Limiting Reactants

Reactants Products• Limiting Reactant: Determines the

amount of products that can be made• Excess Reactant : Left over reactants

Identifying the Limiting Reactant

• Convert each reactant to grams of a product using stoichiometry

• The reactant that makes the least amount of products is the limiting reactant

Sample ProblemWhich is the limiting reactant if

25.0 grams of phosphorus reacts with 50.0 grams of oxygen?

P4 + 5O2 P4O10

P4 is limiting

Sample ProblemA 2.00 g sample of ammonia is mixed with 4.00 g of oxygen. How much NO is produced and which is the limiting

reactant?

4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g)

Sample ProblemAn alkaline battery produces energy according to the following reaction.

Determine the limiting reactant if 25.0 g of Zn and 30.0 g of MnO2 are used. What

mass of Zn(OH)2 is produced? Zn + 2MnO2 + H2O Zn(OH)2 + Mn2O3

MnO2 is limiting

Excess Reactants• The amount of excess reactants

can be determined by using stoichiometry to find the amount of reactants that is needed to react with the limiting reactant.

We're not finished yet though. 1.70 g is the amount of ammonia that reacted, not what is left over. To find the amount of excess reactant remaining, subtract the amount that reacted from the amount in the original sample.

Yield

• The theoretical yield represents the maximum amount of product that can be formed from given amounts of reactants

• The actual yield is the amount of product that actually forms when the reaction is carried out (in the laboratory). It is considered as an experimental value.

Percent Yield

A ratio of actual yield to theoretical yield

Why might percent yield be important?

Percent yield is important, because…

• Higher yield=less waste• Less waste= lower cost for products

Percent Error: how much error there is between

actual and theoretical yield

Sample ProblemAluminum hydroxide is often present in antacids to neutralize stomach acid (HCl). If 14.0 g of aluminum hydroxide is present in an antacid tablet, determine the theoretical yield of aluminum chloride produced when the tablet reacts with stomach acid. If the actual yield of aluminum chloride is 22 g, what is the percent yield?

Al(OH)3 + 3HCl AlCl3 + 3H2O

Al(OH)3 + 3HCl AlCl3 + 3H2O

What is the percent error?