Hydrocarbons
Learn about the three main families of hydrocarbons -- alkanes, alkenes, and alkynes -- their structures, properties, isomerism, and combustion reactions.
The Three Hydrocarbon Families
Hydrocarbons are organic compounds containing only carbon and hydrogen atoms. They are classified by the type of carbon-carbon bond present. Hydrocarbons with only single bonds are saturated; those with double or triple bonds are unsaturated.
Comparing Hydrocarbon Families
Alkanes
CnH2n+2
Single bonds only (saturated)
Suffix: -ane
CH4, C2H6, C3H8
Alkenes
CnH2n
One C=C double bond (unsaturated)
Suffix: -ene
C2H4, C3H6, C4H8
Alkynes
CnH2n-2
One C≡C triple bond (unsaturated)
Suffix: -yne
C2H2, C3H4, C4H6
Saturation test: Alkenes and alkynes decolourise bromine water (turning from orange to colourless) because the double/triple bond reacts with bromine. Alkanes do not react -- they are already saturated.
Structural Isomers
Isomers are molecules with the same molecular formula but different structural arrangements. Structural isomers differ in how their atoms are connected, giving them different physical and sometimes chemical properties.
Isomers of C4H10 (Butane)
Butane (straight chain)
CH3—CH2—CH2—CH3
Boiling point: -1 °C
Methylpropane (branched)
CH3—CH(CH3)—CH3
Boiling point: -12 °C
Both have formula C4H10 but different structures and different boiling points.
Why do isomers have different properties? Branched isomers have a more compact shape, leading to weaker dispersion forces between molecules and therefore lower boiling points compared to their straight-chain counterparts.
Combustion Reactions
Hydrocarbons are important fuels. When they burn in oxygen, they undergo combustion. There are two types of combustion depending on the oxygen supply.
Complete Combustion
Occurs with excess oxygen supply
CH4 + 2O2 → CO2 + 2H2O
Products: Carbon dioxide + Water
Blue flame, maximum energy released
Incomplete Combustion
Occurs with limited oxygen supply
2CH4 + 3O2 → 2CO + 4H2O
Products: Carbon monoxide and/or soot (C) + Water
Yellow/orange flame, less energy, toxic CO produced
Environmental Impact
Burning fossil fuels (hydrocarbons) releases CO2, a greenhouse gas contributing to climate change. Incomplete combustion also produces CO (toxic) and particulate matter (soot), which causes air pollution and respiratory issues. Australia is investing in renewable energy to reduce dependence on hydrocarbon fuels.
Key Vocabulary
Saturated
A hydrocarbon containing only single carbon-carbon bonds. All bonding positions are filled with hydrogen atoms (e.g. alkanes).
Unsaturated
A hydrocarbon containing at least one carbon-carbon double or triple bond. Can undergo addition reactions (e.g. alkenes, alkynes).
Isomer
Molecules with the same molecular formula but different structural arrangements, resulting in different physical or chemical properties.
Combustion
An exothermic reaction where a substance reacts with oxygen to produce heat and light. For hydrocarbons, products include CO2 and H2O.
Worked Examples
Write the balanced equation for the complete combustion of propane (C3H8).
Step 1: Write the unbalanced equation: C3H8 + O2 → CO2 + H2O
Step 2: Balance carbons: 3 C atoms → 3CO2
Step 3: Balance hydrogens: 8 H atoms → 4H2O
Step 4: Balance oxygens: 3(2) + 4(1) = 10 O atoms needed → 5O2
Answer: C3H8 + 5O2 → 3CO2 + 4H2O
Determine the molecular formula for an alkene with 4 carbon atoms.
Step 1: The general formula for alkenes is CnH2n.
Step 2: Substitute n = 4: C4H2(4) = C4H8
Answer: The molecular formula is C4H8 (butene).
How would you distinguish between hexane and hexene using a simple chemical test?
Test: Add bromine water (orange/brown) to each sample and shake.
Hexane (alkane): No colour change -- bromine water stays orange. Alkanes are saturated and do not react.
Hexene (alkene): Bromine water is decolourised (turns colourless). The C=C double bond undergoes an addition reaction with Br2.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
What is the general formula for alkanes?
Question 2
Which products indicate incomplete combustion of a hydrocarbon?
Question 3
What type of bond is present in alkenes that is NOT present in alkanes?
Question 4
Structural isomers have the same:
Question 5
Adding bromine water to an unknown hydrocarbon causes decolourisation. This indicates the compound is:
Key Concepts Summary
- ●Alkanes (CnH2n+2) are saturated hydrocarbons with single bonds; alkenes (CnH2n) have double bonds; alkynes (CnH2n-2) have triple bonds.
- ●Structural isomers share the same molecular formula but have different arrangements of atoms.
- ●Complete combustion (excess O2) produces CO2 + H2O; incomplete combustion (limited O2) produces CO and/or C (soot).
- ●The bromine water test distinguishes saturated from unsaturated hydrocarbons.
- ●Burning fossil fuels releases greenhouse gases and pollutants, contributing to climate change.