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Science lesson plan · 10th grade (Year 11 in England) · 30 minutes

Predict the products of electrolysis

Chemical reactions, acids and bases

CCSS NGSS.HS-PS1-2England: GCSE Combined Science: Chemical changes (electrolysis)

Objective and success criteria

Students will: Predict the products of electrolysis.

  • 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: Build ionic formulae from ion charges; The reactivity series and displacement

Leads to: Oxidation, reduction and half-equations

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 (Build ionic formulae from ion charges; The reactivity series and displacement). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. What is the formula of calcium hydroxide? (Ions: Ca²⁺ and OH⁻) (CaOH₂ / Ca(OH)₂ / Ca(OH)₃ / Ca₂(OH)₂) → Ca(OH)₂Prompt: The total positive charge must equal the total negative charge.
  2. In CaS (calcium sulfide), how many sulfide ions are there for each formula unit? → 1Prompt: Look at the small number after the symbol or bracket.
  3. What is the formula of magnesium sulfate? (Ions: Mg²⁺ and SO₄²⁻) (Mg(SO₄)₂ / Mg₂SO₄ / MgSO₄) → MgSO₄Prompt: The total positive charge must equal the total negative charge.

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: Predict the products of electrolysis. 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: Electrolysis of copper(II) chloride solution: what forms at the cathode (negative electrode)?
  • Step 1: Solution: compare the metal with hydrogen in the reactivity series.
  • Step 2: So the answer is: copper
  • Answer: copper
  • 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. Electrolysis of molten lead bromide: what forms at the cathode (negative electrode)? (copper / lead / silver / zinc) → leadPrompt: Positive ions are attracted to the negative cathode.
  2. Electrolysis of aluminium oxide dissolved in molten cryolite: what forms at the anode (positive electrode)? (oxygen / sulfur / chlorine / bromine) → oxygenPrompt: Negative ions are attracted to the positive anode.

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. Electrolysis of potassium bromide solution: what forms at the cathode (negative electrode)? (lead / zinc / copper / hydrogen) → hydrogenPrompt: Positive ions are attracted to the negative cathode.
  2. Electrolysis of water using renewable electricity makes "green hydrogen". Why is it seen as a low-carbon fuel? (Hydrogen is green in colour / It is made from coal / It absorbs carbon dioxide as it burns / Burning hydrogen produces only water, and the electricity came from renewable sources) → Burning hydrogen produces only water, and the electricity came from renewable sourcesPrompt: What is made when hydrogen burns?

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: Build ionic formulae from ion charges.
  • 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

  • Move on to: Oxidation, reduction and half-equations.
  • 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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Predict the products of electrolysis

Name

Date

Score/ 6

  1. Question 1

    Electrolysis of molten sodium chloride: what forms at the anode (positive electrode)?

    • A. chlorine
    • B. sulfur
    • C. oxygen
    • D. hydrogen

  2. Question 2

    Electrolysis of molten lead bromide: what forms at the cathode (negative electrode)?

    • A. silver
    • B. zinc
    • C. lead
    • D. sodium

  3. Question 3

    During electrolysis, what happens to positive ions at the cathode?

    • A. They lose electrons (are oxidised)
    • B. They gain electrons (are reduced)
    • C. Nothing
    • D. They become more positive

  4. Question 4

    Electrolysis of aluminium oxide dissolved in molten cryolite: what forms at the anode (positive electrode)?

    • A. chlorine
    • B. bromine
    • C. hydrogen
    • D. oxygen

  5. Question 5

    Electrolysis of sodium sulfate solution: what forms at the cathode (negative electrode)?

    • A. silver
    • B. hydrogen
    • C. zinc
    • D. aluminium

  6. Question 6

    Why is extracting aluminium by electrolysis so expensive?

    • A. Aluminium is less reactive than carbon
    • B. The electrodes are made of gold
    • C. It needs huge amounts of electricity to melt the ore and drive the reaction
    • D. Aluminium ore is very rare

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Answer key: Predict the products of electrolysis

  1. 1. A. chlorine

  2. 2. C. lead

  3. 3. B. They gain electrons (are reduced)

  4. 4. D. oxygen

  5. 5. B. hydrogen

  6. 6. C. It needs huge amounts of electricity to melt the ore and drive the reaction