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