What happens to a ratio when an excess quantity of a reactant exists?

May 22, 2017

Nothing. A significant excess of one reactant over another in a two component reaction simply guarantees that the reactant in lower amount is completely consumed.

Explanation:

Regardless of the quantities of reagents mixed in a reaction, the reaction ratio will remain the same. One reactant will be totally consumed and any other reactants will remain in excess unreacted.

Example:

Assume 5 moles of each reactant in the following equation is mixed and the process goes to completion.

$2 N O \left(g\right) + {O}_{2} \left(g\right) \implies 2 N {O}_{2} \left(g\right)$

All of the $N O \left(g\right)$ will be consumed but only 2.5 moles of ${O}_{2} \left(g\right)$ will be consumed leaving 2.5 moles of the oxygen in excess. The mole amount of $N O \left(g\right)$ consumed in the reaction will always be twice the mole amount of ${O}_{2} \left(g\right)$ consumed regardless of the mole amounts of each reagent mixed.