Limiting Reagents

If you mix two chemicals together as reactants it is unlikely that both reactants will be used up completely when forming products. One will be used to completion, this one is the limiting reactant, and will limit how much product can be made. The other will be used up until the other reactant runs out and there will be some left over, so it is an excess reagent. 


If you have 8 cars and 48 tires, which one is limiting? Which one will you run out of first?

In this example it is easy to "see" which is the limiting and which is excess. When looking at quantities of chemicals it is not as easy to "see."

To determine which chemical is limiting, convert from the given reactants to a product (it does not matter which). Whichever reactant produces the least amount of product is limiting. 

4 NH3(g) + 5 O2(g)4 NO(g) + 6 H2O(g)

If you have 2.00 grams of ammonia and 4.00 grams of oxygen gas which is limiting?

Because the oxygen produced less of the product nitrogen monoxide, oxygen is limiting.