Limiting reactant — Science definition

The reactant that runs out first, capping the amount of product a reaction can make.

Where it is taught

In use

The reactant that runs out first is the limiting reactant, because it limits how much product can form.

Limiting Reactants & Percent Yield

Questions using limiting reactant

Problem 1

The limiting reactant in a chemical reaction is the reactant that…
  1. has the largest molar mass
  2. is added in the greatest amount
  3. runs out first and limits how much product forms
  4. is always left over at the end
Answer and explanation

runs out first and limits how much product forms

The limiting reactant is used up first, setting the ceiling on how much product can be made.

Problem 2

For 2H₂ + O₂ → 2H₂O, you have 4 mol of H₂ and 4 mol of O₂. Which is the limiting reactant?
  1. H₂
  2. O₂
  3. both are limiting
  4. neither — they are in perfect ratio
Answer and explanation

H₂

The ratio needs 2 H₂ per 1 O₂. 4 mol H₂ requires only 2 mol O₂ (so O₂ is in excess); 4 mol O₂ would need 8 mol H₂ (not available). H₂ runs out first, so H₂ is limiting.

Problem 3

For N₂+3H₂→2NH₃, begin with 1.50 mol N₂ and 3.00 mol H₂. Find limiting reactant, theoretical NH₃, remaining N₂ and yield if 1.60 mol NH₃ is isolated.
Answer and explanation

H₂ limits. It uses 1.00 mol N₂, forms 2.00 mol NH₃ and leaves 0.50 mol N₂ in the complete model. Isolated yield is 1.60/2.00×100=80.0%; that number alone does not identify the cause of lost target.

H₂ limits. It uses 1.00 mol N₂, forms 2.00 mol NH₃ and leaves 0.50 mol N₂ in the complete model. Isolated yield is 1.60/2.00×100=80.0%; that number alone does not identify the cause of lost target.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 4

Explain the following claim and supply a valid example or counterexample: The smaller initial mass is not necessarily the limiting reactant.
Answer and explanation

H₂ limits; it supports 2molNH₃ and consumes 1molN₂, leaving 1molN₂. Compare potential products, not raw amounts. The smaller initial mass is not necessarily the limiting reactant.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

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