Limiting and Excess Reagents

1. Limiting and Excess Reagents Section 9.3

2. Limiting and Excess Reagents • In order to determine how much of a product can be formed in the lab, we need to determine how much of each reactant is needed. • You can identify a Limiting Reagent Stoichiometry problem because it will have two givens. (You will be given amounts of each reactant that are available.) • You must work a minimum of TWO stoichiometry problems to determine which reactant is the limiting reagent and which is the excess reagent.

3. Definitions • Limiting reactant (or reagent): limits the amount of product that can be formed; is all completely used up in reaction • Excess reactant (or reagent): the reactant that is not completely used up in a chemical reaction; there is an excess or extra left over

4. Synthesis of a Peanut Butter Sandwich Excess Reagent Limiting Reagent Maximum (limited) amount of product formed Maximum (limited) amount of product formed Excess Reagent Limiting Reagent

5. For chemical reactions, you cannot determine the limiting reactant (LR) or excess reactant (XS) without working TWO stoichiometry math problems. • Use each given amount of reactant to work a separate stoich problem, ending with the same product in the same unit.

6. Compare the 2 product amounts, and choose the smaller one. This is the maximum (or limited) amount of product that can be made. You cannot make the larger amount, so draw a line through it. • When looking at your 2 stoich problems, • the reactant that produced the smaller amount of product is the limiting reactant • the reactant that produced the larger amount of product (which you drew a line through) is the excess reagent