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Crude Oil - Cracking
Cracking is an example of a thermal decomposition chemical reaction.

Crude Oil - Cracking

Cracking turns heavy fractions into fuels and useful alkenes. This GCSE Chemistry quiz checks conditions, products, and how cracking helps meet demand for petrol and plastics.

Explore the Topic →
(quiz starts below)

Fascinating Fact:

In catalytic cracking, the vapour is passed over a hot catalyst, often a zeolite. The catalyst lowers the activation energy and makes the process more efficient.

In GCSE Chemistry, cracking explains how we turn long-chain hydrocarbons (less useful, thicker fuels) into shorter chains that are in higher demand. You learn the key conditions, why a catalyst speeds the reaction, and how cracking produces a mixture of alkanes (fuels) and alkenes (for making polymers).

Key Terms

  • Long-chain hydrocarbon: A hydrocarbon with many carbon atoms, usually with a higher boiling point and greater viscosity than short-chain hydrocarbons.
  • Alkene: An unsaturated hydrocarbon containing at least one carbon-carbon double bond, making it more reactive than an alkane.
  • Catalyst: A substance that increases the rate of a reaction by providing an alternative pathway, and is not used up overall in the reaction.

Frequently Asked Questions (Click to see answers)

Why is cracking needed in crude oil processing?

Cracking is needed because crude oil contains many long-chain hydrocarbons, but there is higher demand for shorter-chain fuels like petrol. Cracking helps match supply to demand by making more useful products.

What products are made when hydrocarbons are cracked?

Cracking produces a mixture of smaller hydrocarbons, including alkanes and alkenes. The alkanes can be used as fuels, and the alkenes are valuable starting materials for making polymers and other chemicals.

How can you tell if cracking has produced alkenes?

You can test for alkenes using bromine water. If alkenes are present, the bromine water changes from orange to colourless because the alkene double bond reacts with bromine.

Try These Related Quizzes

1 .
An alkene molecule with one double bond contains 7 carbon atoms. How many hydrogen atoms does it contain?
12
14
16
15
This often comes up in GCSE questions related to crude oil
2 .
The alkene product of cracking...
is an unsaturated compound
has molecules in which each carbon atom has two bonds
is unreactive
will burn to produce carbon dioxide and hydrogen
Unsaturated means that it contains a double bond. In the laboratory, you test for it using bromine water. A positive result is that the bromine water is discoloured
3 .
Which of the following shows only the products that could be directly obtained by cracking C10H24?
Alkanes up to C20H44, hydrogen and ethene
Alkanes up to C8H18, carbon dioxide and ethene
Propene, poly(ethene) and hydrogen
Alkanes up to C8H18, ethene and propene
Remember the smallest possible alkene is ethene. Therefore the largest alkane must be C8H18. You could dismiss the other three answers because at GCSE, you are expected to know that the products of thermal cracking of an alkane will produce only a shorter chain alkane and an alkene so any answer containing other products, like carbon dioxide and hydrogen, are false
4 .
In the cracking process...
the hydrocarbon vapour is heated with iodine
the hydrocarbon is mixed with hydrogen and heated
the hydrocarbon vapour is passed over a hot catalyst
the hydrocarbon is liquefied, then passed over a catalyst
The catalyst used industrially is zeolite based
5 .
The purpose of cracking is to...
split long chain hydrocarbons into more useful short chain molecules
split long chain hydrocarbons into their component molecules
split hydrocarbons into carbon and hydrogen
separate different sized hydrocarbons
It solves the issue of under production and over production at the same time
6 .
What is the general formula for alkenes?
CnH2n+2
CnH2n+3
CnHn+1
CnH2n
Alkenes contain twice as many hydrogen atoms as carbon atoms
7 .
Cracking is an example of what type of chemical reaction?
Reduction
Oxidation
Neutralisation
Thermal decomposition
Thermal = heat; decomposition = break down; so thermal decomposition = break down using heat
8 .
The products of cracking are...
alkane + alkene
alkene + alkene
alkane + alcohol
alkane + alkane
Remember, nothing is being added, so the number and type of atoms must remain the same
9 .
The catalyst used in the cracking process in the lab is...
powdered ceramic pot
nickel
platinum
manganese
Broken pot can also be used. It needs to be powdered or broken to increase the surface area to maximise the speed of reaction
10 .
Select from the list the correct formula for ethene.
C3H6
C2H4
C2H6
C4H2
In organic chemistry, you need to know that any chemicals beginning with 'eth' contains two carbon atoms and you also need to know the general formulae for the alkanes and alkenes
You can find more about this topic by visiting BBC Bitesize - Crude oil, hydrocarbons and alkanes

Author:  Kate Gardiner (Chemistry Educator & GCSE Quiz Writer)
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