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
Because available amounts must be compared with the ratios required by the balanced equation.
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
2 mol O₂ require 4 mol H₂. Since 5 mol H₂ are available, O₂ is limiting and 1 mol H₂ remains in excess.
Avoid this
Never choose the limiting reactant by simply comparing the available mole counts.