11.01.4 Cracking & Alkenes

Syllabus

  • Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules.

  • Cracking can be done by various methods including catalytic cracking and steam cracking.

  • Students should be able to describe in general terms the conditions used for catalytic cracking and steam cracking.

  • The products of cracking include Alkanes and another type of Hydrocarbon called Alkenes.

  • Alkenes are more reactive than Alkanes and react with Bromine water, which is used as a test for alkenes.

  • Students should recall the colour change when Bromine water reacts with an Alkene.

  • There is a high demand for fuels with small molecules and so some of the products of cracking are useful as fuels.

  • Alkenes are used to produce polymers and as starting materials for the production of many other chemicals.

  • Students should be able to balance chemical equations as examples of cracking given the formulae of the reactants and products.

  • Students should be able to give examples to illustrate the usefulness of cracking.

  • They should also be able to explain how modern life depends on the uses of Hydrocarbons.

What does this mean?

Why crack long fractions?

Fractional Distillation separates compounds mixed together in Crude Oil into fractions containing Hydrocarbons of a similar length. It doesn't make any compounds that weren't there already.

Shorter Hydrocarbons evaporate easily (they are volatile) and so burn well without giving off much soot.

So these shorter Hydrocarbons make better fuels (like petrol) and there is a very high demand.

Longer Hydrocarbons contain more energy but are difficult to burn. There is a much lower demand for these.

Fortunately, it is possible to "crack" long Alkanes to produce shorter Alkanes (for fuels) and Alkenes (for making Polymers)

Catalytic Cracking

One way to crack long alkanes is to pass them at high temperature (actual temp not needed) over a suitable catalyst (we use unglazed pottery but knowledge of actual catalyst not needed.)

In general

Long Alkane --> Short Alkane + Alkene

Though we're not in control of where the alkane cracks.

So, we can't control which alkanes & alkenes we make

Steam Cracking

Steam cracking simply combines the long alkanes with very high temperature steam.

It still makes shorter alkanes.

Long Alkane --> Short Alkane + Alkene

But it makes a slightly different blend of products and is less likely to make Carbon (coke) waste

Supply of the shorter fractions is lower than the demand, supply of of the longer fractions is higher than the demand.

Cracking deals with both issues by increasing shorter fractions and using up longer fractions.

Balancing a cracking equation

Examiners like to give you part of a cracking equation and ask for the rest.

You don't need to learn these - it's simple Maths.

eg

C20H42 --> C4H8 + ___________?

There are 16 missing Carbon atoms and 34 missing H atoms on the righthand side.

And only one product - so, all the missing atoms go into the one product.

Answer: C20H42 --> C4H8 + C16H34

eg 2

C30H62 --> C16H34 + 2 ___________?

There are 14 missing Carbon atoms and 28 missing H atoms on the righthand side.

But there are two lots of product - so, divide the missing atoms by two.

Answer: C30H62 --> C16H34 + 2 C7H14

Test for saturation - Proving Alkenes are made during cracking

Notice that in all cracking the long starting Hydrocarbon is saturated (no double bonds), some of the products are unsaturated.

We can test for unsaturation with a colour change that demonstrates that Double bonds are present.

In our experiment, we tested the gases we collected from cracking with Bromine Water to prove that some of it contained a double bond (see Alkenes)

Bromine water is Orange.

If it decolourises then double bonds are present

Videos

Cracking in the lab. Video

Past Paper Questions

2021

2019