Science lesson plan · 10th grade (Year 11 in England) · 45 minutes
Cracking, alkenes and the bromine water test
Chemical reactions, acids and bases
CCSS NGSS.HS-PS1-2England: GCSE Combined Science: Organic chemistry
Objective and success criteria
Students will: Cracking, alkenes and the bromine water test.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Alkanes and the fractions of crude oil
Leads to: the next skill in this area.
Materials: Teacher demo or video, Indicator color chart, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Alkanes and the fractions of crude oil). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 11 carbon atoms? → 24Prompt: Substitute n into 2n + 2.
- Which fraction is the most flammable? (diesel oil / bitumen / heavy fuel oil / refinery gases (shortest chains)) → refinery gases (shortest chains)Prompt: Short-chain hydrocarbons ignite most easily.
- What is the molecular formula of propane, the alkane with 3 carbon atoms? (C₃H₇ / C₃H₈ / C₃H₅ / C₄H₈) → C₃H₈Prompt: Use CₙH₂ₙ₊₂.
Hook · 5 min
- Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
- Share the objective: Cracking, alkenes and the bromine water test. Read the success criteria together.
I do: model it · 10 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: Cracking: C₁₁H₂₄ → C₅H₁₂ + an alkene. How many hydrogen atoms are in one molecule of the alkene?
- Step 1: C: 11 − 5 = 6
- Step 2: H: 24 − 12 = 12
- Step 3: The alkene is C₆H₁₂.
- Step 4: So the answer is: 12
- Answer: 12
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- Cracking: C₁₅H₃₂ → C₉H₂₀ + an alkene. How many hydrogen atoms are in one molecule of the alkene? → 12Prompt: Atoms are conserved: subtract the atoms in the smaller alkane.
- What is the general formula of the alkenes? (CₙH₂ₙ₋₂ / CₙHₙ / CₙH₂ₙ / CₙH₂ₙ₊₂) → CₙH₂ₙPrompt: They have two fewer hydrogen atoms than the alkane with the same carbons.
You do: independent practice · 10 min
- Students work on the student sheet. Circulate and check the first two answers of every student early.
- Students who finish: write a question of their own for a partner, with the answer.
The 8 questions are on the student sheet below, with answers in the key.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Bromine water is shaken with a hydrocarbon and turns from orange to colourless. What does this show? (The bromine has evaporated / The hydrocarbon is an alkene (it has a C=C double bond) / The hydrocarbon contains oxygen / The hydrocarbon is an alkane) → The hydrocarbon is an alkene (it has a C=C double bond)Prompt: Alkenes are unsaturated.
- Alkenes and cracking: which statement is NOT true? (Cracking is a thermal decomposition reaction. / Ethene can be used to make the polymer poly(ethene). / Alkenes are more reactive than alkanes. / Alkanes decolourise bromine water.) → Alkanes decolourise bromine water.Prompt: Which hydrocarbons are saturated?
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Alkanes and the fractions of crude oil.
- Use concrete materials: teacher demo or video.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-10.
Free lesson plan from eurekami.com/lesson-plan-builder