What makes something a limiting reagent

In order to assemble a car, 4 tires and 2 headlights are needed among other things. In this example, imagine that the tires and headlights are reactants while the car is the product formed from the reaction of 4 tires and 2 headlights.

If you have 20 tires and 14 headlights, how many cars can be made? With 20 tires, 5 cars can be produced because there are 4 tires to a car.

With 14 headlights, 7 cars can be built each car needs 2 headlights. Although more cars can be made from the headlights available, only 5 full cars are possible because of the limited number of tires available. In this case, the headlights are in excess. Because the number of cars formed by 20 tires is less than number of cars produced by 14 headlights, the tires are the limiting reagent they limit the full completion of the reaction, in which all of the reactants are used up.

This scenario is illustrated below:. The initial condition is that there must be 4 tires to 2 headlights. The reactants must thus occur in that ratio; otherwise, one will limit the reaction. There are 20 tires and 14 headlights, so there are two ways of looking at this problem. For 20 tires, 10 headlights are required, whereas for 14 headlights, 28 tires are required. Because there are not enough tires 20 tires is less than the 28 required , tires are the limiting "reactant.

The limiting reagent is the reactant that is completely used up in a reaction, and thus determines when the reaction stops. From the reaction stoichiometry , the exact amount of reactant needed to react with another element can be calculated. If the reactants are not mixed in the correct stoichiometric proportions as indicated by the balanced chemical equation , then one of the reactants will be entirely consumed while another will be left over. The limiting reagent is the one that is totally consumed; it limits the reaction from continuing because there is none left to react with the in-excess reactant.

There are two ways to determine the limiting reagent. The mass of product formed in a reaction depends upon the mass of the limiting reactant. This is because no more product can form when the limiting reactant is all used up.

How much aluminium chloride is formed when 2. This particular resource used the following sources:. Skip to main content. Mass Relationships and Chemical Equations. Search for:. Limiting Reagents. Learning Objective Determine the limiting reagent and the amount of a product formed in a given reavion.

Key Points The limiting reagent is the reactant that is used up completely. This stops the reaction and no further products are made. Given the balanced chemical equation that describes the reaction, there are several ways to identify the limiting reagent. One way to determine the limiting reagent is to compare the mole ratios of the amounts of reactants used.

Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Updated December 23, Featured Video. Cite this Article Format. Helmenstine, Anne Marie, Ph. Limiting Reactant Definition Limiting Reagent. Theoretical Yield Definition in Chemistry.

Stoichiometry Definition in Chemistry.