What is a limiting reactant?

A limiting reactant is defined as the reactant that is completely consumed first during a chemical reaction, which subsequently limits the amount of product that can be formed. When one reactant is used up before the others, it halts the reaction because there are no more molecules of that reactant available to react, regardless of the quantities of the other reactants.

In practical terms, identifying the limiting reactant is crucial in stoichiometry because it allows chemists to calculate the maximum amount of product that can be produced from a given set of reactants. The availability of the limiting reactant directly dictates the extent of the reaction; once it is gone, the reaction stops.

The other concepts presented in the choices do not accurately define a limiting reactant. The reactant that is in excess refers to those that remain after the reaction has completed, the reactant that determines the reaction rate pertains to the kinetic aspects and does not necessarily correlate with stoichiometric limits, and the reactivity of a substance does not define it as limiting or not in a specific reaction context.