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Science lesson plan · 12th grade (Year 13 in England) · 45 minutes

Find the limiting reactant and how much product forms

Chemical reactions, acids and bases

CCSS NGSS.HS-PS1-7England: GCSE Chemistry: Quantitative chemistry (limiting reactants, Higher)

Objective and success criteria

Students will: Find the limiting reactant and how much product forms.

  • 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: Moles and reacting masses; Percentage yield

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 · 5 min

  • Quick retrieval from earlier learning (Moles and reacting masses; Percentage yield). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. How many moles are in 120 g of magnesium oxide (MgO, Mr = 40)? → 3 molPrompt: moles = mass ÷ Mr
  2. A reaction could make 64 g of product in theory, but 38.4 g was actually collected. What is the percentage yield? → 60 %Prompt: percentage yield = actual mass ÷ theoretical mass × 100
  3. What is the mass of 2 mol of calcium carbonate (CaCO₃, Mr = 100), in g? → 200 gPrompt: mass = moles × Mr

Hook · 5 min

  • Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
  • Share the objective: Find the limiting reactant and how much product forms. Read the success criteria together.

I do: model it · 10 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: CHâ‚„ + 2Oâ‚‚ → COâ‚‚ + 2Hâ‚‚O. 1 mol of methane is mixed with 1 mol of oxygen. Which is the limiting reactant?
  • Step 1: methane: 1 ÷ 1 = 1; oxygen: 1 ÷ 2 = 0.5
  • Step 2: So the answer is: oxygen
  • Answer: oxygen
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 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. Mg + 2HCl → MgCl₂ + H₂. 4 mol of magnesium is mixed with 5 mol of hydrochloric acid. Which is the limiting reactant? (magnesium / hydrochloric acid) → hydrochloric acidPrompt: Divide each amount by its number in the equation.
  2. CH₄ + 2O₂ → CO₂ + 2H₂O. 2 mol of methane reacts with 9 mol of oxygen. What is the maximum amount of carbon dioxide that can form, in mol? Give 2 decimal places if needed. → 2 molPrompt: Find the limiting reactant first.

You do: independent practice · 10 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 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. Mg + 2HCl → MgCl₂ + H₂. 3 mol of magnesium is mixed with 4 mol of hydrochloric acid. Which is the limiting reactant? (magnesium / hydrochloric acid) → hydrochloric acidPrompt: Divide each amount by its number in the equation.
  2. N₂ + 3H₂ → 2NH₃. 5 mol of nitrogen reacts with 4 mol of hydrogen. What is the maximum amount of ammonia that can form, in mol? Give 2 decimal places if needed. → 2.67 molPrompt: Find the limiting reactant first.

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: Moles and reacting masses.
  • 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-12.

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

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Find the limiting reactant and how much product forms

Name

Date

Score/ 8

  1. Question 1

    N₂ + 3H₂ → 2NH₃. 5 mol of nitrogen reacts with 8 mol of hydrogen. What is the maximum amount of ammonia that can form, in mol? Give 2 decimal places if needed.

    Answermol

  2. Question 2

    In a car engine with too little air, which reactant is limiting and what is the risk?

    • A. nothing is limiting
    • B. nitrogen is limiting, so no reaction happens
    • C. oxygen is limiting, so incomplete combustion makes carbon monoxide
    • D. fuel is limiting, so too much oxygen forms

  3. Question 3

    2H₂ + O₂ → 2H₂O. 2 mol of hydrogen is mixed with 5 mol of oxygen. Which is the limiting reactant?

    • A. hydrogen
    • B. oxygen

  4. Question 4

    2H₂ + O₂ → 2H₂O. 2 mol of hydrogen reacts with 5 mol of oxygen. What is the maximum amount of water that can form, in mol? Give 2 decimal places if needed.

    Answermol

  5. Question 5

    Why is one reactant often used in excess in industry?

    • A. To make the product impure on purpose
    • B. To slow the reaction down
    • C. To make sure all of the more expensive reactant reacts
    • D. Because excess reactants are free

  6. Question 6

    N₂ + 3H₂ → 2NH₃. 4 mol of nitrogen is mixed with 8 mol of hydrogen. Which is the limiting reactant?

    • A. nitrogen
    • B. hydrogen

  7. Question 7

    2Na + Cl₂ → 2NaCl. 4 mol of sodium reacts with 3 mol of chlorine. What is the maximum amount of sodium chloride that can form, in mol? Give 2 decimal places if needed.

    Answermol

  8. Question 8

    Doubling the amount of the reactant in excess will…

    • A. stop the reaction
    • B. halve the product
    • C. double the product
    • D. not change the amount of product

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Answer key: Find the limiting reactant and how much product forms

  1. 1. 5.33 mol

  2. 2. C. oxygen is limiting, so incomplete combustion makes carbon monoxide

  3. 3. A. hydrogen

  4. 4. 2 mol

  5. 5. C. To make sure all of the more expensive reactant reacts

  6. 6. B. hydrogen

  7. 7. 4 mol

  8. 8. D. not change the amount of product