Limiting Reactants (Reagents)
1. Two equations for the combustion of ethene, C2H4, are possible.
C2H4 + 3O2  2CO2 + 2H2O (complete combustion) AND
C2H4 + 2O2  2CO + 2H2O (incomplete combustion)
What will occur for each reaction if 2.70 mol of C2H4 is reacted with 6.30 mol of O2?
a. Identify the limiting reagent.
b. Calculate the moles of water produced.
c. Calculate the moles of excess reagent remaining.
2. Hydrogen gas can be produced in the laboratory by the reaction of magnesium metal
with hydrochloric acid.
Mg + 2HCl  MgCl2 + H2
How many grams of hydrogen can be produced when 4.00 g of HCl is
g of Mg?
added to 3.00
3. What is the limiting reactant for a process?
4. What is the theoretical yield for a reaction, and how does this quantity depend on the
limiting reactant?
5. Explain how one determines which reactant in a process is the limiting reactant. Does
this depend only on the masses of the reactant present? Is the mole ratio in which the
reactants combine involved?
6. What do we mean when we say that a reactant is present "in excess" in a reaction?
Does the presence of an excess of one or more reactants affect the theoretical yield of the
reaction?