The limiting reactant (or limiting reagent) is the substance in a chemical reaction that gets completely used up first. Once it is gone, the reaction stops, and no more products can be formed, even if other reactants are still available.
To determine the limiting reactant, follow these steps:
- Write and balance the chemical equation.
Make sure the equation obeys the law of conservation of mass. - Convert the given amounts of reactants to moles.
Use molar masses to change grams into moles, if needed. - Use the mole ratio from the balanced equation.
Compare how much of one reactant is required to react with the other. - Identify which reactant produces fewer moles of product.
The reactant that produces the smaller amount of product is the limiting reactant. - The other reactant is in excess.
Example:
Reaction: 2H₂ + O₂ → 2H₂O
Suppose we have 5 moles of H₂ and 2 moles of O₂.
- From the mole ratio (2:1), 5 moles of H₂ need 2.5 moles of O₂.
- But only 2 moles of O₂ are available.
- Therefore, O₂ is the limiting reactant, and some H₂ will remain unused.
The limiting reactant determines the maximum yield of the product.