The shape of the lesson
By the end of the lesson, learners can
- Unreactive, but they burn: Alkanes do not react with acids, alkalis or bromine water in the dark.
- Substitution with chlorine: Mix methane and chlorine in the simulator and shine ultraviolet light on them.
- Alkanes: CnH2n+2, saturated, generally unreactive.
- Complete combustion: CO2 + H2O; balance C, H, then O.
- Substitution: Cl2 in UV light; CH4 + Cl2 → CH3Cl + HCl.
- Extended: photochemical, UV light supplies the activation energy.
One picture every idea returns to
Alkanes are saturated hydrocarbons with the general formula CnH2n+2; every bond is a single covalent bond. They are generally unreactive, except for combustion and substitution by chlorine. In complete combustion an alkane burns in plenty of oxygen to give carbon dioxide and water, releasing a lot of energy, which is why alkanes are used as fuels. In substitution a hydrogen atom is replaced by a chlorine atom: CH4 + Cl2 → CH3Cl + HCl. (Extended) This is a photochemical reaction: ultraviolet light provides the activation energy.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Saturated and mostly unreactive.. Alkanes are the main hydrocarbons in crude oil and natural gas. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Unreactive, but they burn | Alkanes do not react with acids, alkalis or bromine water in the dark. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Substitution with chlorine | Mix methane and chlorine in the simulator and shine ultraviolet light on them. Project tab 2 of the lab (Hydrocarbons lab, substitution with chlorine) and let a student drive it while the class predicts. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Substitution with chlorine, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Substitution makes two products: do not forget the HCl. | Alkanes do not undergo addition, because they have no C=C bond to open. The condition is ultraviolet light, not heat with a catalyst, and the chlorine replaces a hydrogen, never a carbon. |
Alkanes: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
State the general formula of the alkanes and explain why they are described as saturated.
Write a balanced equation for the complete combustion of butane, C4H10.
30 cm3 of propane is burned completely: C3H8 + 5O2 → 3CO2 + 4H2O. Calculate the volume of oxygen used and the volume of carbon dioxide made, all volumes at room temperature and pressure.
Ethane reacts with chlorine. State the condition needed, write the equation and name the organic product.
(Extended) A mixture of methane and chlorine is kept in the dark and nothing happens. When it is placed in sunlight, it reacts. Explain these observations.
A student writes CH4 + Cl2 → CH4Cl2 and calls it addition. Identify two errors.
Answer key and marking notes
- CnH2n+2.
- They contain only single carbon-carbon bonds (no C=C).
- C4H10 + 6½O2 → 4CO2 + 5H2O (O on the right = 8 + 5 = 13).
- Doubled: 2C4H10 + 13O2 → 8CO2 + 10H2O.
- Oxygen: 5 × 30 = 150 cm3.
- Carbon dioxide: 3 × 30 = 90.0 cm3.
- Ultraviolet light.
- C2H6 + Cl2 → C2H5Cl + HCl.
- Chloroethane.
- The reaction is photochemical.
- Ultraviolet light (in sunlight) provides the activation energy needed to start the reaction; in the dark there is not enough energy.
- Methane is saturated, so it reacts by substitution, not addition.
- The products are CH3Cl and HCl: one hydrogen is replaced and leaves as hydrogen chloride (CH4Cl2 would need carbon to make six bonds).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is the general formula of the alkanes?
- ACnH2n+2
- BCnH2n
- CCnH2n+1OH
- DCnH2n−2
Why are alkanes described as generally unreactive?
- Athey contain only strong C−C and C−H single bonds
- Bthey contain a reactive C=C double bond
- Cthey are ionic compounds
- Dthey do not burn
Which is the balanced equation for the complete combustion of propane?
- AC3H8 + 5O2 → 3CO2 + 4H2O
- BC3H8 + 3O2 → 3CO2 + 4H2O
- CC3H8 + 5O2 → 3CO + 4H2O
- DC3H8 + 2O2 → 3C + 4H2O
Methane reacts with chlorine. Which condition is needed?
- Aultraviolet light
- Ba nickel catalyst
- Cphosphoric acid catalyst at 300 °C
- Dyeast at 30 °C
Which equation shows the reaction of methane with chlorine?
- ACH4 + Cl2 → CH3Cl + HCl
- BCH4 + Cl2 → CH4Cl2
- CCH4 + Cl2 → CH2Cl2 + H2
- DCH4 + Cl2 → C + 2HCl + H2
Ethane reacts with chlorine in ultraviolet light. Which statement is correct?
- Ait is a substitution reaction and a mixture of products with different numbers of chlorine atoms can form
- Bit is an addition reaction that gives only one product
- Cit is a substitution reaction that gives only C2H5Cl, whatever the amount of chlorine
- Dit is an addition reaction because ethane is unsaturated
Which is the balanced equation for the complete combustion of butane?
- A2C4H10 + 13O2 → 8CO2 + 10H2O
- BC4H10 + 6O2 → 4CO2 + 5H2O
- C2C4H10 + 9O2 → 8CO + 10H2O
- DC4H10 + 13O2 → 4CO2 + 5H2O
A gas fire burns methane in a poorly ventilated room. Why is this dangerous?
- Aincomplete combustion produces carbon monoxide, which is toxic
- Bcomplete combustion produces carbon dioxide, which is highly toxic in small amounts
- Cmethane reacts with nitrogen in the air to form chloromethane
- Dthe water produced is poisonous as a vapour
Quiz key
With n = 1, CnH2n+2 gives CH4, and with n = 2 it gives C2H6, so it is the alkane formula. CnH2n is the alkene formula. CnH2n+1OH is the alcohol formula. CnH2n−2 has even fewer hydrogens and is not an alkane.
Alkanes are saturated: they have only single C−C and C−H bonds, which are strong and not easily attacked, so apart from burning they react with very little. A C=C double bond is what makes alkenes reactive. Alkanes are covalent, not ionic. Alkanes do burn well, which is why they are used as fuels.
3 C give 3CO2 and 8 H give 4H2O. The right side then has 6 + 4 = 10 oxygen atoms, so 5O2 are needed. With only 3O2 the oxygen does not balance. The equation giving CO describes incomplete combustion and is not balanced (it has 7 O on the right). Producing carbon (soot) is also incomplete combustion, not complete.
Methane and chlorine react in a substitution reaction only in ultraviolet light, which provides the energy to start the reaction. A nickel catalyst is used to add hydrogen to alkenes. Phosphoric acid at 300 °C is for adding steam to ethene. Yeast is used for fermentation of sugars.
One chlorine atom replaces one hydrogen atom, forming chloromethane, and the hydrogen that is removed joins the other chlorine atom as hydrogen chloride. CH4Cl2 would need carbon to form six bonds, which is impossible, and it describes addition, which saturated alkanes cannot do. Hydrogen gas is not a product. Breaking methane down into carbon is not what happens.
Chlorine replaces hydrogen atoms one at a time. The first product, C2H5Cl, still has C−H bonds, so it can react further to give C2H4Cl2 and so on, giving a mixture. Ethane is saturated, so it cannot undergo addition. Saying only C2H5Cl forms ignores further substitution.
One C4H10 gives 4CO2 and 5H2O, which contain 8 + 5 = 13 O atoms, so 6½ O2 are needed; doubling gives 2C4H10 + 13O2 → 8CO2 + 10H2O. With 6O2 there are 12 O on the left and 13 on the right. The CO equation is balanced (18 O on each side), but carbon monoxide is a product of incomplete combustion, not complete combustion. The last option has 26 O on the left and only 13 on the right.
With a limited oxygen supply, combustion is incomplete and carbon monoxide forms. CO is toxic because it reduces the ability of blood to carry oxygen. Carbon dioxide is the product of complete combustion and is not the main hazard here. Chloromethane needs chlorine, not nitrogen. Water vapour is not poisonous.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.