Cracking Hydrocarbons
Cracking Hydrocarbons
Lesson duration is: 60 minItems in this lesson
Cracking Hydrocarbons
Open with the idea that crude oil contains many long-chain hydrocarbons, but industry often needs smaller, more useful molecules.
Learning goal
I can explain the process of cracking, explain why cracking is important, and work out simple cracking equations.
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What words come to mind when you hear cracking hydrocarbons?
Use this to surface prior ideas such as heat, splitting, fuels, alkenes, crude oil, or catalyst. Accept simple everyday words as well as scientific terms.
What cracking is
Cracking is the breaking down of large hydrocarbon molecules into smaller, more useful molecules.
It turns long-chain alkanes into a mixture of shorter alkanes and alkenes.
Stress that cracking is a chemical change because new substances are formed.
What is being cracked?
The starting substances are usually long-chain alkanes
These come from fractions of crude oil
Long-chain molecules are less useful because they are harder to ignite and less in demand
Link back to fractional distillation if students have met it before, but keep the focus on why some fractions need changing.
How cracking happens
Hydrocarbon vapours are heated until they evaporate
The vapours pass over a hot catalyst
The molecules split into smaller molecules
This needs a high temperature
At this level, students need the general process: heat, vapour, catalyst, split. Do not overload with industrial detail.
Conditions for cracking
High temperature provides energy
A catalyst speeds up the reaction
Common catalysts are solids such as silica or alumina
Students do not need detailed catalyst chemistry; the key idea is that the catalyst helps the long molecules break apart.
Products of cracking
Cracking often makes:
short-chain alkanes, used as fuels
alkenes, used to make polymers
This means one long molecule can become two or more useful products.
Reinforce that alkenes are unsaturated and valuable as feedstock for making plastics.
What is cracking?
Burning hydrocarbons
Cooling vapours
Splitting large molecules
Mixing fractions
Checks the core definition from slides 2–6. Distractors reflect confusion with combustion, condensation, or fractional distillation.
Complete the process
In cracking, long-chain alkanes are heated until they form vapours. The vapours pass over a hot catalyst at a high temperature. The large molecules then split into smaller molecules, including shorter alkanes and alkenes.
The blanks target the load-bearing terms of the process: alkanes, catalyst, molecules.
Why cracking is important
There is more demand for short-chain hydrocarbons than long-chain ones
Cracking helps match the supply from crude oil to what people need
It makes useful fuels such as petrol-sized molecules
It also makes alkenes for the chemical industry
This is the economic reason: crude oil does not naturally contain the exact balance of fractions industry wants.
Short chains and long chains
Short-chain hydrocarbons are usually more useful as fuels
They are more flammable
They have lower boiling points
Long-chain hydrocarbons are often used less as fuels
Keep this comparative rather than going deep into intermolecular forces. The purpose is to explain demand.
Why alkenes matter
A second reason cracking is important is that it makes alkenes.
Alkenes are used to make many materials, especially polymers such as plastics.
If helpful, mention examples like poly(ethene), but keep the main point on alkenes as chemical feedstock.
Why is cracking important?
It makes atoms disappear
It gives more long chains
It matches supply to demand
It lowers all boiling points
The key idea is economic usefulness: more demand for short-chain hydrocarbons and alkenes than for long-chain fractions.
Complete the explanation
Cracking is important because short-chain hydrocarbons are more useful as fuels and are more flammable. It also makes alkenes for making polymers such as plastics.
This reinforces both reasons for cracking: useful fuels and alkene feedstock for polymers.
A simple cracking equation
Example:
C10H22 → C8H18 + C2H4
The long-chain alkane splits into a shorter alkane and an alkene
The number of atoms must balance on both sides
Point out that 10 carbons stay 10 carbons, and 22 hydrogens stay 22 hydrogens.
Balancing cracking equations
To work out a simple cracking equation:
start with the large alkane
choose one product
count the carbons left for the second product
count the hydrogens left
check both sides balance
Students often guess products without checking atom totals. Emphasise counting carefully.
Worked example
Start with C12H26
If one product is C3H8:
carbons left = 9
hydrogens left = 18
second product = C9H18
Equation: C12H26 → C3H8 + C9H18
You can point out that C9H18 fits the alkene general formula, but students can also solve by subtraction alone.
Spotting the products
One product is often an alkene
Alkenes follow the formula CnH2n
The other product is often an alkane
Alkanes follow the formula CnH2n+2
This pattern helps students sense-check answers, though balancing by atom counting remains the safest method.
Checking your equation
A cracking equation is correct when:
the total number of carbon atoms matches
the total number of hydrogen atoms matches
the products are smaller molecules than the reactant
at least one product is often an alkene
Remind students that chemical equations must conserve atoms; cracking changes how atoms are arranged, not how many there are.
Which equation is balanced for cracking C10H22?
C5H12 + C5H10
C5H10 + C5H10
C6H14 + C4H10
C8H18 + C2H6
Only C5H12 + C5H10 keeps 10 carbons and 22 hydrogens and includes an alkene product.
Complete the method
To work out a cracking equation, start with the large alkane and choose one product. Then count the carbons left and the hydrogens left for the second product. At the end, check both sides balance.
This checks the step-by-step method from slides 11–14.
How do you check a cracking equation is right?
Expected elements: atoms must balance on both sides; products should be smaller than the reactant; one product is often an alkene. Accept simple phrasing.
What is one thing you learned today?
Reflection only. Invite short personal responses about key learning, such as the process, why cracking matters, or balancing equations.
What helped you understand cracking best?
Reflection only. Students may mention worked examples, counting atoms, or the step-by-step method.
Which part of this lesson felt hardest?
Reflection only. Likely answers include balancing equations, remembering alkane/alkene formulas, or why alkenes are useful.
Glossary
cracking - the process of splitting large hydrocarbon molecules into smaller molecules
hydrocarbon - a compound made only of hydrogen and carbon
alkane - a saturated hydrocarbon with only single bonds
alkene - an unsaturated hydrocarbon with at least one double bond
catalyst - a substance that speeds up a reaction without being used up
fraction - a group of hydrocarbons with similar boiling points collected from crude oil
polymer - a very large molecule made from many small molecules joined together
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