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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 4 carbon atoms? → 10Prompt: Substitute n into 2n + 2.
  2. Which fraction is the most flammable? (heavy fuel oil / bitumen / diesel oil / refinery gases (shortest chains)) → 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 6 carbon atoms? → 14Prompt: 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: C₁₃H₂₈ → C₇H₁₆ + an alkene. How many hydrogen atoms are in one molecule of the alkene? → 12Prompt: Atoms are conserved: subtract the atoms in the smaller alkane.
  2. Which conditions are used for catalytic cracking? (adding acid at room temperature / dissolving the oil in water / vaporised hydrocarbons passed over a hot catalyst / freezing the oil) → vaporised hydrocarbons passed over a hot catalystPrompt: Cracking is a thermal decomposition.

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. Alkenes and cracking: which statement is NOT true? (Alkenes contain a carbon–carbon double bond. / Alkanes decolourise bromine water. / Alkenes are more reactive than alkanes. / Cracking is a thermal decomposition reaction.) → Alkanes decolourise bromine water.Prompt: Which hydrocarbons are saturated?
  2. Which conditions are used for catalytic cracking? (freezing the oil / adding acid at room temperature / dissolving the oil in water / vaporised hydrocarbons passed over a hot catalyst) → vaporised hydrocarbons passed over a hot catalystPrompt: Cracking is a thermal decomposition.

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

    Why do refineries crack long-chain hydrocarbons?

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

  2. Question 2

    Alkenes and cracking: which statement is NOT true?

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

  3. Question 3

    Cracking: C₈H₁₈ → C₃H₈ + an alkene. How many hydrogen atoms are in one molecule of the alkene?

    Answer

  4. Question 4

    Bromine water is shaken with a hydrocarbon and turns from orange to colourless. What does this show?

    • A. The bromine has evaporated
    • B. The hydrocarbon is an alkene (it has a C=C double bond)
    • C. The hydrocarbon contains oxygen
    • D. The hydrocarbon is an alkane

  5. Question 5

    Cracking: C₁₆H₃₄ → C₁₄H₃₀ + an alkene. How many carbon atoms are in one molecule of the alkene?

    Answer

  6. Question 6

    Which conditions are used for catalytic cracking?

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

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Answer key: Cracking, alkenes and the bromine water test

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

  2. 2. C. Cracking makes longer molecules from shorter ones.

  3. 3. 10

  4. 4. B. The hydrocarbon is an alkene (it has a C=C double bond)

  5. 5. 2

  6. 6. D. vaporised hydrocarbons passed over a hot catalyst