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Science lesson plan · 11th grade (Year 12 in England) · 30 minutes

Atom economy and greener reactions

Chemical reactions, acids and bases

CCSS NGSS.HS-PS1-7CCSS NGSS.HS-ETS1-3England: GCSE Chemistry: Quantitative chemistry (atom economy)

Objective and success criteria

Students will: Atom economy and greener reactions.

  • 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: Relative formula mass

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 (Relative formula mass). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Calculate the relative formula mass (Mr) of NH₃. Relative atomic masses: N = 14, H = 1. → 17Prompt: Multiply each relative atomic mass by the number of those atoms.
  2. Calculate the relative formula mass (Mr) of aluminium (aluminum) oxide, Al₂O₃. Relative atomic masses: Al = 27, O = 16. → 102Prompt: Count the atoms of each element first, including any brackets.
  3. What is the percentage by mass of hydrogen in H₂SO₄? Relative atomic masses: H = 1, S = 32, O = 16. Give 1 decimal place. → 2 %Prompt: Work out Mr first.

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: Atom economy and greener reactions. 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: CH₄ + H₂O → CO + 3H₂. The useful product is hydrogen (Mr 2). Relative masses of reactants: CH₄ = 16, H₂O = 18. What is the atom economy? Give 1 decimal place.
  • Step 1: useful product: 6; reactants: 34
  • Step 2: 6 ÷ 34 × 100 = 17.65
  • Step 3: So the answer is: 17.6 %
  • Answer: 17.6 %
  • 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. Zn + 2HCl → ZnCl₂ + H₂. The useful product is zinc chloride (Mr 136). Relative masses of reactants: Zn = 65, HCl = 36.5. What is the atom economy? Give 1 decimal place. → 98.6 %Prompt: atom economy = total Mr of the useful product ÷ total Mr of all reactants × 100
  2. What does a 100 % atom economy mean? (The reaction is very fast / The yield is always 100 % / No energy is needed / All the atoms in the reactants end up in the desired product) → All the atoms in the reactants end up in the desired productPrompt: Is there a waste product?

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. Atom economy: which statement is NOT true? (Reactions with only one product have 100 % atom economy. / Green chemistry aims to use reactions with high atom economy. / Selling or using by-products makes a process less wasteful. / A low atom economy means the reaction is fast.) → A low atom economy means the reaction is fast.Prompt: How is atom economy different from yield?
  2. Why does a low atom economy matter for sustainability? (It removes pollution / More resources are turned into waste products / It increases the yield / It makes products cheaper) → More resources are turned into waste productsPrompt: Where do the other atoms go?

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: Relative formula mass.
  • 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-11.

Free lesson plan from eurekami.com/lesson-plan-builder

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Atom economy and greener reactions

Name

Date

Score/ 6

  1. Question 1

    What does a 100 % atom economy mean?

    • A. All the atoms in the reactants end up in the desired product
    • B. The yield is always 100 %
    • C. The reaction is very fast
    • D. No energy is needed

  2. Question 2

    Atom economy: which statement is NOT true?

    • A. Atom economy is calculated from the balanced equation.
    • B. Atom economy is the same as percentage yield.
    • C. Reactions with only one product have 100 % atom economy.
    • D. Selling or using by-products makes a process less wasteful.

  3. Question 3

    Zn + 2HCl → ZnCl₂ + H₂. The useful product is zinc chloride (Mr 136). Relative masses of reactants: Zn = 65, HCl = 36.5. What is the atom economy? Give 1 decimal place.

    Answer%

  4. Question 4

    Ethanol can be made by fermentation (atom economy 51 %) or by reacting ethene with steam (100 %). Why might a company still choose fermentation?

    • A. It produces purer ethanol
    • B. It has a higher atom economy
    • C. It uses renewable sugar crops instead of crude oil
    • D. It is faster

  5. Question 5

    C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The useful product is ethanol (Mr 46). Relative masses of reactants: C₆H₁₂O₆ = 180. What is the atom economy? Give 1 decimal place.

    Answer%

  6. Question 6

    Why does a low atom economy matter for sustainability?

    • A. More resources are turned into waste products
    • B. It increases the yield
    • C. It makes products cheaper
    • D. It removes pollution

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Answer key: Atom economy and greener reactions

  1. 1. A. All the atoms in the reactants end up in the desired product

  2. 2. B. Atom economy is the same as percentage yield.

  3. 3. 98.6 %

  4. 4. C. It uses renewable sugar crops instead of crude oil

  5. 5. 51.1 %

  6. 6. A. More resources are turned into waste products