"Liquid paraffin" is a colourless oily liquid which contains alkane molecules with around 20 carbon atoms. In this case, you need to add another 3 carbon atoms and 6 hydrogens.Ī simple demonstration of cracking in the lab Count the carbons and hydrogens on the left-hand side, and add a new hydrocarbon with enough of each to balance it. There are other possibilities, including it breaking more than once.ĭon't worry about this! A simple exam question might just give you this partial equation and ask you to complete it. The structural formula for C 3H 6 is CH 2=CHCH 3. It might break off a 3-carbon fragment from the right-hand end instead, so you could get: There's no reason, of course, why the nonane should only break at this point. Should you want to write an ordinary equation for this, it would be So you have ended up with a smaller alkane, and a usefully reactive alkene. The two carbon atoms in the right-hand fragment can satisfy their bonding requirements by forming a double bond. If a hydrogen atom from the far right CH 3 group transfers to the CH 2 group at the end of the left-hand fragment, you get this: There needs to be some rearranging of the atoms. If you think about the carbon atoms in the two CH 2 groups at the broken ends, each of those carbon atoms is only forming 3 bonds - 1 to another carbon atom, and 2 to two hydrogen atoms. That's why I have drawn this random one above.īut these fragments aren't viable as they stand. They can rotate freely about all the carbon-carbon single bonds, and can take up all sorts of different shapes. We always draw these hydrocarbons as if they were tidy straight chains, but they aren't. For example, consider the molecule nonane, C 9H 20. The hydrocarbons in crude oil are mainly alkanes and, apart from burning, these aren't actually very reactive.Ĭracking produces molecules containing a carbon-carbon double bond (alkenes) and these are much more reactive and can be turned into useful products.īig molecules are broken at a carbon-carbon single bond. In particular, you need a lot more shorter hydrocarbons in order to meet the demand for petrol (gasoline) than there are in crude oil. The proportions of the various fractions that you get aren't in the same proportion that you ideally need to meet demand. There are two problems with the output from fractional distillation: It looks at a simple experiment to demonstrate cracking in the lab as well as some industrial details.Īlkanes are hydrocarbons which only contain carbon-carbon single bonds.Īlkenes are hydrocarbons which contain a carbon-carbon double bond. This page is about how you can break longer hydrocarbons into more useful shorter ones. Cracking long hydrocarbons to make smaller useful ones
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