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Grade 9 · lesson 4 of 4

Find the Limiting Reactant in a Particle Model

Count complete reaction sets and leftover particles.

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Supplied materials

  • Use the supplied fictional observations or model. Paper and pencil are optional; spoken answers are welcome. No equipment or real experiment is needed.

Learn the idea

A balanced equation specifies what one complete reaction set uses. A reactant can be plentiful in total count yet still limit the number of sets because each set needs several of its particles. Compare each available amount with its required coefficient. The smallest possible number of complete sets controls the maximum product in this simplified model.

Use N₂ + 3 H₂ → 2 NH₃ with a fictional packet containing 4 N₂ molecules and 9 H₂ molecules. Assume all possible complete sets react and no other reactions occur. N₂ could supply four sets, but H₂ supplies only 9/3 = three. Three sets consume 3 N₂ and 9 H₂, make 6 NH₃, and leave 1 N₂. This is a paper particle model, not an instruction to obtain gases.

  1. List the molecule requirements for one complete set.
  2. Calculate the possible sets from each reactant.
  3. Choose the smaller set count and multiply by the product coefficient.
  4. Calculate leftovers and verify N and H atom counts including leftovers.
Worked example

See the method in action

Three sets form 6 NH₃ and leave 1 N₂. Before: 8 N atoms and 18 H atoms. After: 6 N and 18 H in NH₃ plus 2 N in leftover N₂, preserving both element counts.

Your turn

Check your reasoning

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If a new packet has 2 N₂ and 9 H₂, what is the complete-set result?

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See the answer and explanation

4 NH₃ and 3 H₂ left

N₂ limits the packet to two sets. They use 6 H₂ and make 4 NH₃, leaving 9 − 6 = 3 H₂.

Take it further

Change the packet to 5 N₂ and 12 H₂. H₂ supplies four sets, so 8 NH₃ form and 1 N₂ remains. Atom check: 10 N and 24 H before; 8 N and 24 H in product plus 2 N leftover.

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