The shape of the lesson
By the end of the lesson, learners can
- Testing for unsaturation: Add bromine to ethene in the simulator and watch the double bond open.
- Addition reactions: Extended.
- Alkenes: CnH2n, unsaturated, C=C.
- Test: aqueous bromine, orange to colourless.
- Extended, hydrogen: nickel catalyst, gives an alkane.
- Extended, steam: H3PO4 catalyst, 300 °C, 6000 kPa, gives an alcohol.
One picture every idea returns to
Alkenes are unsaturated hydrocarbons with the general formula CnH2n and a C=C double bond as the functional group. To tell an alkene from an alkane, shake it with aqueous bromine: an alkene turns it from orange to colourless, an alkane leaves it orange. (Extended) Alkenes undergo addition reactions, in which the double bond opens and two atoms or groups add on, giving a single product: with bromine (ethene → 1,2-dibromoethane), with hydrogen using a nickel catalyst (ethene → ethane), and with steam using an acid catalyst (ethene → ethanol).
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, The double bond that opens.. Alkenes carry a C=C double bond, and that one feature makes them far more reactive than alkanes. 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: Testing for unsaturation | Add bromine to ethene in the simulator and watch the double bond open. Project tab 4 of the lab (Hydrocarbons lab, addition of bromine) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Addition reactions | Extended. Project tab 5 of the lab (Hydrocarbons lab, alkene reactions) and let a student drive it while the class predicts. | Jot Thoughts |
| 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, 15 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 Addition reactions, 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.
Jot Thoughts
Teams write ideas on slips, one idea per slip, as many as they can in two minutes, saying each aloud as they place it in the middle. Then they sort the slips, which is where the structure of the topic appears.
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 |
|---|---|
| An addition reaction gives one product only: there is no HBr or H 2 O left over, unlike substitution. | Learn the catalysts as pairs: hydrogen with nickel, steam with an acid (phosphoric acid). And the bromine test result is orange to colourless, never "clear": clear means transparent, and orange bromine water is already clear. |
Alkenes: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Describe a test to show that ethene is unsaturated but ethane is not. Give the reagent and both results.
Give the molecular formula and structural formula of propene.
(Extended) Write an equation for the reaction of ethene with bromine and name the product.
(Extended) State the conditions used to make ethanol from ethene and write the equation.
(Extended) But-1-ene, CH2=CHCH2CH3, reacts with hydrogen. Name the catalyst, give the structural formula of the product and name it.
(Extended) Explain why the reaction of ethene with bromine is called an addition reaction, while the reaction of ethane with chlorine is a substitution reaction.
Answer key and marking notes
- Shake each gas with aqueous bromine (bromine water).
- Ethene: orange to colourless.
- Ethane: stays orange.
- C3H6.
- CH3CH=CH2.
- CH2=CH2 + Br2 → CH2BrCH2Br.
- 1,2-dibromoethane.
- Steam with an acid catalyst (phosphoric acid).
- 300 °C and 6000 kPa.
- C2H4 + H2O → C2H5OH.
- Nickel catalyst.
- CH3CH2CH2CH3.
- Butane.
- Ethene: the C=C opens and both bromine atoms add on, giving a single product.
- Ethane: a hydrogen atom is replaced by a chlorine atom, so two products form (chloroethane and hydrogen chloride).
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.
A hydrocarbon is shaken with bromine water. The bromine water turns from orange to colourless. What does this show?
- Athe hydrocarbon is unsaturated
- Bthe hydrocarbon is saturated
- Cthe hydrocarbon is an alcohol
- Dthe hydrocarbon has been completely burned
What is the product when ethene reacts with bromine?
- ACH2BrCH2Br
- BCH2=CHBr + HBr
- CCH3CH2Br
- DCH3CH3 + Br2
Ethene reacts with hydrogen to form ethane. Which catalyst is used?
- Anickel
- Biron
- Cphosphoric acid
- Dvanadium(V) oxide
Which reaction makes ethanol from ethene?
- Aaddition of steam
- Baddition of hydrogen
- Csubstitution with chlorine
- Dfermentation
Which statement explains why alkenes are more reactive than alkanes?
- Aalkenes have a C=C double bond that can open up so other atoms add on
- Balkenes have more hydrogen atoms than alkanes
- Calkenes are ionic
- Dalkenes have longer carbon chains than alkanes
Which conditions are used in industry to make ethanol from ethene and steam?
- A300 °C, 6000 kPa, phosphoric acid catalyst
- B30 °C, atmospheric pressure, yeast
- C450 °C, 200 kPa, vanadium(V) oxide catalyst
- D150 °C, nickel catalyst
Propene, CH2=CHCH3, reacts with bromine. What is the name of the product?
- A1,2-dibromopropane
- B1-bromopropane
- C1,3-dibromopropane
- D2-bromopropene
Steam is added to propene, CH2=CHCH3. Why are two different alcohols formed?
- Athe −OH can add to either carbon of the double bond, giving propan-1-ol or propan-2-ol
- Bpropene contains two double bonds
- Csome propene is substituted and some undergoes addition
- Done product is an alcohol and the other is a carboxylic acid
Quiz key
Bromine adds across a C=C double bond, so the orange colour disappears. This shows the hydrocarbon is unsaturated (an alkene). A saturated alkane leaves the bromine water orange. An alcohol is not a hydrocarbon, and the test does not involve burning.
The C=C opens and one Br atom adds to each carbon: CH2=CH2 + Br2 → CH2BrCH2Br (1,2-dibromoethane). Forming HBr would be substitution, which is how alkanes react. CH3CH2Br is the product with hydrogen bromide, not bromine. Ethane is made by adding hydrogen, not bromine.
Hydrogen adds across the C=C bond in the presence of a nickel catalyst (at about 150 °C). Iron catalyses the Haber process. Phosphoric acid catalyses the addition of steam to ethene. Vanadium(V) oxide catalyses the Contact process.
Steam (H2O) adds across the C=C bond: C2H4 + H2O → C2H5OH. Adding hydrogen gives ethane. Substitution with chlorine is an alkane reaction. Fermentation also makes ethanol, but from sugar, not from ethene.
The C=C double bond in an alkene is the reactive site: it can open and allow atoms to add (addition reactions). An alkene has fewer hydrogens than the alkane with the same number of carbons, not more. Alkenes are covalent. Chain length is not the difference, as ethene and ethane both have two carbons.
Ethene and steam are passed over a phosphoric acid catalyst at about 300 °C and 6000 kPa. Yeast at about 30 °C is fermentation of sugars. 450 °C and V2O5 are Contact process conditions. A nickel catalyst at 150 °C is for adding hydrogen to alkenes.
The C=C is between carbons 1 and 2, so one Br adds to each of these carbons: CH2BrCHBrCH3, 1,2-dibromopropane. 1-bromopropane has only one Br and would come from substitution or from HBr. 1,3-dibromopropane puts Br on a carbon that was not part of the double bond. 2-bromopropene still has a C=C, which would mean substitution, not addition.
The double bond is between carbons 1 and 2. If the −OH adds to carbon 1 the product is propan-1-ol; if it adds to carbon 2 it is propan-2-ol. These are structural isomers. Propene has only one C=C bond. Steam adds to alkenes; it does not substitute. Adding H2O cannot form a carboxylic acid, which would need an extra oxygen atom.
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.