The limiting reactant — Chemistry, 14–17 years
Why a reaction stops when one reactant runs out, even if other reactants are still left. Chemistry, 14–17 years.
The ingredient that runs out
In a reaction, the limiting reactant is the substance that gets used up first. Its amount sets the maximum amount of product that can form; extra amounts of the other reactants cannot continue the reaction alone. Think of making sandwiches: two slices of bread and one slice of cheese make one sandwich, so the ingredient with fewer complete sets limits the number made.
Why find it?
Real reactions rarely begin with exactly the required amounts. A chemist needs to know which reactant will run out so the predicted product amount is not too high. This idea grew from the practical problem of planning mixtures: it identifies waste, explains leftover material and shows which ingredient must be added if production is to increase.
Worked example: making ammonia
Use N₂ + 3H₂ → 2NH₃. Suppose there are 2.0 mol N₂ and 5.0 mol H₂. The nitrogen could make 4.0 mol NH₃, but 5.0 mol H₂ can make only 10 ÷ 3 = 3.33 mol NH₃. Therefore hydrogen is limiting, and the maximum product is 3.33 mol NH₃. Nitrogen remains unused: 2.0 − 5.0 ÷ 3 = 0.33 mol.
Choosing the smaller number
A common shortcut is to call the reactant with fewer moles the limiter. That can be wrong because the equation may require different numbers of particles: one mole of N₂ needs three moles of H₂. The safer method is to divide each reactant amount by its coefficient and compare those results. The smallest quotient shows how many reaction batches are possible.
Less waste, better recipes
Manufacturers use limiting-reactant calculations when making fertilisers, medicines, fuels and food ingredients. They can choose amounts so expensive materials are not left in large excess, while still allowing for safe control of the process. In cooking, the same logic explains why having lots of flour cannot make more cakes when the eggs have run out.
Keep exploring
Other languages
Loading MyLeoNes™…