Limiting Reagent:
In a chemical reaction, the limiting reagent is the reactant that is completely consumed during the reaction, thus limiting the amount of product that can be formed. The reactant that is in excess is called the excess reagent. The concept of limiting reagents is based on the principle of stoichiometry, which describes the quantitative relationship between reactants and products in a chemical reaction.

Explanation:
When a chemical reaction occurs, the reactants combine to form products in specific ratios determined by the balanced chemical equation. These ratios are based on the stoichiometric coefficients of the reactants and products. However, the actual amounts of reactants used in a reaction may not always match these ratios. In such cases, one of the reactants will be completely consumed before the others, limiting the amount of product that can be formed.

Example:
Let's consider the reaction between hydrogen gas (H2) and oxygen gas (O2) to form water (H2O):

2H2 + O2 → 2H2O

If we have 3 moles of hydrogen gas and 2 moles of oxygen gas, we can determine the limiting reagent by comparing the stoichiometric ratios. According to the balanced equation, 2 moles of hydrogen gas react with 1 mole of oxygen gas to form 2 moles of water.

Since we have an excess of oxygen gas (2 moles) compared to the hydrogen gas (3 moles), the oxygen gas will be the excess reagent. The hydrogen gas will be completely consumed, limiting the amount of water that can be formed.

Importance:
Identifying the limiting reagent is crucial for determining the theoretical yield of a reaction, which is the maximum amount of product that can be obtained. By knowing the limiting reagent, we can calculate the amount of product that will be formed based on the stoichiometric ratios. This information is essential for various applications, such as industrial production processes and laboratory experiments.

Conclusion:
The concept of limiting reagents is essential in understanding the quantitative aspects of chemical reactions. By identifying the limiting reagent, we can determine the maximum amount of product that can be obtained and calculate the theoretical yield. This knowledge helps in planning and optimizing chemical reactions in various fields.