[Music] hi and welcome back to freesciencelessons.com.uk by the end of this video you should be able to describe what's meant by cracking and then state the conditions for cracking you should then be able to describe what's meant by an alkene and describe the test for alkenes and finally you should be able to balance chemical equations to show cracking over the last few videos we've seen that crude oil is a mixture of hydrocarbons i'm showing you the structure of a hydrocarbon here now as we've seen hydrocarbons like this are called alkanes and there are two key features of alkanes firstly alkanes have the general formula cnh2n plus two and as we said before it's very important that you learn that secondly alkanes only have single covalent bonds between the carbon atoms now as you can see this alkane is a long chain hydrocarbon in other words it's got a lot of carbon atoms in a previous video we saw that long chain hydrocarbons are not very flammable and this is one reason why long chain hydrocarbons do not make good fuels now that's a problem as there's a high demand for short chain hydrocarbons to be used as fuels so to solve that problem we need to convert long chain hydrocarbons into shorter chain hydrocarbons and to do that we use a process called cracking in cracking a long chain alkane is broken down or cracked to produce smaller more useful molecules like this as you can see we've made a shorter chain alkane shorter chain arcane such as this one are used for fuels for example in cars now in this reaction we've also made a different type of molecule this is called an alkene we're going to be looking at alkenes in more detail later in this video now in the exam you could be asked to describe the conditions for cracking so we're going to look at those now there are two ways we can carry out cracking the first way is called catalytic cracking in catalytic cracking we use high temperature and a catalyst and the job of the catalyst is to speed up the reaction i should point out that you're not expected to know the name of the catalyst the second type of cracking is called steam cracking in steam cracking we use high temperature and steam coming up we'll take a closer look at alkenes and learn how to balance cracking equations okay as we said cracking makes hydrocarbons called alkenes i'm showing you an alkene here this is called ethene now as you can see alkenes have a double covalent bond between two carbon atoms and it's really important that you learn that alkenes are very useful molecules and in the exam you could be asked why firstly alkenes are used to make chemicals called polymers alkenes are also used as a starting material for other useful chemicals as well now one key factor about alkenes is that they're more reactive than alkanes we can use this to test for alkenes and it's really important that you learn this test we test for alkenes using bromine water which is orange if we shake our alkene with bromine water then the bromine water turns colorless like this now in the exam it's really important that you say that the bromine water has turned colorless do not say it's turned clear as that's not correct okay now you could be asked to balance an equation for cracking this is quite straightforward i'm showing you a cracking equation here we're cracking a long chain hydrocarbon into two shorter molecules we've been given one of the product molecules and we've got to work out the number of carbon atoms and hydrogen atoms in the other product molecule now the key fact is that the number of carbon and hydrogen atoms must be the same on both sides of the equation on the left hand side we've got 25 carbon atoms and 52 hydrogen atoms on the right hand side we've got 20 carbon atoms and 42 hydrogen atoms this means that the second product molecule must have five carbon atoms and ten hydrogen atoms here's one for you to try work out the number of carbon atoms and hydrogen atoms in the first product of this reaction so pause the video now and try this yourself okay on the left hand side we've got 40 carbon atoms and 82 hydrogen atoms and on the right hand side we've got seven carbon atoms and 14 hydrogen atoms this means that our first product molecule must have 33 carbon atoms and 68 hydrogen atoms remember you'll find plenty of questions on cracking and alkenes in myovision workbook and you can get that by clicking on the link above [Music] you