Edexcel IGCSE Chemistry · Spec 4.17, 4.26-4.27
Cracking Conditions, Alkene Isomers and Bromine
Covers state cracking conditions precisely, Draw and name alkenes to four carbons and Predict bromine-addition products.
Chemistry revision video
Cracking Conditions, Alkene Isomers and Bromine
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Explained
Cracking, and what comes out of it
Fractional distillation separates crude oil but cannot change what is in it, and what is in it does not match demand. Long chain fractions are plentiful and less useful; short chains are wanted for fuels and for making plastics.
Cracking solves that. It breaks long chain alkanes into shorter, more useful molecules, and unlike distillation it is a chemical change: bonds break and new substances form.
The conditions, precisely
Vaporise the long chain alkane and pass the vapour over a hot catalyst.
The temperature is 600 to 700 degrees Celsius. The catalyst is silica or alumina, also written as silicon dioxide or aluminium oxide, and zeolites are accepted as well.
These two numbers and names are worth learning exactly, for the reason set out below.
What is produced
Every cracking reaction produces at least one alkene, and that is not a coincidence. There are not enough hydrogen atoms to go round once the chain is split, so at least one product has to manage with fewer, which it does by forming a carbon to carbon double bond.
A typical equation: C14H30 gives C8H18 plus 3C2H4. Octane for petrol, and three ethene molecules for making poly(ethene).
Balance these by counting carbons first, then hydrogens. The carbons on the right must total the carbons on the left, and the same for hydrogens, because cracking neither adds nor removes atoms.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme gives the conditions as two separate marks: 600 to 700 degrees Celsius inclusive, and silica or alumina as the catalyst. It accepts zeolites, SiO2, Al2O3, aluminosilicates, silicon dioxide and aluminium oxide.
An examiner report on an earlier paper says of the same conditions that many candidates knew the correct temperature and that silica or alumina was often answered, then adds a rare piece of direct advice: learn these conditions, as it is often tested.
Note the word inclusive on the temperature. A range is being marked, so 600 and 700 both count, but a vague answer such as very hot does not.
On the equation, the same mark scheme allows multiples or fractions, so a correctly balanced equation scaled differently still earns the mark.
On why the products are useful, it wants a chain rather than a fact: octane can be used as petrol or as fuel for cars, because it undergoes combustion, releasing heat energy. Naming the use without saying what makes it useful is half the answer.
Naming and drawing alkenes to four carbons
Ethene is C2H4, propene C3H6, butene C4H8, following the general formula C n H 2n.
From butene onwards the position of the double bond matters, so but-1-ene has it between the first and second carbons and but-2-ene between the second and third. Those two are structural isomers: the same molecular formula, a different arrangement.
When you draw one, show the double bond as two lines and make sure every carbon has four bonds in total. A carbon in a double bond has two bonds to its partner and two others, which is where drawings usually go wrong.
Bromine addition
Bromine adds across the double bond. The bond opens, one bromine atom attaches to each of the two carbons, and nothing else is produced.
Ethene plus bromine gives dibromoethane. The di is not optional, because bromine arrives as two atoms and both end up in the product.
This is also the test for unsaturation. Orange bromine water turns colourless with an alkene and stays orange with an alkane, and colourless rather than clear is the word that earns the mark.
Spec 4.17, 4.26-4.27
What you need to know
- State cracking conditions precisely
- Draw and name alkenes to four carbons
- Predict bromine-addition products
Active recall
Quick check
Answer each question before opening the answer.
Name a cracking catalyst
Silica or alumina
Name the two unbranched C4H8 alkenes
But-1-ene and but-2-ene
What is the product of ethene and bromine?
1,2-dibromoethane
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