1. Quantities in chemical reactions
7.2 The Limiting Reactant

2. Introduction - Limiting Reactant
In real life chemical reactions rarely have reactants in stoichiometric amounts.
Ex - Your BBQ has the following chemical reaction:
C3H8(g) + 5 O2(g)  3 CO2(g) + 4H2O(g)
What happens when you run out of propane?

3. Introduction - Limiting Reactant
Definitions:
Limiting Reactant (LR)
This reactant in a chemical reaction yields the least amount of product.
This reactant is completely used up (consumed) in the chemical reaction.
Excess Reactant (ER)
This reactant remains after the reaction is over.

4. Examples
In the following scenarios, which reactants are in excess and which are limiting?
2 car bodies, 7 tires
No gasoline in your vehicle but oxygen is still present for combustion
4 hamburgers patties, 2 hamburger buns
6 crackers, 28 chocolate squares, marshmallows
Perfect S’more
2 Graham Crackers
4 Chocolate Squares
1 Marshmallow

5. Examples
In the following scenarios, which reactants are in excess and which are limiting?
2 car bodies, 7 tires
No gasoline in your vehicle but oxygen is still present for combustion
4 hamburgers patties, 2 hamburger buns
6 crackers, 28 chocolate squares, marshmallows
Perfect S’more
2 Graham Crackers
4 Chocolate Squares
1 Marshmallow

6. Practice
1. If 4.80 g of calcium are mixed with 2.00 g of nitrogen, which is the limiting reactant, Ca or N2? 3Ca(s) + N2(g)  Ca3N2(s)

7. mass Ca moles Ca mole ratio moles Ca3N2
3Ca(s) N2(g)  Ca3N2(s)
mass Ca
moles Ca
mole ratio
moles Ca3N2
4.80 g Ca produces mol Ca3N2

8. mass N2 moles N2 moles Ca3N2 mole ratio
3Ca(s) N2(g)  Ca3N2(s)
mass N2
moles N2
mole ratio
moles Ca3N2
2.00 g N2 produces mol Ca3N2

9. Which is the limiting reactant, Ca or N2?
The limiting reactant is Ca
mol Ca3N2 < mol Ca3N2

10. Practice:
2. We have a sample of 2.00 g of ammonia and 4.00 g of oxygen. The products of the reaction are water and nitrogen monoxide. a) What mass of nitrogen monoxide will be produced? b) What amount of excess reactant remains after the reaction is complete?
4 NH3(g) + 5 O2(g)  6 H2O(g) + 4 NO(g) 

11. The limiting reactant is
Step-2
Determine the limiting reactant by calculating how much NO is produced by the amount of each reactant.
Note:
The reactant that yields a smaller amount of NO is the limiting reactant (LR).
2.00 g NH3 produces mol NO
The limiting reactant is O2
4.00 g O2 produces mol NO

12. Step-3
Determine the mass of NO is produced by the limiting reactant by converting the moles of NO into mass.
4.00 g O2 produces 3.00g NO

13. Step-4 Determine the mass of excess reactant (NH3) that remains by:
i) calculating how many moles of NH3 react.
ii) calculating how many moles of NH3
remain unreacted.
iii) convert the moles of NH3 into mass.
0.29 g
NH3
remain
0.100 mol NH3 react
0.017 mol NH3 remain unreacted

14. SUMMARY Strategy for Solving Limiting Reactant Problems:
Calculate how much product can be produced from each of the reactants.
Compare the molar amounts of the product.
The substance that produces the least amount of the product is the limiting reactant.
-This reactant governs how much product is formed.
Mass of product can be determined by multiplying moles by molar mass.

15. Questions

16. - Complete prediction (show your calculations)
Textbook p. 254 #
p #
Investigation 7A
- Complete prediction (show your calculations)
- Copy observation tables (2)
- Read procedure
NOTE: TEST UNIT 3 (Ch 5-7) in 5 school days
(see suggested textbook review)