Réactif limitant

The limiting reactant is the substance that is completely consumed during a chemical reaction and determines the maximum amount of product formed. Once the limiting reactant is used up, the reaction stops, even if other reactants remain. In contrast, the excess reactant is present in a quantity greater than necessary to react completely with the limiting reactant.

To identify the limiting reactant, convert the masses of all reactants to moles using their molar masses. Then compare the mole ratio of reactants provided to the stoichiometric ratio from the balanced equation. The reactant with a ratio lower than the stoichiometric ratio is the limiting reactant. This approach relies on understanding reaction stoichiometry stoichiometric coefficient and ratio relationships.

An alternative approach involves calculating the amount of product formed in moles from each reactant separately, using the balanced equation's stoichiometry. Compare the product amounts calculated for each reactant. The reactant that produces the least amount of product is the limiting reactant, while others are in excess.

Once you identify the limiting reactant, use its mole amount and the stoichiometric ratio from the balanced equation to calculate product moles. Convert the product moles to grams using the product's molar mass. This gives the theoretical maximum amount of product that can form from the available limiting reactant.

Excess reactant remains because it is present in a quantity greater than the stoichiometric amount needed to react completely with the limiting reactant. The reaction stops when the limiting reactant is entirely consumed, leaving unreacted excess reactant behind. The amount of unreacted excess can be calculated by subtracting the moles consumed from the initial moles available.

Yes. In a recipe, if one cup of flour, two eggs, and three tablespoons of sugar make five waffles, and you have three cups of flour, four eggs, and eight tablespoons of sugar, eggs become the limiting ingredient. Flour and sugar are in excess because eggs run out first, limiting waffle production to ten instead of the maximum possible amount.

Stoichiometric ratios from a balanced equation show the proportions in which reactants should combine. If the actual mole ratio of reactants differs from the stoichiometric ratio, one reactant will be limiting. For example, if hydrogen and oxygen should combine in a 2:1 ratio but are provided in a 2.5:1 ratio, oxygen becomes the limiting reactant.