Limiting Reactants
Chapter 13
Plan of the day
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Intro to Limiting Reactants
S’mores
Example Problems
Work through homework problems
– Ch 13 HW Assignment: #1-9, 12,13
• Chapter 13 Test: 5/4
Limiting Reactants—you will
be able to…. (objectives)
• Define a limiting reactant
• Articulate the importance of a
limiting reactant
• Identify the limiting reactant in a
chemical reaction
• Calculate limiting reactant problems.
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… just listen)
• 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
S’mores Stoichiometry
• Limiting Reactants
• Complete S’mores wkst. #1-4
–We’ll get started together
–When you are done, get a
“S’mores Kit”
S’mores Kit Stoichiometry
•
Record on back of your worksheet:
1. Obtain a bag with s’more ingredients
2. Write the balanced equation for S’mores!
3. Using Stoichiometry, determine how many
S’mores you can make with each
ingredient given.
4. What is your limiting reactant?
5. Using stoichiometry, determine the
amount of your excess reactants.
6. Wash your hands, and make yourself a
S’more as a reward for a job well done 
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.
5.
Start with a Balanced Equation
Change grams of reactants to moles – this is
how much you have.
Pick one of the reactants you just converted
to mols (how much you have) & use Stoich
from the balanced eqn to determine the
number of mols of other reactant needed.
Compare mols of other reactant needed to
what you have of that reactant… do you
have enough?
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  mols of other
reactant. This is what you need…Do you have
enough?
• If “yes,” then the other is your LR… if no than
that one is your LR
• 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 (g) of limiting
reactant, use stoichiometry
(balanced equation) to find out the
amount (g) of product!
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 is
used up.
• Calculate the amounts of each
product that will be produced.
Reflection
• Looking at the objectives, give me a
finger vote. 1—still feel clueless 5—
makes perfect sense!
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
IMPORTANT INFO…
• Chapter 13 HW:
– #1-9, 12,13
• Chapter 13 Test Dates
–5/4/15 (A-day)