Hydrocarbons - Alkanes and Alkenes

(GCSE chemistry)
Hope you're all-kene to read!

A hydrocarbon is any chemical made from only carbon and hydrogen. Hydrocarbons can come in two forms; alkenes and alkanes, these have similar names for the molecules but vary in the number of hydrogens and bonds. Hydrocarbons are made from crude oil which is found in the ground. This oil was formed over millions of years ago as mud, sand and dead animal/plant matter lay at the beds of the oceans. As time went on, this matter built up and the original matter became deeper and deeper in the mud and sand. The temperatures built and pressure on this matter increased which changed the and and mud into rock, whilst the dead matter below turned into crude oil and gas.

Crude oil is a mixture of chemicals which consists mostly of hydrocarbons. Crude oil is frequently used for fuels, plastics, polymers. solvents and lubricants. So what are the two types of hydrocarbon?

Alkanes are hydrocarbons which contain only single carbon bonds. The relationship between the carbons and hydrogens can be looked at mathematically using an nth term. The formula for alkanes is; CnH2n+2. We have names for many of the alkanes and it's helpful to know the names of the first few. 

Some funky oil which is 100% related (Daniel Olah)

• Methane CH4
• Ethane C2H6
• Propane C3H8
• Butane C4H10
• Pentane C5H12
• Hexane C6H14

We can use alkanes for fuel for a number of reasons. Hydrocarbons are good for fuel as the waste product is water. Hydrocarbons also release a significant quantity of energy when we combust them however they are hard to extract which means they aren't economically viable. The problem with crude oil is, the longer the hydrocarbon chain the less flammable it is. To use hydrocarbons for fuel, we can use cracking. Cracking is the process of breaking large alkanes into smaller molecules.

Cracking the alkanes requires lots of energy therefore we must use high energy conditions or find a way to lower the energy requirement. We can use two main methods of cracking;

1. Hydration of alkanes. Long alkanes can be mixed with steam and then heated to very high temperatures to break them into smaller molecules. This energy from the high temperatures can be very expensive.
2. Catalytic cracking. Long alkanes can be passed across the surface of a hot catalyst, where it breaks down into smaller molecules. This is more economically viable because lower energy is required.

Here's a reminder! A catalyst is a substance that speeds up a reaction by lowering the activation energy and proving an alternative pathway for a reaction. 

When cracking alkanes, we always make at least one alkane and alkene. But what is an alkene? Alkenes are hydrocarbons which contain a double carbon bond between two of the carbon atoms. We call them unsaturated because they have a double carbon bond which allows it to create an extra bond with other molecules (this is how polymerisation works). Alternatively, we call alkanes saturated because all the bond valencies of the atoms are used up. The formula for alkenes is CnH2n. Alkenes include:

• Ethene C2H4
• Propene C3H6
• Butene C4H8
• Pentene C5H10
• Hexene C6H12

Both alkanes and alkenes are in the same homologous series meaning they have the same functional group.