Limiting Reagent

The following scenario demonstrates the importance of limiting the number of reagents used. In order to put together a car, four tyres and two headlights are required (among other things). Consider the following scenario: the tyres and headlights are reactants, and the car is the product of the reaction between four tyres and two headlights. If you have 20 tires and 14 headlights, how many cars can you make with those materials? Because there are four tyres to a car, a set of 20 tyres can be used to produce five cars. Seven automobiles can be constructed from 14 headlights (each car needs 2 headlights). Despite the fact that more cars can be constructed from the available headlights, only five complete automobiles are possible due to the limited number of tyres available. In this instance, the number of headlights is excessive. Because the number of cars formed by 20 tyres is less than the number of cars formed by 14 headlights, the tyres serve as the limiting reagent in the car formation process (they limit the full completion of the reaction, in which all of the reactants are used up).

Limiting reagents are substances that are completely consumed by a chemical reaction before it can be completed successfully. They are also referred to as limiting agents or limiting reactants in some instances. When it comes to chemical reactions, the stoichiometry of the reaction states that a fixed amount of reactants is required for the reaction to be completed. 

Explanation of Reagents 

In most cases, this reactant determines when the reaction will come to an end. Because of the stoichiometry of the reaction, it is possible to calculate the exact amount of reactant that will be required to react with a different element. The limiting reagent is determined by the mole ratio of the reactants present, rather than the masses of the reactants present.

Using Reagents to a Minimum

Reducing the amount of reagent used before and after the reaction

It can be seen in the illustration above that the limiting reactant is the reason that the reaction cannot continue because there is nothing left to react with the excess reactant after the limiting reactant has been consumed. During the course of the reaction, it is the reactant that is completely depleted of all of its energy.

Examples of Reagents 

Consider the following scenario: 1 mol of oxygen and 1 mol of hydrogen are present for the reaction to take place.

2H2O is formed by the reaction of 2H2 and O2.

Because hydrogen depletes the reaction’s supply twice as quickly as oxygen, hydrogen would be the reaction’s limiting reactant.

As an illustration, 100g of hydrochloric acid is mixed with 100g of zinc. Calculate the amount of hydrogen gas that has been produced under standard laboratory conditions.

Solution:

The chemical equations for these reactions are shown in the following section.

The reaction 2HCl(aq) + Zn(s) produces ZnCl2(aq) + H2 (g)

Because excessive amounts of zinc chloride are formed, hydrochloric acid serves as the limiting reagent in this reaction.

22.4 litres of water from 73 grammes of HCl

100g of HCl is equal to yL of H2.

y/22.4 = 100/73 y = (100 x 22.4)/73 y = 30.6L y/22.4 = 100/73 y = (100 x 22.4)/73 y = 30.6L

As a result, under standard laboratory conditions, 33.6L of H2 is produced per hour.

Best way to locate Limiting Reagent

It is common practice for the determination of the limiting reactant to be just one piece of a larger puzzle. While solving most limiting reactant-stoichiometry problems, the ultimate goal should be determining the amount of product that could be formed from a specific reactant mixture. Identifying the limiting reactant or reagent can be accomplished through one of two methods.

Using the mole ratio as a guideline

Taking a product-oriented approach

The balanced equation must be written first in order to calculate the mass of the product, and then it must be determined which reagent has an excess amount. Calculate the mass of the finished product by using the limiting reagent.

Attempt to identify the limiting reagent

In cases where there are only two reactants, write a balanced chemical equation and determine the amount of reactant B that is required for the reaction to occur between reactants A and B. When the amount of reactant B is greater than the amount of reactant A, the reactant A serves as the limiting reagent.

The limiting reactant is the reactant that is present in a lower concentration than that required by stoichiometric equations.

An alternative method of determining the limiting agent involves calculating the amount of product formed by each reactant.

The limiting reactant is the reactant that results in the formation of the smallest amount of product possible.

If we calculate the amount of one reactant that is required to react with another reactant, the reactant that is in short supply would be the limiting reactant that is required to complete the reaction.

As a result, the limiting reagent for the reaction can be determined by referring to the information provided above. When calculating the percentage yield of a given reaction, the presence of these reagents is critical to success.