How much can I make?Maximizing Chemical Quantities
Limiting and Excess ReactantsPercent 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 mol Cu
63.5 g Cu
1 mol S32.1 g S
1.26
1.09
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 11.26
2 mol Cu1 mol S 0.63
0.631.09
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 ________ = 2 mol Cu
1 mol Cu2S
1 mol Cu2S
159.1 g Cu2S 100 g 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
100.1 g CaCO3
1 mol CaCO3
1 mol CaCO3
1 mol CaO
1 mol CaO
56 g 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