How much can I make? Maximizing Chemical Quantities Limiting and Excess Reactants Percent Yield

What is a limiting reactant? Limiting Reactant - reactant in a chemical reaction that limits the amount of product that can be formed The reaction will stop once all of the limiting reactant is consumed.

What is an excess reactant? Excess Reactant - reactant in a chemical reaction that remains when a reaction stops The excess reactant remains because there is nothing to react with it!

How many cars can be made? No matter how many tires there are, if there are only 8 car bodies, then only 8 cars can be made. 

Example #1: Copper metal reacts with sulfur to form copper (I) sulfide according to the balanced equation. _____ Cu + ____ S  _____ Cu2S What is the limiting reactant when 80 g Cu reacts with 25 g S? 80 g Cu x _________ = mol of Cu 25 g of S x _______ = mol of S 2 1 1 1.26 1 mol Cu 63.5 g Cu 1 mol S 1.09 32.1 g S

Example #1: 2 Cu + 1 S  1 Cu2S What is the mole ratio of Cu to ? ____ Cu : ____ S mol of Cu x ___________ = mol of S Translation: I only need mol of S to fully react with copper. But I was given ________ mol of S. Excess = Sulfur Limiting = Copper 2 1 1.26 1 mol S 0.63 2 mol Cu 0.63 1.09

What’s the max amount of product that I can yield? Use Stoichiometry to determine the max amount of product that can be formed. 1.26 mol Cu x _______ x ________ = 1 mol Cu2S 159.1 g Cu2S 100 g Cu2S 2 mol Cu 1 mol Cu2S

Theoretically vs. Actually

Percent Yield Theoretical Yield = maximum amount of product that could be formed from a given amount of reactants Actual Yield = amount actually formed when a reaction is carried out

Why don’t reactions always go to completion? purity of reactants loss of product formed during filtration competing reactions measurement error

Theoretical Yield Example What is the theoretical yield of CaO if 24.8 g of CaCO3 is heated? _____ CaCO3  _____ CaO + ______ CO2 24. 8 g CaCO3 x ________ x _________ x ________ = 13.9 g CaO Theoretical Yield = 13.9 g CaO 1 mol CaCO3 1 mol CaO 56 g CaO 100.1 g CaCO3 1 mol CaCO3 1 mol CaO

Percent Yield Example What is the percent yield if 13.1 g of CaO was actually produced when 24.8 g of CaCO3 was heated? % Yield= 13.1 g x 100 = 94.2 % 13.9 g