Crude oil: a finite mixture of hydrocarbons formed from ancient plankton

1. Crude oil is a finite resource found in rocks.
2. A finite resource forms far more slowly than we use it, so its supply will eventually run out.
3. Crude oil formed over millions of years from the buried remains of ancient biomass, mostly tiny sea organisms called plankton.
4. The plankton were covered by mud, and over a very long time high temperature and pressure turned their remains into crude oil.
5. Crude oil is a mixture, meaning it contains many different compounds that are not chemically bonded together.

Hydrocarbons: molecules built from carbon and hydrogen only

1. Most of the compounds in crude oil are hydrocarbons.
2. A hydrocarbon is a molecule that contains carbon and hydrogen atoms only.
3. If a compound contains any other element, such as oxygen or chlorine, it is not a hydrocarbon.
4. Most of the hydrocarbons in crude oil belong to a family called the alkanes.

Alkanes: one general formula, $C_n{H}_{2n+2}$, describes the whole family

1. The alkanes are a homologous series, a family of hydrocarbons that all share the same general formula.
2. The general formula of the alkanes is $C_n{H}_{2n+2}$.
3. In this formula $n$ is the number of carbon atoms in one molecule.
4. You find the number of hydrogen atoms by working out $2n+2$.
5. The first four alkanes
   1. Methane has one carbon, so $n=1$ gives ${CH}_4$.
   2. Ethane has two carbons, so $n=2$ gives $C_2{H}_6$.
   3. Propane has three carbons, so $n=3$ gives $C_3{H}_8$.
   4. Butane has four carbons, so $n=4$ gives $C_4{H}_{10}$.
6. You need to recall the names methane, ethane, propane and butane, but not the names of larger alkanes.
7. To test a formula, work out $2n+2$ from the number of carbons and check it against the number of hydrogens.
   1. For $C_5{H}_{12}$, $2n+2=(2\times 5)+2=12$, which matches, so it is an alkane.
   2. For $C_4{H}_8$, $2n+2=(2\times 4)+2=10$, but there are only 8 hydrogens, so it is not an alkane.

Reading formulae: molecular and displayed forms both reveal an alkane

1. A molecular formula tells you how many of each atom are in one molecule, such as $C_2{H}_6$ for ethane.
2. A displayed formula draws out every atom and every covalent bond, so you can see how the atoms are joined.
3. To decide whether a molecule is an alkane, count its carbon and hydrogen atoms and check whether they fit $C_n{H}_{2n+2}$.
4. The displayed formula of ethane is shown here.
   1. $\begin{array}{ccccc}& H& & H& \\ & |& & |& \\ H-& C& -& C& -H\\ & |& & |& \\ & H& & H& \end{array}$
5. Ethane's displayed formula shows two carbon atoms and six hydrogen atoms, giving the molecular formula $C_2{H}_6$.
6. This fits $C_n{H}_{2n+2}$ when $n=2$, confirming that ethane is an alkane.