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.
- What is the molecular formula of methane, the alkane with 1 carbon atom? (CH₂ / CH₄ / CH₃ / C₂H₄) → CH₄Prompt: Use CₙH₂ₙ₊₂.
- Why is crude oil called a finite (non-renewable) resource? (It never runs out / It is made in factories / It is made from sunlight each day / It formed over millions of years and is used much faster than it forms) → It formed over millions of years and is used much faster than it formsPrompt: Crude oil came from ancient plankton buried in mud.
- What is the molecular formula of ethane, the alkane with 2 carbon atoms? (C₂H₄ / C₂H₅ / C₂H₆ / C₃H₆) → C₂H₆Prompt: 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: C₁₀H₂₂ → C₈H₁₈ + an alkene. How many carbon atoms are in one molecule of the alkene? → 2Prompt: Atoms are conserved: subtract the atoms in the smaller alkane.
- Why do refineries crack long-chain hydrocarbons? (There is more demand for shorter chains such as petrol, and alkenes make polymers / To make crude oil / Long chains are too useful / To remove all the carbon) → There is more demand for shorter chains such as petrol, and alkenes make polymersPrompt: Compare supply and demand of fractions.
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.
- Cracking: C₁₅H₃₂ → C₁₃H₂₈ + an alkene. How many hydrogen atoms are in one molecule of the alkene? → 4Prompt: Atoms are conserved: subtract the atoms in the smaller alkane.
- What is the general formula of the alkenes? (CₙH₂ₙ₊₂ / CₙHₙ / CₙH₂ₙ₋₂ / CₙH₂ₙ) → CₙH₂ₙPrompt: They have two fewer hydrogen atoms than the alkane with the same carbons.
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