Limiting reactant
The limiting reactant is consumed first according to stoichiometric proportions. It sets the maximum amount of product.
The essential idea
The limiting reactant is consumed first according to stoichiometric proportions. It sets the maximum amount of product.
Example
With 3 mol H₂ and 1 mol O₂ for 2 H₂ + O₂ → 2 H₂O, O₂ is limiting: it consumes 2 mol H₂ and leaves 1 mol H₂ in excess.
Mini problem
Why isn't the reactant present in the smaller amount always limiting?
Show the solution
Answer: Because available amounts must be compared with the ratios required by the balanced equation.
Explanation: Never choose the limiting reactant by simply comparing the available mole counts.
Method: Connect the definition to the mechanism described in the lesson: identify what changes and what remains fixed before drawing the conclusion.
Common mistake
Do not replace reasoning with a memorized rule. Always check geometry, units, coefficients or model assumptions.
Guided exercise
With 5 mol H₂ and 2 mol O₂, which reactant limits 2 H₂ + O₂ → 2 H₂O?
Show the detailed solution
Answer: 2 mol O₂ require 4 mol H₂. Since 5 mol H₂ are available, O₂ is limiting and 1 mol H₂ remains in excess.
Explanation: With 3 mol H₂ and 1 mol O₂ for 2 H₂ + O₂ → 2 H₂O, O₂ is limiting: it consumes 2 mol H₂ and leaves 1 mol H₂ in excess. Never choose the limiting reactant by simply comparing the available mole counts.
Method: Start from the relation used, substitute each quantity with its unit, then check the order of magnitude and the final unit.
Avoid this
Never choose the limiting reactant by simply comparing the available mole counts.