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Science lesson plan · 10th grade (Year 11 in England) · 30 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 · 4 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.
  1. Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 10 carbon atoms? → 22Prompt: Substitute n into 2n + 2.
  2. Which fraction is the most flammable? (diesel oil / heavy fuel oil / refinery gases (shortest chains) / bitumen) → refinery gases (shortest chains)Prompt: Short-chain hydrocarbons ignite most easily.
  3. Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 8 carbon atoms? → 18Prompt: Substitute n into 2n + 2.

Hook · 3 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 · 7 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 · 6 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. Cracking: C₁₅H₃₂ → C₁₂H₂₆ + an alkene. How many carbon atoms are in one molecule of the alkene? → 3Prompt: Atoms are conserved: subtract the atoms in the smaller alkane.
  2. Why do refineries crack long-chain hydrocarbons? (There is more demand for shorter chains such as petrol, and alkenes make polymers / To remove all the carbon / To make crude oil / Long chains are too useful) → There is more demand for shorter chains such as petrol, and alkenes make polymersPrompt: Compare supply and demand of fractions.

You do: independent practice · 7 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 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. Why do refineries crack long-chain hydrocarbons? (There is more demand for shorter chains such as petrol, and alkenes make polymers / To remove all the carbon / Long chains are too useful / To make crude oil) → There is more demand for shorter chains such as petrol, and alkenes make polymersPrompt: Compare supply and demand of fractions.
  2. Alkenes and cracking: which statement is NOT true? (Ethene can be used to make the polymer poly(ethene). / Alkenes contain a carbon–carbon double bond. / Methene is the smallest alkene. / Alkenes are more reactive than alkanes.) → Methene is the smallest alkene.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

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Cracking, alkenes and the bromine water test

Name

Date

Score/ 6

  1. Question 1

    Cracking: C₁₃H₂₈ → C₈H₁₈ + an alkene. How many carbon atoms are in one molecule of the alkene?

    Answer

  2. Question 2

    Why do refineries crack long-chain hydrocarbons?

    • A. To make crude oil
    • B. To remove all the carbon
    • C. Long chains are too useful
    • D. There is more demand for shorter chains such as petrol, and alkenes make polymers

  3. Question 3

    Alkenes and cracking: which statement is NOT true?

    • A. Alkenes are more reactive than alkanes.
    • B. Cracking makes longer molecules from shorter ones.
    • C. Alkenes contain a carbon–carbon double bond.
    • D. Cracking is a thermal decomposition reaction.

  4. Question 4

    Cracking: C₁₂H₂₆ → C₆H₁₄ + an alkene. How many carbon atoms are in one molecule of the alkene?

    Answer

  5. Question 5

    Which conditions are used for catalytic cracking?

    • A. adding acid at room temperature
    • B. dissolving the oil in water
    • C. vaporised hydrocarbons passed over a hot catalyst
    • D. freezing the oil

  6. Question 6

    Cracking: C₉H₂₀ → C₃H₈ + an alkene. How many hydrogen atoms are in one molecule of the alkene?

    Answer

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Answer key: Cracking, alkenes and the bromine water test

  1. 1. 5

  2. 2. D. There is more demand for shorter chains such as petrol, and alkenes make polymers

  3. 3. B. Cracking makes longer molecules from shorter ones.

  4. 4. 6

  5. 5. C. vaporised hydrocarbons passed over a hot catalyst

  6. 6. 12