Limiting Reagents

Limiting Reagents Rarely are reactants in nature available in the ratios specified by a chemical equation. Usually 1 or more reactants are in excess and the reaction will continue until 1 gets used up.

Why do reactions stop? The amount of product formed is determined by the reactant that is limiting. This is what makes the limiting reagent important. It runs out first so it controls when the reaction stops and how much product is made. Once you figure out what the limiting reagent is, you will use it for any other calculations. You have actually already been doing limiting reagent problems. If 40 grams of calcium carbonate combines with excess HCl ………. Calcium carbonate is your limiting reagent because the other reactant is in excess.

Vocabulary Word limiting reactant: a reactant that is totally consumed during a chemical reaction. It limits the reaction and determines the amount of product formed

Vocabulary Word Excess reactant: reactant that remains after a chemical reaction has stopped.

Excess and Limiting Reactants Product formed: 2 molecules NH3 Excess: 2 molecules N2 Reactants: 3 molecules N2 3 molecules H2

Steps for Determining which reactant is limiting: Calculate mass to mass relationship of each reactant with any product (the same product for each). 2. The reactant that gives the smaller amount of product is the limiting reactant. The reactant with the biggest number is the excess reactant.

Steps for Determining which reactant is limiting: Example: 4 NH3(g) + 5 O2(g)  4 NO(g) + 6 H2O(g) If you have 2g of ammonia and 4g of oxygen gas which one is the limiting reagent?

To find the amount of excess reactant: Calculate mass to mass relationship between the limiting reactant and the excess reactant. Subtract the original amount of excess reactant from the amount of excess reactant that was used up in the reaction.