DSE Chemistry | Alkanes and alkenes lesson plan (for tutors)

DSE Chemistry | Alkanes and alkenes lesson plan (for tutors) | Isomers and addition products

2026-11-03

Tutor lesson-prep guides · Secondary Chemistry

Students can draw ethene but fall apart when asked how many isomers butane has. This lesson uses drawing and counting as the backbone and links formula, structure and reaction.

Objectives — Students can: (1) draw every structural isomer from a molecular formula (C4H10 has 2, C5H12 has 3); (2) state that alkanes only substitute while alkenes add, and separate them with bromine water; (3) compute that 0.10 mol of ethene gives 18.79 g of dibromoethane (M = 187.9 g per mol) and that the ethene occupies 2.4 dm3 at room temperature.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Moles and mass	Mass of 0.10 mol when M is 187.9?	18.79 g
Room-temperature molar volume	Volume of 0.10 mol of gas?	About 2.4 dm3
Single and double bonds	Can a double bond take more atoms?	Yes, it opens to a single bond

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Formula opener	Give C4H10 and ask how many different structures exist	They draw 2
10-24 min	Systematic isomer drawing	Longest chain first, then shorten and branch	C5H12 gives 3
24-38 min	Alkane versus alkene	CₙH₂ₙ₊₂ against CₙH₂ₙ, substitution against addition	State the difference with one example each
38-48 min	Test and addition calculation	Bromine water decolourises; 0.10 mol adds bromine	Get 18.79 g
48-56 min	Combustion and uses	Propane needs 5 mol of oxygen; fuels and plastics	Balance the equation
56-60 min	Exit check	Mark on the spot	Three correct

03 Teaching script (read it out)

Scene 1: Isomers come from a procedure, not inspiration

C4H10: draw four carbons in a chain (butane), then shorten the main chain to three and hang one carbon on it (2-methylpropane); that is 2. For C5H12: straight chain, one branch in two positions, and two branches, giving 3. Shorten the chain each time and fill branch positions left to right, deleting repeats. The same procedure extends to C6.

Ask them to: apply the same procedure to C6H14 (five isomers).

Scene 2: One sentence separates the two families

Alkanes are saturated with single bonds and only substitute under light. Alkenes carry a double bond and add. Test: add bromine water, orange-brown; an alkene decolourises it, an alkane does not. Write “decolourised” and “addition”, never just “reacts”.

Ask them: why does the alkene decolourise bromine water and the alkane does not?

Scene 3: From equation to mass and volume

CH2=CH2 + Br2 gives CH2BrCH2Br in a 1:1 ratio. 0.10 mol of ethene gives 0.10 mol of product; with M = 187.9 g per mol the mass is 18.79 g, and the ethene itself occupies about 2.4 dm3 at room temperature. Combustion: C3H8 + 5O2 gives 3CO2 + 4H2O, balanced in the order carbon, hydrogen, oxygen.

Remind: addition takes one Br2 unless the question says excess.

04 Question types and worked steps

Type	Example	Steps	Answer
Drawing	All isomers from a formula	Shorten the main chain	C4H10 has 2
Identification	Chemical test for alkane versus alkene	Bromine water	Alkene decolourises it
Calculation	Mass of addition product	1:1 ratio, then molar mass	18.79 g
Equation	Balancing complete combustion	Carbon, hydrogen, oxygen	Propane needs 5 mol of oxygen

Sanity check (teach this): M(C2H4Br2) = 2 x 12.01 + 4 x 1.008 + 2 x 79.90 = 187.9 g per mol. The formula CₙH₂ₙ only holds for an open-chain alkene with one double bond; cycloalkanes share the formula but not the chemistry, so say “open-chain alkene with one double bond” in answers.

05 Independent practice (3 questions)

Question	Answer	What to watch
Mass of product from 0.10 mol of ethene	18.79 g	One to one mole ratio
Number of structural isomers of C5H12	3	Chains of five, four and three
Reagent to tell hexane from hexene	Bromine water	The alkene decolourises it

06 Common mistakes and what to say

Mistake	Student writes	Say this
Double counting isomers	Answering 3 for C4H10	“Reversing 2-methylpropane gives the same molecule; count from one end.”
Calling addition a substitution	Writing that ethene substitutes bromine	“The double bond opens and adds atoms: addition.”
Using the reactant molar mass	Using 28.1 for the product	“Two bromines were added, so the product must be heavier.”
Unbalanced combustion	Writing 3O2 for propane	“Count three carbons and eight hydrogens, so oxygen is 3 + 2 = 5.”

07 Homework (paper, section and question numbers)

DSE chemistry practice paper (organic chemistry) — Section B structure and addition calculations
Secondary chemistry chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Number of structural isomers of C4H10	2	All three right: move to alcohols
Mass of product from 0.10 mol of ethene	18.79 g	One wrong: reteach the mole ratio
Reagent to separate alkane from alkene	Bromine water	Two wrong: restart act one

09 Tutor FAQ

Must the number of isomers be memorised?

No; use the shorten-the-chain procedure and the count follows.

Alkenes barely dissolve in water, so how does bromine water work?

The reaction happens at the interface or with dissolved bromine; solubility is not the barrier.

Do more branches raise or lower the boiling point?

Lower, because branching reduces contact area and weakens van der Waals forces.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.