Download Limiting Reactant

Limiting Reactants
• Substances can only react in a chemical reaction until
the reactants are used up.
• Reactions only occur when reactants are in a specific
ratio to one another and in multiples thereof.
o If 1 pencil reacts with 4 pieces of paper, then 2 pencils will
react with 8 pieces of paper. 3 pencils will react with 12
pieces of paper etc. at a continuous ratio of 1:4.
o If I have 32 sheets of paper, how many pencils do I need to
use them all up?
o If I have 1236 pencils and 5000 sheets of paper, which one
will be used up first?
• The one used up first is the limiting reactant.
Steps to Solve Limiting Reactants Problems
1.
2.
Identify what you know and what you do not know and write it down.
Convert any masses given to moles.
•
3.
Mass of substance (g)
= Number of Moles
Molar Mass of substance (g/mol)
Determine the Mole Ratio needed.
• Coefficient of unknown substance in BALANCED chemical equation
Coefficient of known substance in BALANCED chemical equation
• mol unknown molecule
# mol known molecule
• # mol comes from BALANCED chemical equation NOT the problem itself.
4.
Use the mole ratio to determine the amount of product the known
reactant will produce.
• Known Moles X Mole Ratio = Moles of Product
5.
Convert moles of product to grams.
• Number of Moles X Total Molar Mass = Mass of substance (g)
Steps to Solve Limiting Reactant Problems
…continued
6.
Compare the amount of “Unknown” calculated and compare it to the amount of
“Unknown” given in the problem.
•
•
If there is more “Unknown” calculated than given, then it is the limiting reactant.
If there is more “Unknown” given than calculated, then it is the excess reactant and the “Known” is
the limiting reactant.
Extension: If the problem continues to ask specifically how much one of the products in the
equation will be formed by the reaction…Steps 1-6 Same as Above and then:
7.
Determine your new Mole Ratio using the Limiting Reactant identified above.
•
•
•
8.
Use the mole ratio to determine the amount of substance the Limiting Reactant will
produce.
•
9.
Coefficient of unknown substance in BALANCED chemical equation
Coefficient of Limiting Reactant in BALANCED chemical equation
mol unknown molecule
# mol Limiting Reactant
# mol comes from BALANCED chemical equation NOT the problem itself.
Known Moles of Limiting Reactant X Mole Ratio = Moles of Unknown
Convert moles of product to grams.
•
Number of Moles X Total Molar Mass = Mass of substance (g)
Limiting Reactants Practice
I
1. Calcium hydroxide, used to neutralize acid spills,
reacts with hydrochloric acid according to the
following equation: Ca(OH)2 + 2HCl → CaCl2 + 2H2O.
If you have spilled 6.3 mol of HCl and put 2.8 mol of
Ca(OH)2 on it, which substance is the limiting
reactant?
We
Limiting Reactants Practice …continued
2. Aluminum oxidizes according to the following
equation: 4Al + 3O2 → 2Al2O3.
0.048 mol of powdered Al is placed into a
container containing 0.030 mol O2. What is
the limiting reactant?
We
Limiting Reactants Practice …continued
3. Chlorine can replace bromine in bromide
compounds forming a chloride compound and
elemental bromine. 2KBr + Cl2 → 2KCl + Br2 is
an example of this reaction.
When 0.855 g of Cl2 and 3.205 g of KBr are
mixed in solution, which is the limiting
reactant? How many grams of Br2 are formed?
You
Limiting Reactants Practice …continued
4. A process by which zirconium metal can be
produced from the mineral zirconium(IV)
orthosilicate, ZrSiO4 , starts by reacting it with
chlorine gas to form zirconium(IV) chloride.
ZrSiO4 + 2Cl2 → ZrCl4 + SiO2 + O2
What mass of ZrCl4 can be produced if 862 g
of ZrSiO4 and 950.0 g of Cl2 are available? (You
must first determine the limiting reactant.)
End Day 3