Reagente limitante (2 reagentes)

Limiting reagent: whichever runs out first

When two reagents go into a reaction, the one that runs out first caps how much product you can make. That one is the limiting reagent, and whatever doesn't get used stays behind as excess. Finding it isn't hard. Take the moles you have of each reagent, divide by its stoichiometric coefficient, and the smaller ratio wins. Try it on 2 H₂ + O₂ → 2 H₂O with 4 mol H₂ and 3 mol O₂. You get H₂/2 = 2 and O₂/1 = 3, so H₂ comes in lower and limits the reaction while O₂ is left over. The ceiling is 4 mol H₂O (a 1:1 match with H₂), and 1 mol of O₂ never reacts. Because the limiting reagent fixes the product amount, piling on more of the other one buys you nothing.

Applications

Stoichiometry homework leans on this constantly, and so does batch design in industrial chemistry, where the usual play is to keep the cheap reagent in excess so the expensive one converts all the way. Green chemistry runs the same reasoning to trim waste. In pharmaceutical synthesis a single reagent is often the cost that decides whether a route is even worth running.

FAQ

Why divide moles by coefficient? The coefficient tells you how many moles feed a single "reaction unit". If you hold 4 mol of a reagent whose coefficient is 2, you only cover 2 reaction units; 3 mol with a coefficient of 1 covers 3. Whichever has the lower capacity does the limiting.

Can I just compare masses? No. Comparing masses misses the fact that different substances have different molar masses. Convert everything to moles first, then divide by the coefficients.

What happens to the reagent in excess? It stays unreacted in the mixture and is removed during purification. A lot of the time the excess is on purpose: keep the cheap reagent abundant, and the expensive one reacts all the way through.