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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 NaOH. Relative atomic masses: Na = 23, O = 16, H = 1. → 40Prompt: Multiply each relative atomic mass by the number of those atoms.
  2. Calculate the relative formula mass (Mr) of ethanol, C₂H₅OH. Relative atomic masses: C = 12, H = 1, O = 16. → 46Prompt: Count the atoms of each element first, including any brackets.
  3. What is the percentage by mass of oxygen in MgO? Relative atomic masses: Mg = 24, O = 16. Give 1 decimal place. → 40 %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: CHâ‚„ + Hâ‚‚O → CO + 3Hâ‚‚. The useful product is hydrogen (Mr 2). Relative masses of reactants: CHâ‚„ = 16, Hâ‚‚O = 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. 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. → 17.6 %Prompt: atom economy = total Mr of the useful product ÷ total Mr of all reactants × 100
  2. What does a 100 % atom economy mean? (No energy is needed / All the atoms in the reactants end up in the desired product / The yield is always 100 % / The reaction is very fast) → 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. Ethanol can be made by fermentation (atom economy 51 %) or by reacting ethene with steam (100 %). Why might a company still choose fermentation? (It produces purer ethanol / It has a higher atom economy / It is faster / It uses renewable sugar crops instead of crude oil) → It uses renewable sugar crops instead of crude oilPrompt: Where does ethene come from?
  2. CaCO₃ → CaO + CO₂. The useful product is calcium oxide (Mr 56). Relative masses of reactants: CaCO₃ = 100. What is the atom economy? Give 1 decimal place. → 56 %Prompt: atom economy = total Mr of the useful product ÷ total Mr of all reactants × 100

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

    Atom economy: which statement is NOT true?

    • A. Atom economy is the same as percentage yield.
    • B. Atom economy is calculated from the balanced equation.
    • C. Green chemistry aims to use reactions with high atom economy.
    • D. Reactions with only one product have 100 % atom economy.

  2. Question 2

    Atom economy and percentage yield are different. Which statement is correct?

    • A. Atom economy is measured with a balance
    • B. They are always equal
    • C. Yield comes from the equation only
    • D. Atom economy comes from the equation; yield comes from what you actually collect

  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

    What does a 100 % atom economy mean?

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

  5. Question 5

    Fe₂O₃ + 3CO → 2Fe + 3CO₂. The useful product is iron (Ar 56). Relative masses of reactants: Fe₂O₃ = 160, CO = 28. What is the atom economy? Give 1 decimal place.

    Answer%

  6. Question 6

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

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Answer key: Atom economy and greener reactions

  1. 1. A. Atom economy is the same as percentage yield.

  2. 2. D. Atom economy comes from the equation; yield comes from what you actually collect

  3. 3. 98.6 %

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

  5. 5. 45.9 %

  6. 6. A. It uses renewable sugar crops instead of crude oil