Organic chemistry · GCSE Chemistry
Alkanes
GCSE Chemistry revision on alkanes: saturated hydrocarbons, the general formula CnH2n+2, displayed formulae, combustion, and how boiling point, viscosity and flammability change with chain length.
Saturated means C–C single bonds only. General formula CnH2n+2. Longer chain: higher boiling point, more viscous, less flammable. Complete combustion gives CO₂ and H₂O; limited oxygen also gives CO or soot.
The important bits
What you need to know
- 1
Alkanes are a homologous series of saturated hydrocarbons. Saturated means every carbon–carbon bond is a single bond; the molecule contains as much hydrogen as those carbons can hold.
- 2
The general formula is CnH2n+2. The first four are methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. Each member differs from the next by a –CH₂– unit.
- 3
A homologous series has the same general formula, similar chemical reactions, and a gradual trend in physical properties as the chain gets longer.
- 4
A molecular formula shows how many atoms of each element are present. A displayed formula shows every atom and every covalent bond. Learn both for methane, ethane, propane and butane.
- 5
Alkanes are relatively unreactive except for combustion, which is why they make useful fuels. Complete combustion: hydrocarbon + oxygen → carbon dioxide + water.
- 6
Incomplete combustion in a limited supply of oxygen produces carbon monoxide (toxic, colourless, odourless) and/or carbon (soot), and releases less energy than complete combustion.
- 7
As the carbon chain lengthens, intermolecular forces between molecules get stronger, so boiling point and viscosity increase. Flammability decreases because the longer molecules are harder to ignite.
- 8
Crude-oil fractions are mostly alkanes. Short-chain alkanes are gases used in bottled fuel; medium chains are petrol and diesel; very long chains are thick fuel oils and bitumen.
Quotations worth analysing
Short evidence. Real method.
“Alkanes are saturated hydrocarbons with the general formula CnH2n+2.”
“Saturated” and the general formula are separate marks. Do not write CnH2n — that is alkenes. Hydrocarbon means hydrogen and carbon only.
“Complete combustion of a hydrocarbon produces carbon dioxide and water.”
Name both products. Incomplete combustion also produces carbon monoxide and/or carbon. State “limited oxygen” when you explain why.
“As the chain length increases, boiling point and viscosity increase and flammability decreases.”
Give the particle reason: stronger intermolecular forces between larger molecules. Do not say covalent bonds get stronger — those stay inside each molecule.
Go deeper
Homologous series is a three-part definition
Examiners want the same general formula, similar chemical properties, and a trend in physical properties, with each member differing by CH₂. Methane to butane is the named set you must be able to draw. Count the bonds on carbon: four, always. If your displayed formula shows a carbon with three bonds, you have invented a radical, not an alkane. Molecular formulae such as C₃H₈ do not show the chain shape, so a question that says “draw” wants displayed or structural. Chemical similarity comes from the same C–C and C–H single bonds. Physical trends come from size: more electrons, stronger intermolecular forces, more energy to separate the molecules. That is boiling point, not bond enthalpy.
Go deeper
Combustion equations must balance oxygen last
Write the hydrocarbon and oxygen on the left, CO₂ and H₂O on the right. Balance carbon, then hydrogen, then oxygen. Propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. If the oxygen count is odd, double everything. Incomplete combustion is not a different fuel; it is the same alkane with not enough O₂, so some carbon ends as CO or C. A yellow, sooty flame is the practical clue. Carbon monoxide is dangerous because it binds to haemoglobin. Soot is particulates. Both appear in atmosphere questions as well as organic ones. Always say which product forms in excess oxygen versus limited oxygen.
Go deeper
Trends are intermolecular, not covalent
Students write “longer alkanes have stronger bonds so they are harder to burn”. The covalent bonds inside methane and inside decane are similar C–C and C–H bonds. What changes is the force between molecules. Larger molecules have stronger intermolecular forces, so they boil at higher temperatures and pour more slowly (higher viscosity). Short molecules vaporise easily and mix with air, so they ignite readily — high flammability. That is why refinery gases and petrol are fuels you light easily, while bitumen is a tar for roads. In the exam, name the property, the direction of the trend, and intermolecular forces. Leave covalent bonds for combustion and cracking, where bonds actually break.
See the idea in action
Write a balanced equation for the complete combustion of butane, C₄H₁₀. Carbon: 4CO₂. Hydrogen: 10 H atoms → 5H₂O. Oxygen atoms on the right: (4 × 2) + 5 = 13, so 13/2 O₂. Double the equation: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. In limited oxygen the same butane can form CO or C as well, for example 2C₄H₁₀ + 9O₂ → 8CO + 10H₂O. The fuel has not changed; the oxygen supply has.
Exam technique
Turn knowledge into marks
State saturated, hydrocarbon, and CnH2n+2 in the opening line. Draw displayed formulae with four bonds on every carbon. For trends, blame intermolecular forces, not covalent bonds. For combustion, name CO₂ and H₂O, then add CO or C if oxygen is limited.
Common mistakes
Do not give these marks away
- 01
Using the alkene formula CnH2n for alkanes, or calling alkanes unsaturated.
- 02
Saying boiling point rises because covalent bonds get stronger as the chain lengthens.
- 03
Writing only “carbon dioxide” for combustion, or balancing C and H but leaving O₂ wrong.
What is the general formula of the alkanes?
ACnH2n
BCnH2n+2
CCnH2n−2
DCnH2n+1OH
Show the answer
CnH2n+2. Alkanes are saturated hydrocarbons. Each carbon forms four single bonds, which leads to the general formula CnH2n+2. CnH2n is alkenes.
Quick questions
If this is the bit you searched
What is the general formula of alkanes GCSE?
CnH2n+2. Methane is CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. They are saturated hydrocarbons.
What does saturated mean in organic chemistry?
The molecule contains only carbon–carbon single bonds, so it holds the maximum number of hydrogen atoms. Alkanes are saturated; alkenes are unsaturated.
Why do longer alkanes have higher boiling points?
Larger molecules have stronger intermolecular forces, so more energy is needed to separate them. The covalent bonds inside the molecules are not the reason.
What is produced when an alkane burns in limited oxygen?
Carbon monoxide and/or carbon (soot) as well as water, instead of only carbon dioxide and water. Less energy is released than in complete combustion.