Unlock the logic behind four key hydrocarbon reactions — learn why alkanes substitute, alkenes add, all hydrocarbons burn, and how monomers chain into polymers.
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
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. Do not readily add bromine — but can undergo substitution with halogens when UV light initiates the reaction.
The electron-rich C=C bond is the reactive site. Bromine, hydrogen, hydrogen halides, and steam can add across it.
All hydrocarbons burn in oxygen. Full oxygen → CO2 + H2O. Limited oxygen → 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.
| 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 |
Same fuel, different oxygen supply, different products.
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.
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 |
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.
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.
Monomer and repeating unit must not be confused in exam responses.
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.
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.
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.
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.
"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.
Bonding decides reactivity: single bonds only usually substitute; double bonds add; all hydrocarbons combust.
In limited oxygen, incomplete combustion of propane produces carbon monoxide (CO) and/or carbon (C, soot) instead of CO₂. CO forms when there is insufficient O₂ to fully oxidise carbon from its +2 oxidation state to +4 (CO₂ requires one more oxygen atom per carbon than CO). Carbon soot forms when oxygen is so limited that carbon cannot be oxidised at all. This explains why gas heaters need ventilation — CO is produced in enclosed spaces.
Complete the Learn phase to unlock Practice.
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.
A. Bromine water stays orange.
B. Bromine water rapidly decolourises.
C. Clean blue flame; CO₂ and water vapour detected on a cold surface.
D. The product tests positive for an OH group.
For each compound or description below, identify the functional group, name the compound using IUPAC nomenclature, and draw its structural formula.
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:
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:
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. 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].
Back at the start, you were shown two colourless gases bubbled into separate tubes of bromine water — one tube stayed orange and the other instantly turned clear. Now you know exactly why.
The gas that decolourised the bromine water is an alkene (or alkyne — any unsaturated hydrocarbon). It contains a reactive C=C double bond. Bromine adds across the double bond in an addition reaction, forming a dibromoalkane and consuming the bromine — which is why the orange colour disappears: C2H4 + Br2 → C2H4Br2.
The gas that stayed orange is an alkane. Alkanes contain only C–C and C–H single bonds — no pi bond for bromine to add across. Without UV light to initiate free radical substitution, bromine does not react with alkanes under normal conditions. The colour remains.
The bromine water test is therefore a test for unsaturation: decolourisation confirms a C=C (or C≡C) bond is present. An alkane simply does not provide the reactive site that makes addition possible.
What are the products of complete combustion of a hydrocarbon?
What observation confirms that an alkene is present when bromine water is used?
Why is UV light described as an "energy source" rather than a "catalyst" in alkane halogenation?
What is the repeating unit for the addition polymer of propene?
What are the health hazards of incomplete combustion products?
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