, and the water changes from orange to colourless. What does this tell us?

The gas is ethane

The gas is ethene

We cannot be sure what the gas is

Look at each of these formulas, and decide if each one is an alkane, an alkene or neither.

 alkanealkeneneither
C4H8
C4H10
C5H8
C5H10
C5H12

What is produced when we crack a long alkane molecule? Tick all the possible answers.

catalyst

shorter alkane

alkene

carbon dioxide

Why are alkenes useful?

They are sustainable.

We can make plastics from them.

They release much larger amounts of energy when burnt.

  • Question 1

What is the name of the process that breaks large hydrocarbon molecules into smaller ones?

CORRECT ANSWER
cracking
EDDIE SAYS
It's cracking, because we are breaking something into smaller pieces, but not losing anything in the process.
  • Question 2

What conditions are needed to make cracking work?

CORRECT ANSWER
High temperatures
Catalyst or steam
EDDIE SAYS
High temperatures are needed because we are doing a kind of thermal decomposition to break up the molecules. We need a catalyst or steam to make the reaction go fast enough to be useful.
  • Question 3

Think about this reaction some more;

How many atoms of each element are there on the products side of this reaction? Type in your answers as numbers, not words.

 

CORRECT ANSWER
EDDIE SAYS
The products are ethene, octene and octane. There are (2 + 8 + 8 =) 18 carbon atoms, and (4 + 16 + 18 =) 38 hydrogen atoms on the products side of the reaction. Those are the same numbers as on the reactants side, so the reaction is balanced and nothing has been lost.
  • Question 4

Look at this molecule.

What can we say about this structure? Pick a half-sentence from each section.

CORRECT ANSWER
EDDIE SAYS
The bond between the carbons is double (C=C), so this is an alkene.
  • Question 5

Look at this molecule.

What is its name?

CORRECT ANSWER
pentene
EDDIE SAYS
There are five carbons, so it is pent-something. There is a double C=C bond (look at the top two carbon atoms), so it is an alkene. That means the name is pentene.
  • Question 6

Suppose we have a gas, and want to tell if it is ethane or ethene. What experiment can we do to check?

CORRECT ANSWER
Add the gas to bromine water
EDDIE SAYS
Bromine water is the test for compounds with C=C double bonds, so it lets us tell alkanes and alkenes apart.
  • Question 7

Suppose we have a gas, and want to tell if it is ethane or ethene. We bubble the gas through bromine water, and the water changes from orange to colourless. What does this tell us?

CORRECT ANSWER
The gas is ethene
EDDIE SAYS
Ethene has a double C=C bond, so it can react with bromine water. Ethane only has a singe C-C bond, so it does nothing. If we see the colour change, the gas must be ethene.
  • Question 8

Look at each of these formulas, and decide if each one is an alkane, an alkene or neither.

CORRECT ANSWER
 alkanealkeneneither
C4H8
C4H10
C5H8
C5H10
C5H12
EDDIE SAYS
Alkenes have exactly twice as many hydrogen atoms as carbon atoms. Alkanes have twice as many hydrogen atoms as carbon atoms, plus two more hydrogen atoms.
  • Question 9

What is produced when we crack a long alkane molecule? Tick all the possible answers.

CORRECT ANSWER
shorter alkane
alkene
EDDIE SAYS
Cracking breaks a long alkane molecule into one or more shorter alkanes and one or more alkenes. We can't make carbon dioxide, because there is no oxygen in the original alkane.
  • Question 10

Why are alkenes useful?

CORRECT ANSWER
We can make plastics from them.
EDDIE SAYS
The key use of alkenes is that we can join them together to make polymers, which are the basis of plastics. There is more on that in another activity.
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