Which statement best describes a limiting reactant?

A limiting reactant is defined as the reactant that gets consumed first during a chemical reaction, ultimately determining the maximum amount of product that can be formed. As the reaction proceeds, the limiting reactant is used up completely while other reactants may remain in excess. This consumption halts further reaction because there are no longer sufficient quantities of that reactant to continue forming products.

In chemical calculations, identifying the limiting reactant is crucial for determining yields and stoichiometry in reactions. This concept reflects the fact that in any given reaction, it is typically the limiting reactant that dictates how much product can be synthesized due to its rapid depletion.

The other statements do not accurately capture the essence of a limiting reactant. The idea of something being in excess pertains to reactants that remain once the limiting reactant is entirely consumed, and the initiation of a reaction pertains more closely to catalysts or reactive agents that may facilitate a reaction's progression rather than the limiting reactant itself. Similarly, the product formed in the smallest quantity is not a valid characterization of a limiting reactant, as it does not directly describe a reactant's role in the consumption process during the reaction.