The demonstration will take about 15—20 minutes, including testing of the gases. A class experiment should take about 45 minutes. Liquid paraffin is vaporised and passed over a hot pumice stone catalyst, producing a mixture of gaseous short-chain hydrocarbons. The mixture is flammable and decolourises bromine water and acidified permanganate ions. Equipment required for cracking liquid paraffin over porcelain chips and collecting the product gas. The demand for petrol is greater than the gasoline fraction obtained by distilling crude oil.
Cracking larger hydrocarbons produces smaller alkanes that can be converted into petrol. It also produces small alkenes, which are used make many other useful organic chemicals petrochemicals , especially plastics. This experiment models the industrial cracking process. You need to be particularly vigilant and lift the apparatus out of the water if suck-back starts to occur and cannot be reversed by stronger heating. It is very important to stress that students should not stop heating the boiling tube at the end of the experiment until the delivery tube is out of the water and contains no water.
The gas mixture which collects has a characteristic smell, burns with a yellow flame, and decolourises bromine water and acidified potassium manganate VII solution. This shows the presence of unsaturated molecules.
Students will find it helpful to build molecular models to understand the reaction and be able to write an equation for the reaction. The slideshow describes the process of catalytic cracking. The structure of hexane. The hexane is passed over a hot catalyst. Butane and ethene are produced. Cracking is important for two main reasons:. The supply is how much of a fraction an oil refinery produces. The octane is one of the molecules found in petrol gasoline.
Modern cracking uses zeolites as the catalyst. These are complex aluminosilicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge. They are, of course, associated with positive ions such as sodium ions. You may have come across a zeolite if you know about ion exchange resins used in water softeners. The zeolites used in catalytic cracking are chosen to give high percentages of hydrocarbons with between 5 and 10 carbon atoms - particularly useful for petrol gasoline.
It also produces high proportions of branched alkanes and aromatic hydrocarbons like benzene. There isn't any single unique reaction happening in the cracker.
The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds. One possible reaction involving the hydrocarbon C 15 H 32 might be:. This is only one way in which this particular molecule might break up. The ethene and propene are important materials for making plastics or producing other organic chemicals.
The octane is one of the molecules found in petrol gasoline. Modern cracking uses zeolites as the catalyst. These are complex aluminosilicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge. They are, of course, associated with positive ions such as sodium ions. You may have come across a zeolite if you know about ion exchange resins used in water softeners. The zeolites used in catalytic cracking are chosen to give high percentages of hydrocarbons with between 5 and 10 carbon atoms - particularly useful for petrol gasoline.
It also produces high proportions of branched alkanes and aromatic hydrocarbons like benzene. For UK A level and equivalent purposes, you aren't expected to know how the catalyst works, but you may be expected to know that it involves an ionic intermediate.
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