we saw in the last video that alkanes are a homologous series of hydrocarbons that contain only carbons and hydrogen atoms with no double bonds and we covered the names of the first four alkanes in the series in this video though we're going to talk about the properties of alkanes and then see how we can write equations for their combustion although all alkanes have similar properties there are trends they need to know about as the length of the carbon chain increases the first is that the boiling point increases with chain length so these four here all have fairly low boiling points and so exist as gases at room temperature because they have the shortest chains however longer alkanes with more than four carbons are liquids at room temperature and if the chains are really long then they could be solid this feature actually explains their next property as well which is that shorter alkanes are more volatile meaning that they evaporate more easily because of that low boiling point we just talked about meanwhile the longer alkanes are more viscous which just means that they're kind of thick and sticky like honey the last property you need to know is that shorter alkanes are also more flammable which means that they're easier to ignite or burn as we'll see when we come to crude oil the next thing we need to look at is combustion reactions one of the main uses of hydrocarbons like arcanes is for fuel because they release loads of energy when they're burned with oxygen as long as there's enough oxygen available we get complete combustion which is when a hydrocarbon reacts with oxygen to form carbon dioxide and water releasing loads of energy in the process so we could say that it's an exothermic reaction we could also say that the hydrogen and carbon in the hydrocarbon are being oxidized as each of the elements is combining with oxygen to form a co2 and h2o respectively now when it comes to exams you need to be able to write out the equations and balance them for example you could be asked to write the balanced equation for the complete combustion of propane we saw earlier that propane is a c3h8 so we'll react this with o2 to form a co2 and h2o then all we need to do is balance it we know that on the left we have three carbons so on the right we'll need three co2s we also have eight hydrogens on the left so because each h2o molecule contains two hydrogens we know we'll need four of them on the right then lastly we need to balance the oxygens on the right we now have six oxygens in the co2s plus four oxygens from the waters so 10 in total so to get 10 on the left we'll need five oxygen molecules and finally we just need to double check that all the numbers balance which they do so we're done as a last example let's write the balanced equation for the combustion of none which has the molecular formula c9h20 the first step is to write out the unbalanced equation then because nonain has nine carbons we know that there will be nine co2s on the right and as it has 20 hydrogens we'll have to form 10 h2os then lastly we just need to balance the oxygens so there's 9 times 2 or 18 in the carbon dioxides and 10 in the waters which is 28 in total so on the left we must have 14 o2 molecules because that totals 28 oxygen atoms and that's it our equation now balances the key point to take away from this video is that the properties of hydrocarbons like alkanes depend on the length of their carbon chain with the shortest hydrocarbons having the lowest boiling points and so being the most volatile and flammable which makes them great fuels and whenever we use a hydrocarbon as a fuel we call the process combustion with complete combustion only taking place if there's enough oxygen available which will produce carbon dioxide and water releasing loads of energy in the process anyway that's all for this video so hope you enjoyed it and we'll see you next time you