Hydrocarbons are only useful because they react in predictable ways. A chemist can tell whether a molecule will burn, substitute, add across a double bond, or polymerise simply by looking at its bonding and functional groups.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Two colourless gases are bubbled into separate test tubes of bromine water. One tube stays orange. The other rapidly turns colourless. A third sample is ignited and produces a sooty flame.
Before reading on, predict what kind of hydrocarbons could produce each observation. What does each test tell you about the bonding inside the molecule?
📚 Core Content
A fast way to organise this lesson is to stop memorising isolated equations and start classifying reactions by the bond being targeted. All hydrocarbons combust. Alkanes mainly substitute. Alkenes mainly add.
Single bonds only. They do not readily add bromine because there is no C=C bond to open, but they can undergo substitution with halogens when UV light initiates the reaction.
The electron-rich C=C bond is the reactive site. Reagents such as bromine, hydrogen, hydrogen halides, and steam can add across the double bond.
Hydrocarbons burn in oxygen to release energy. Product identity depends on oxygen supply: full oxygen gives CO2 and H2O; limited oxygen gives CO and/or soot.
Alkenes can repeatedly add to each other. The double bond opens and forms a long carbon chain polymer such as polyethene.
Combustion is the most economically important hydrocarbon reaction, but it is also the one students answer too vaguely. Good responses distinguish complete from incomplete combustion, state the products, and explain the environmental or health consequences of each.
With excess oxygen, a hydrocarbon undergoes complete combustion to form carbon dioxide and water. With limited oxygen, incomplete combustion forms carbon monoxide and/or solid carbon particles (soot) as well as water. In practice, real flames often produce a mixture depending on how efficiently oxygen mixes with the fuel.
| Condition | Main products | Observation | Impact |
|---|---|---|---|
| Complete combustion | CO2 + H2O | Cleaner blue flame | CO2 contributes to greenhouse warming |
| Incomplete combustion | CO + H2O and/or C + H2O | Yellow luminous sooty flame | CO is toxic; soot damages lungs and air quality |
Alkanes are relatively unreactive because they contain only strong sigma bonds and no electron-rich multiple bond. When they do react with halogens such as chlorine, the reaction is a substitution: one hydrogen atom is replaced by a halogen atom.
The classic HSC example is methane reacting with chlorine under UV light. The UV energy breaks the Cl-Cl bond and initiates a free-radical chain process. The syllabus focus is the equation and reaction type, not the full mechanism.
1. Check whether the hydrocarbon is saturated.
2. Look for a halogen reagent such as Cl2 or Br2 with UV light.
3. Compare reactant and product formulas: one H is gone, one halogen has replaced it.
4. Name the by-product: hydrogen halide such as HCl or HBr.
CH4 is a saturated alkane with only C-H and C-C single bonds.
Cl2 supplies the halogen atom that will replace one hydrogen atom.
Ultraviolet light provides the energy needed to initiate the substitution process.
The hydrocarbon framework remains intact while one hydrogen is replaced.
The double bond is the reactive feature that distinguishes alkenes from alkanes. In an addition reaction, the pi bond is broken and new single bonds are formed to the added atoms or groups.
| Reagent | Example with ethene | Product | Use in HSC |
|---|---|---|---|
| Br2 | C2H4 + Br2 | 1,2-dibromoethane | Test for unsaturation |
| H2 | C2H4 + H2 | Ethane | Hydrogenation |
| HCl / HBr | C2H4 + HCl | Chloroethane | Haloalkane formation |
| H2O (steam) | C2H4 + H2O | Ethanol | Hydration pathway |
Switch between common hydrocarbon reactions and compare the bond changes, conditions, and exam clues.
Addition polymerisation is repeated addition chemistry. Each alkene monomer opens its double bond and links to neighbouring monomers, creating a long carbon chain. The atoms of the monomer all remain in the polymer repeating unit.
For ethene, the polymer is polyethene. The monomer is CH2=CH2. The repeating unit is written as [−CH2−CH2−]n. Brackets show the repeating pattern and n shows that the unit repeats many times.
A small alkene molecule before reaction, for example ethene.
The structural pattern inside the polymer chain shown in brackets.
The complete macromolecule made of many repeating units joined together.
Use this sequence whenever a question gives you a structural formula and a reagent list.
Step 1: Identify whether the organic molecule is saturated or unsaturated.
Step 2: Identify the reagent and any condition such as UV light or oxygen supply.
Step 3: Match structure + reagent to reaction family: combustion, substitution, or addition.
Step 4: Predict the product by tracking which bond breaks and what atoms are added or replaced.
✍️ Activities
A chemist carries out four experiments and records the observations below. For each experiment, identify the reaction type and justify your answer using the evidence.
“Bromine water tests for all hydrocarbons.” Wrong. It is primarily a test for unsaturation, especially alkenes.
“Substitution and addition both just mean chemicals react together.” Wrong. Addition opens a multiple bond; substitution replaces one atom or group with another.
“The repeating unit is the same as the monomer.” Wrong. The repeating unit shows the structure after the C=C bond has opened and linked into the chain.
🧠 Worked Examples
Problem: Ethene is bubbled through bromine water. Explain the observation and write the equation.
Ethene is an alkene, so it contains a reactive C=C bond.
Bromine adds across the double bond, so the orange bromine colour disappears.
The balanced equation is C2H4 + Br2 → C2H4Br2.
Answer: Bromine water decolourises because bromine adds across the C=C bond in ethene. This is an addition reaction: C2H4 + Br2 → C2H4Br2.
Problem: Write the complete combustion equation for butane, C4H10.
Write the skeleton: C4H10 + O2 → CO2 + H2O.
Balance C first: 4 carbons means 4CO2.
Balance H next: 10 hydrogens means 5H2O.
Count oxygen atoms on the right: 8 + 5 = 13 O atoms, so use 13/2 O2, then multiply through by 2.
Answer: 2C4H10 + 13O2 → 8CO2 + 10H2O.
Problem: Draw or describe the repeating unit formed when propene polymerises.
Start with the monomer: CH2=CHCH3.
The double bond opens during addition polymerisation.
The carbon skeleton remains, so the repeating unit becomes [−CH2−CH(CH3)−]n.
Answer: The repeating unit is [−CH2−CH(CH3)−]n. The CH3 side group remains attached to every second carbon in the polymer chain.
🧠 Check Your Understanding
1. Which observation is the best evidence that an unknown hydrocarbon contains a C=C bond?
2. Methane reacting with chlorine under UV light is best classified as:
Methane reacting with chlorine under UV light is best categorised as:
3. Which set of products indicates incomplete combustion of a hydrocarbon?
4. Which statement about addition polymerisation is correct?
5. Ethene reacts with hydrogen chloride to form chloroethane. This is an example of:
Ethene reacts with hydrogen chloride to form chloroethane. This is an instance of:
📝 Short Answer
1. Explain why bromine water can distinguish between ethane and ethene. 3 MARKS
2. Propene reacts with steam to form an alcohol. Identify the reaction type and explain the bond changes that occur. 4 MARKS
3. A yellow smoky flame is observed when a hydrocarbon burns in a limited oxygen supply. Explain what this observation suggests about the reaction products and why those products are concerning. 5 MARKS
A. No reaction (or very slow substitution only if UV is present). Hexane is saturated — it has no C=C bond — so bromine cannot add across a double bond. Without UV light, substitution is not initiated either, so the orange colour remains.
B. Addition reaction. Hex-1-ene contains a C=C bond. Bromine adds across the double bond to form a dibromoalkane, consuming Br₂ and removing its orange colour.
C. Complete combustion. Excess oxygen means all carbon is fully oxidised to CO₂ and all hydrogen to H₂O. The clean blue flame and absence of soot confirm complete combustion.
D. Addition (hydration). Ethene reacts with water (steam) across its C=C bond to form ethanol. The OH group detected confirms an alcohol product formed by addition of water across the double bond.
1. Ethene decolourises bromine water because it contains a C=C bond, allowing bromine to add across the double bond. Ethane has only single bonds and does not undergo this rapid addition reaction under normal test conditions.
2. CH4 + Cl2 → CH3Cl + HCl. This is a substitution reaction because one hydrogen atom on methane is replaced by chlorine.
3. Complete combustion of propane gives CO2 and H2O. Incomplete combustion gives CO and/or C together with H2O because there is insufficient oxygen to fully oxidise all carbon atoms to CO2.
4. Polypropene monomer: CH2=CHCH3. Repeating unit: [−CH2−CH(CH3)−]n.
1. C — Rapid bromine-water decolourisation is the clearest sign of a C=C bond and unsaturation.
2. B — A hydrogen atom on methane is replaced by chlorine, so the reaction is substitution.
3. D — Incomplete combustion forms partially oxidised carbon products such as CO and soot.
4. A — Addition polymers form when alkene double bonds open and join into long chains.
5. B — H and Cl add across the double bond, so the reaction is addition.
Q1 (3 marks): Ethene contains a carbon-carbon double bond [1]. Bromine adds across this double bond, so bromine water is decolourised [1]. Ethane is saturated and has no C=C bond, so it does not rapidly react with bromine water under normal conditions [1].
Q2 (4 marks): This is an addition reaction [1]. Propene contains a reactive C=C bond [1]. During hydration, the double bond opens and the atoms of water add across the bond [1]. The product is an alcohol because an -OH group is introduced into the molecule [1].
Q3 (5 marks): A yellow smoky flame suggests incomplete combustion due to limited oxygen supply [1]. Instead of all carbon atoms forming CO2, some form carbon monoxide and/or solid carbon soot [1]. Carbon monoxide is dangerous because it binds strongly to haemoglobin and reduces oxygen transport in the blood [1]. Soot particles are also harmful because they contribute to respiratory disease and poor air quality [1]. The flame appears smoky because glowing carbon particles are present [1].
Return to your original response. You should now be able to sharpen it into a full HSC-style explanation:
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Put your knowledge of Hydrocarbon Reactions — Combustion, Substitution, Addition & Polymerisation to the test. Answer correctly to deal damage — get it wrong and the boss hits back. Pool: lessons 1–5.
Tick when you’ve finished the activities and checked the model answers.