Crude oil, hydrocarbons and alkanes
What you'll learn
- What crude oil is and why it is described as a finite resource.
- Why crude oil is a mixture, not a pure substance.
- What hydrocarbons and alkanes are.
- How to recognise alkanes from molecular formulae and displayed formulae.
Crude oil: the big picture
Crude oil is a dark, sticky liquid found in rocks underground. It is important because it is a source of fuels and a feedstock — a raw material used to make other useful chemicals.
Crude oil is a finite resource, meaning there is only a limited amount available and it forms far too slowly to replace what humans use. It is not being made quickly on a human timescale.
Crude oil formed from ancient biomass, which means material from living organisms. For crude oil, the biomass was mainly plankton — tiny organisms that lived in ancient seas. When they died, they were buried in mud. Over millions of years, conditions underground changed this buried material into crude oil.
Crude oil is not an underground lake
Crude oil is found in rocks. It is usually trapped in tiny spaces inside porous rock, with impermeable rock above stopping it from escaping.
Pure substances, mixtures and compounds
Before we describe crude oil properly, you need three key words.
A substance is a type of matter with a particular composition. For example, water is a substance.
A compound is a substance made from atoms of two or more different elements chemically bonded together. For example, carbon dioxide contains carbon and oxygen atoms bonded together.
A mixture contains two or more substances that are not chemically bonded to each other. The substances in a mixture can often be separated by physical methods.
Crude oil as a mixture
Crude oil is a mixture of a very large number of compounds. This means it is not one single chemical with one formula.
This matters because different compounds in crude oil have different properties, such as boiling points. Later in this topic, you will learn how these differences allow crude oil to be separated into useful fractions.
Hydrocarbons
Most of the compounds in crude oil are hydrocarbons.
Hydrocarbon
A hydrocarbon is a molecule made up of hydrogen atoms and carbon atoms only.
A molecule is a group of atoms joined together by covalent bonds. In this topic, you will often see molecular formulae such as C2H6\text{C}_2\text{H}_6C2H6. This formula means one molecule contains 2 carbon atoms and 6 hydrogen atoms.
Recognising a hydrocarbon
Decide whether C3H8\text{C}_3\text{H}_8C3H8 and C2H5OH\text{C}_2\text{H}_5\text{OH}C2H5OH are hydrocarbons.
- Check which elements are present in each formula. C3H8\text{C}_3\text{H}_8C3H8 contains carbon and hydrogen only, while C2H5OH\text{C}_2\text{H}_5\text{OH}C2H5OH contains carbon, hydrogen and oxygen.
- Apply the definition: a hydrocarbon must contain hydrogen and carbon atoms only.
- Therefore, C3H8\text{C}_3\text{H}_8C3H8 is a hydrocarbon, but C2H5OH\text{C}_2\text{H}_5\text{OH}C2H5OH is not a hydrocarbon because it contains oxygen.
Hydrocarbon test
If a formula contains any element other than carbon and hydrogen, it is not a hydrocarbon.
Alkanes
Most of the hydrocarbons in crude oil are alkanes.
An alkane is a type of hydrocarbon whose molecules contain only single covalent bonds. A covalent bond is a bond where atoms share electrons. In displayed formulae, a single covalent bond is shown as one line.
Alkanes form a homologous series. A homologous series is a family of similar compounds with the same general formula and similar chemical properties.
For alkanes, the general formula is:
CnH2n+2\text{C}_n\text{H}_{2n+2}CnH2n+2Here, nnn is the number of carbon atoms in the molecule. The number of hydrogen atoms is then 2n+22n + 22n+2.
General formula
A general formula is a formula that works for every member of a family of compounds. For alkanes, CnH2n+2\text{C}_n\text{H}_{2n+2}CnH2n+2 links the number of carbon atoms to the number of hydrogen atoms.
The first four alkanes
You need to know the names of the first four alkanes:
- Methane: CH4\text{CH}_4CH4
- Ethane: C2H6\text{C}_2\text{H}_6C2H6
- Propane: C3H8\text{C}_3\text{H}_8C3H8
- Butane: C4H10\text{C}_4\text{H}_{10}C4H10
You do not need to know the names of specific alkanes beyond methane, ethane, propane and butane for this part of the specification.
Remembering the first four
The first four alkane names match 1, 2, 3 and 4 carbon atoms: meth-, eth-, prop-, but-.
Using the alkane general formula
The general formula is very useful because it lets you work out whether a formula could be an alkane.
For an alkane:
number of hydrogen atoms=2n+2\text{number of hydrogen atoms} = 2n + 2number of hydrogen atoms=2n+2where nnn is the number of carbon atoms.
Checking whether a formula is an alkane
Decide whether C6H14\text{C}_6\text{H}_{14}C6H14 and C6H12\text{C}_6\text{H}_{12}C6H12 could be alkanes.
- Both formulae contain 6 carbon atoms, so use n=6n = 6n=6.
- For an alkane with 6 carbon atoms, calculate the expected number of hydrogen atoms: 2n+2=2(6)+2=142n + 2 = 2(6) + 2 = 142n+2=2(6)+2=14.
- Compare with the formulae: C6H14\text{C}_6\text{H}_{14}C6H14 matches the alkane formula, but C6H12\text{C}_6\text{H}_{12}C6H12 does not.
Hydrocarbon does not always mean alkane
A molecule can contain only carbon and hydrogen and still not be an alkane. For example, C2H4\text{C}_2\text{H}_4C2H4 is a hydrocarbon, but it does not fit CnH2n+2\text{C}_n\text{H}_{2n+2}CnH2n+2, so it is not an alkane.
Representing alkane molecules
Alkane molecules can be shown in different ways.
A molecular formula shows how many atoms of each element are in one molecule. For example, C2H6\text{C}_2\text{H}_6C2H6 tells you that ethane has 2 carbon atoms and 6 hydrogen atoms.
A displayed formula shows all the atoms and all the bonds in a molecule. Each line represents one covalent bond.
In GCSE displayed formulae for alkanes:
- carbon atoms are shown as C
- hydrogen atoms are shown as H
- every carbon atom makes 4 bonds in total
- every hydrogen atom makes 1 bond
- alkanes have single bonds only
Spotting an alkane in displayed form
An alkane displayed formula should contain carbon and hydrogen only, show single bonds only, and have each carbon atom making 4 bonds.
Molecular modelling kits are a good way to practise this. If your model has carbon atoms joined in a chain and every carbon has 4 bonds overall, you are building the right type of structure for an alkane.
Reading a displayed alkane formula
A displayed formula shows three carbon atoms joined in a chain by single bonds, with enough hydrogen atoms so that each carbon has 4 bonds. Identify the alkane.
- Count the carbon atoms: there are 3 carbon atoms, so n=3n = 3n=3.
- Use the alkane formula to calculate the hydrogen atoms: 2n+2=2(3)+2=82n + 2 = 2(3) + 2 = 82n+2=2(3)+2=8.
- The molecular formula is C3H8\text{C}_3\text{H}_8C3H8, which is propane.
Quick summary
Crude oil is a finite resource found in rocks. It formed from ancient biomass, mainly plankton, buried in mud millions of years ago.
Crude oil is a mixture of many compounds. Most of these compounds are hydrocarbons, which contain hydrogen and carbon atoms only.
Most hydrocarbons in crude oil are alkanes. Alkanes follow the general formula:
CnH2n+2\text{C}_n\text{H}_{2n+2}CnH2n+2The first four alkanes are methane, ethane, propane and butane.
In the exam
- If asked about crude oil formation, mention ancient biomass, mainly plankton, buried in mud, changed over millions of years.
- If asked whether something is a hydrocarbon, check that the formula contains carbon and hydrogen only.
- If asked whether something is an alkane, use CnH2n+2\text{C}_n\text{H}_{2n+2}CnH2n+2 and compare the expected hydrogen count with the formula given.
Check yourself
- Why is crude oil described as a finite resource?
- What is the difference between a hydrocarbon and an alkane?
- How would you check whether C4H10\text{C}_4\text{H}_{10}C4H10 is an alkane?