we saw in the last video that we can think of crude oil as a mixture of lots of different hydrocarbons and that we can separate out these different hydrocarbons by the length of their carbon chains using a process called fractional distillation we also saw that the shortest hydrocarbons are the most flammable and so they make the best fuels whereas the longer chain hydrocarbons are thick viscous liquids which makes them comparatively less useful so in today's video we're going to look at how these longer less useful hydrocarbons can be broken down into shorter more flammable hydrocarbons in a process we call cracking now cracking is a thermal decomposition reaction and if you think about that word thermal means heating and decomposition or to decompose means breaking down so thermal decomposition means breaking down molecules by heating them and they're actually two different methods you need to know about one is catalytic cracking and the other is steam cracking regardless of which one of these we're going to do though the first step is to heat our long chain hydrocarbons and vaporize them which just means to turn them into a gas and if we were using catalytic cracking we would have some hot powdered aluminium oxide ready as our catalyst we then pass our hydrocarbon vapor over the powder and as the long hydrocarbons come into contact with the catalyst that split apart into two smaller hydrocarbons on the other hand for steam cracking once we've vaporized our hydrocarbons we mix them with steam and then we heat them to a very high temperature which again causes the long hydrocarbon chains to split apart into shorter ones now as well as knowing these different types of cracking you also need to be able to write balanced chemical equations for the cracking of long chain alkanes the equation for this is that you take your long chain alkane for example decane and it will go to form a shorter alkane molecule like heptane and an alkene which in this case would be propine the important thing to remember here is that the number of carbons and hydrogens on each side has to be the same just like with any other balanced equation so as decane is 10 carbons long and one of the products is heptane which has seven carbons then another product must have three carbons and if you did the same thing for hydrogens you'd find that there aren't enough hydrogens left for both products to be saturated and have all single bonds so instead one of our products has to have a double bond which is what makes it an alkene rather than an alkane instead of being a waste product though these alkenes can be used to produce polymers and also be used as starting materials for producing loads of other chemicals let's now have a look at a proper example decane c10h22 can be cracked into ethene c2h4 and one other hydrocarbon write a balanced equation for this reaction so the first thing we do is write the equation with everything that we know so far so decane goes to form some unknown alkane plus ethene then all we have to do is work out how many carbons and hydrogens our unknown alkane must have to do this we just take away the number of carbons and hydrogens of our known product ethene away from our reactant decking and whatever's left is the number of carbon and hydrogen atoms our unknown alkane must have so 10 minus two carbons means that our alkane must have eight carbons and 22 minus 4 hydrogens means our alkane must have 18 hydrogens so its formula is c8h18 and so we just write that into our equation now before we finish we just need to point out that alkenes are another example of a homologous series so a group of similar compounds with similar properties just like alkanes are and the only structural difference between these two groups is that alkenes have a double bond while alkanes only have single bonds so we can say that alkenes are unsaturated while alkanes are saturated and this difference leads to two important features of alkenes one is that they're more reactive than alkanes so they're able to react with bromine and specifically you need to know that if we add bromine water to a solution of alkenes then they'll decolorize the bromine water from orange to colorless and this is actually the test for alkenes they need to remember the other feature of alkenes is that they can be added together to make polymers because a double bonds can break to form two more bonds and so they can bond to the adjacent molecules anyway that's all for today so hope you enjoyed it and we'll see you next time