Question

What is the limiting reactant for a process? Why does a reaction stop when the limiting reactant is consumed, even though there may be plenty of the other reactants present?

Short answer

A limiting reactant is the reactant in a chemical reaction that is completely consumed first, determining when the reaction stops. Once the limiting reactant is consumed, the reaction stops, even if there are other reactants present in excess, because reactants must be present in their appropriate stoichiometric ratios for the reaction to proceed. For example, in the reaction \[ N_2 + 3H_2 \rightarrow 2NH_3 \], if 2 moles of N2 and 8 moles of H2 are available, N2 is the limiting reactant and will be completely consumed, stopping the reaction even though there are 2 moles of H2 remaining.

Step-by-step solution

Introduction to Limiting Reactant

A limiting reactant (or limiting reagent) is the reactant in a chemical reaction that is completely consumed first, which determines when the reaction stops. Because the limiting reactant has a limited amount, once it is consumed, a reaction will stop, even though there may be plenty of other reactants present.

Example of a Limiting Reactant

Consider a simple reaction in which hydrogen gas (H2) reacts with nitrogen gas (N2) to form ammonia (NH3): \[ N_2 + 3H_2 \rightarrow 2NH_3 \] This balanced equation indicates that 1 mole of N2 reacts with 3 moles of H2 to produce 2 moles of NH3. If we have 2 moles of N2 and 8 moles of H2, we can determine which reactant is the limiting reactant.

Identify the Limiting Reactant

To identify the limiting reactant, we must compare the available moles of each reactant with their stoichiometric ratios in the balanced equation. We have 2 moles of N2 and the balanced equation shows that 1 mole of N2 requires 3 moles of H2. Therefore, we need 2 moles × (3 moles H2/1 mole N2) = 6 moles of H2. However, we have 8 moles of H2 available, which is more than enough for the reaction. In this case, N2 is the limiting reactant because it will be completely consumed after the reaction has taken place.

Reaction Stops when Limiting Reactant is Consumed

Once the limiting reactant (N2) is consumed, there will be no more N2 available to react with H2, effectively stopping the reaction. This occurs even if there is an excess of H2 present because reactants must be present in their appropriate stoichiometric ratios for the reaction to proceed. In this example, after 2 moles of N2 react with 6 moles of H2, 2 moles of NH3 will be produced, and there will be no more N2 molecules to react with the excess 2 moles of H2. Therefore, the reaction stops when the limiting reactant is consumed. In summary, a limiting reactant is the reactant that gets consumed first in a chemical reaction, effectively determining when the reaction stops. Once the limiting reactant is consumed, the reaction will stop, even if there are other reactants present in excess, because reactants must be present in their appropriate stoichiometric ratios for the reaction to proceed.

Key concepts

Chemical Reaction

A chemical reaction is a process where substances, called reactants, are transformed into different substances, known as products. During this transformation, the atoms in the reactants are rearranged to form new compounds. This process can involve breaking and forming chemical bonds, which often leads to energy changes manifested in various forms such as heat, light, or even sound.

In a typical chemical reaction, it's essential to consider the amount of each reactant present because it directly influences the products formed. This is where the concept of the limiting reactant comes into play. When one reactant is entirely used up before the others, it limits the amount of product that can be produced, effectively causing the reaction to cease. For instance, if you are making sandwiches and you run out of bread, you can no longer make more sandwiches, even if you still have a lot of cheese and vegetables left. The bread, in this case, is the limiting reactant.

Stoichiometry

Stoichiometry is the branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. It's all about measuring and calculating the proportions of elements and compounds involved in a reaction to ensure it proceeds as desired. By using stoichiometry, we can calculate how much of each reactant is needed or how much of a product will be formed. This is particularly useful in laboratory settings and industrial applications where maximizing efficiency and minimizing waste are crucial operations.

The stoichiometric ratio is derived from the balanced chemical equation, which specifies the exact proportions of molecules that participate in the reaction. For example, if the reaction equation is \( N_2 + 3H_2 \rightarrow 2NH_3 \), it tells us that one molecule of nitrogen reacts with three molecules of hydrogen to form two molecules of ammonia. Recognizing stoichiometric ratios allows us to predict the outcomes of reactions, and identify the limiting reactant by comparing the available quantities to what the ratios dictate.

Balanced Equation

A balanced chemical equation is crucial to accurately represent a chemical reaction. It ensures that the number of atoms for each element remains constant throughout the process, adhering to the law of conservation of mass. This law states that matter cannot be created or destroyed in an isolated system. Therefore, balancing equations is essential to reflect reality where atoms are merely rearranged and not lost or generated anew.

Take the balanced chemical equation for the synthesis of ammonia: \( N_2 + 3H_2 \rightarrow 2NH_3 \). Each side of the equation must have the same number of nitrogen and hydrogen atoms.

• Nitrogen: 1 molecule of \( N_2 \) on both sides.
• Hydrogen: 6 atoms (3 molecules of \( H_2 \)) on the left equals the 6 atoms in the 2 \( NH_3 \) molecules on the right.

Balancing the equation is essential for carrying out calculations related to the limiting reactant and stoichiometry effectively, as it ensures that we have the correct ratios necessary for the calculation.