Crude Oil, Hydrocarbons and Alkanes

Crude oil is a thick, dark liquid pumped from rocks deep underground. It formed over millions of years from the remains of ancient biomass, mostly tiny sea creatures called plankton, that died and were buried in mud on the sea bed.

Over a very long time, high pressure and heat, in the absence of oxygen, turned this buried biomass into crude oil and natural gas. Because it forms so slowly, we are using it up far faster than it is replaced.

Crude oil is a finite resource: it is being used up much faster than it forms, so the supply will eventually run out.

Crude oil is not a single substance. It is a mixture of a very large number of different compounds. The compounds are not chemically bonded to each other, which is what makes it possible to separate them later by fractional distillation.

Most of the compounds in crude oil are hydrocarbons.

A hydrocarbon is a molecule made of hydrogen and carbon atoms only, with no other elements.

This is a definition examiners test directly. If a molecule contains oxygen, nitrogen, chlorine or any element other than hydrogen and carbon, it is not a hydrocarbon. For example, ethanol (C₂H₅OH) contains oxygen, so it is not a hydrocarbon even though it also contains hydrogen and carbon.

Compound	Formula	Hydrocarbon?	Reason
Methane	CH₄	Yes	Hydrogen and carbon only
Propane	C₃H₈	Yes	Hydrogen and carbon only
Water	H₂O	No	Contains oxygen
Ethanol	C₂H₅OH	No	Contains oxygen
Carbon dioxide	CO₂	No	Contains oxygen, and no hydrogen

Most of the hydrocarbons in crude oil belong to a family called the alkanes.

Alkanes are saturated hydrocarbons. Saturated means every carbon atom is joined to its neighbours by single bonds only, so the molecule holds the greatest possible number of hydrogen atoms.

Alkanes form a family that all share the same general formula:

$C_n{H}_{2n+2}$

Here $n$ is the number of carbon atoms. Once you know how many carbons a molecule has, this formula tells you exactly how many hydrogens it must have to be an alkane.

Worked example — an alkane has 5 carbon atoms. How many hydrogen atoms does it have?

Substitute $n=5$ into the general formula:

$H=2n+2=(2\times 5)+2=12$

So the molecular formula is C₅H₁₂. You can use the same rule in reverse to test whether a formula is an alkane: check that the number of hydrogens equals twice the carbons plus two.

To recognise an alkane from a formula, apply $C_n{H}_{2n+2}$. If the hydrogens fit this rule, it is an alkane; if there are fewer hydrogens, it is not saturated.

You need to know the names and formulae of the first four members of the alkane family. Their names always end in -ane.

Name	Carbons ($n$)	Molecular formula	Hydrogens ($2n+2$)
Methane	1	CH₄	4
Ethane	2	C₂H₆	6
Propane	3	C₃H₈	8
Butane	4	C₄H₁₀	10

Notice the pattern: each alkane has one more carbon atom and two more hydrogen atoms than the one before. That constant difference is what makes them a homologous series, a family of compounds with the same general formula and similar chemical properties.

You do not need to name any alkanes beyond these four, but you must be able to recognise a much larger alkane, such as C₈H₁₈, as an alkane by checking it against $C_n{H}_{2n+2}$.

A molecular formula such as C₃H₈ tells you how many of each atom there are. A displayed formula goes further: it shows every atom and every bond drawn out as lines, so you can see how the atoms are joined.

For the first three alkanes, the displayed formulae are:

Methane        Ethane              Propane

    H           H   H            H   H   H
    |           |   |            |   |   |
H — C — H   H — C — C — H    H — C — C — C — H
    |           |   |            |   |   |
    H           H   H            H   H   H

Every carbon atom forms exactly four bonds and every hydrogen atom forms exactly one bond. Counting the atoms in the propane diagram gives 3 carbons and 8 hydrogens, matching C₃H₈. When you draw a displayed formula, always check that each carbon has four lines coming from it and no hydrogen has more than one.

1. Saying crude oil is a compound

Crude oil is a mixture of a very large number of compounds, not a single compound. The compounds are not chemically bonded together, which is exactly why they can be separated by physical means later.

2. Calling any molecule with hydrogen and carbon a hydrocarbon, even with other atoms

A hydrocarbon contains hydrogen and carbon only. Ethanol (C₂H₅OH) has hydrogen and carbon but also oxygen, so it is not a hydrocarbon. Check the whole formula, not just that H and C are present.

3. Getting the general formula wrong

The alkane general formula is $C_n{H}_{2n+2}$, not $C_n{H}_{2n}$. The extra 2 hydrogens matter: C₄H₁₀ is butane (an alkane), while C₄H₈ has too few hydrogens to be a saturated alkane.

4. Drawing a carbon with the wrong number of bonds

Each carbon atom must have exactly four bonds and each hydrogen exactly one. A displayed formula with a carbon showing three or five bonds is wrong, however tidy it looks.

5. Confusing biomass origin with a quick, renewable process

Crude oil forms from ancient plankton over millions of years, so it is treated as finite and non-renewable. Do not describe it as something that replenishes on a human timescale.