Transcript Notes

Limiting Reactants
Chapter 13
A real-life example
• Cookies anyone?
• Given the following recipe for chocolate
chip cookies…
2 ¼ cups flour
1 tsp salt
1 tsp baking soda
1 c butter (2 sticks)
¾ cup brown sugar
¾ cup granulated sugar
2 eggs
2 tsp vanilla
3 oz chocolate chips
Yield: 4 dozen cookies
• What if we only have 1 egg? How many
cookies could we make? What would we
need to do? What would we have left
over?
Demo: Bolts & Washers
2Mg + O2  2MgO
•1 bolt = 1 mole of Mg
•2 washers = 1 mole of O2
•What would 1 mole MgO look like?
•Let’s “make” our reaction with bolts
and washers…
Demo: Bolts & Washers
+
2Mg
+
=
O2
=
2MgO
Demo: Bolts & Washers
• When does that “reaction” stop?
• What did you run out of?
– Liming Reactant
• What is left over?
– Excess Reactant
Definitions
• Reactant
– Is a substance that takes part in a
chemical reaction to make a product
• Limiting Reactant
– The reactant that is completely used up
in the reaction and limits the amount of
product
• Excess Reactant
– The reactant that is in excess, not all
used up.
Why do we care?
(Don’t write…let’s discuss)
• Scientific Reasons
– Determine how much reactant you
need
– Determine how much product will be
produced
• Economic (Show me the money!)
– Keep production cost’s minimal
– Maximize output in production
Let’s Review…
Steps in solving stoichiometry problems…
(how do we find mass or volume of a product given mass
or volume of a reactant?)
1.Write and Balance Equation
2.Identify known and unknown
3.Convert mass (or volume) to moles
4.Use mole ratio to convert from moles of
known to moles of unknown
5.Convert moles of unknown to mass (or
volume) of unknown
Steps in Solving LR
problems!
When two amounts of reactants are given in a
problem, we need to identify the limiting reactant to
solve for the amount of product possible!
1.
2.
3.
4.
Start with a Balanced Equation
Convert grams of each reactant to moles
(this is how much you have of each)
Use mole ratio to convert moles of one react
to moles of the other and compare to the
original number of moles obtained in step
two (what you need)
Decide which is the limiting reactant
Another Method
1.
2.
3.
4.
Always begin with a BALANCED EQUATION!!
Convert the amount (grams/volume) of each
reactant to moles of reactants.
Convert the number of moles of reactant to
moles of product (doesn’t matter which one as
long as you use the same product for both
reactants)
Which ever reactant produces the least
product is the limiting reactant.
Example 1
• The reaction begins with 2.51g of HF
and 4.56g of SiO2.
HF + SiO2  SiF4 + H2O
• Balance Equation
• Convert g  mols of each (What you have)
• Pick one & convert mols  moles of other
reactant. This is what you need…Do you have
enough?
• If “no”, then that is your Limiting Reactant… if
yes than the other is your LR
Example 1
1. Balance the Equation
4HF
+
SiO2

SiF4
+ 2H2O
2. Calculate moles of each reactant (what you have)
2.51 g HF 1 mol HF = 0.1255 mol HF
20.0 g HF
4.56 g SiO2 1 mol SiO2 = 0.059 mol SiO2
76.05 g SiO2
Example 1
3. Use mole ratio to convert from one reactant to
the other.
0.1255 mol HF 1 mol SiO2
4 mol HF
= 0.031 mol SiO2
Compare to moles of SiO2 we have (from Step 2)
0.059 mol SiO2 (have) > 0.031 mol SiO2 (need)
4. Determine the limiting reactant
• If you have more than you need, this is not the
limiting reactant
• This means the other reactant (HF) must be
limiting
• How much excess reactant is left over?
– Determine the amount of the excess
reactant used then subtract from the
starting amount.
How much product can be formed?
• Start with the amount (mass or
volume) of limiting reactant
• Use stoichiometry (mole ratio
conversions) to find the amount
(mass or volume) of product left over
Important Info
• Chapter 11 Homework:
– Red Book #23-27
• Next Class:
– S’mores Lab!
– Don’t forget to sign up and bring
ingredients 
Example 2
Determine the limiting reactant, if an 80.0g
solution of NaOH, which is 40.0% NaOH by
mass, reacts with a 75.0g solution of
H2SO4, which contains 45.0% water by
mass.
NaOH + H2SO4  Na2SO4 + H2O
1. Balance Equation
2. Calculate grams of NaOH and H2SO4
3. g  mols of each (What you have)
4. Pick one & compare (What you need)
5. Do you have enough?
• So…the reaction will go to
completion until all the NaOH (LR) is
used up.
• Calculate the amounts of each
product that will be produced.
Example
•
If 40.0 g of H3PO4 react with 60.0 g of
MgCO3 calculate:
a. g of Mg3(PO4)2 produced
b. g of CO2 produced
c. Volume of CO2 at STP
H3PO4 + MgCO3  Mg3(PO4)2 + CO2 + H2O