Stoichiometry - Fall River Public Schools

Stoichiometry
Chemistry
Ms. Piela
Stoichiometry
• The quantitative study of reactants
and products in a chemical reaction
▫ Chemical reactions give info about
how molecules combine, and can be
used to determine amounts of
substances that form
Mole to Mole Calculations
• When using a chemical equation to determine
the amount of the substance, you must relate
them by their mole ratios
• Example #1:
____ H2 + ____ O2 → ____ H2O
If you start with 4 moles of O2 how many moles
of H2O would form?
Mole to Mole Calculations
• Example #2:
___ Fe + ____ S → ___Fe2S3
• How many moles of iron (III) sulfide, Fe2S3,
forms if the reaction initially starts with 8 moles
of iron, Fe?
Mass to Mass Calculations
• Example #1:
___ H2 + ___ O2 → ___ H2O
If 2.4 g of O2 are reacted with excess H2 gas, how
many moles of H2O would form?
0.15 mol H2O
Mass to Mass Calculations
• Example #2:
___ H2 + ___ O2 → ___ H2O
In the above equation, if I started with 3 grams of
H2, how many grams of H2O would form?
26.8 g H2O
Mass to Mass Calculations
• Example #3:
_ NH3 (g) + _ CO2 (g) → _ (NH2)2CO(aq) + _H2O(l)
When 2.00 grams of ammonia, NH3, react with
excess CO2, what mass of water (in grams) is
formed?
1.06 g H2O
Limiting Reactant
• The reactant used up first in a
chemical reaction
▫ Also called limiting reagent
• After a reaction, excess reactants are
left
▫ These are reactants present in greater
quantities that remain after a reaction
takes place
Limiting Reactant
• Chair analogy:
It’s your lucky day! Mr. Nelson has hired you to
make chairs! The chairs follow the “reaction”:
4 legs + 1 seat + 10 screws → 1 chair
Here are the supplies you have: 10 legs, 5 seats,
and 100 screws. How many chairs be made?
What is the limiting reactant?
Limiting Reactant Simulation
• http://phet.colorado.edu/en/simulation/reacta
nts-products-and-leftovers
Limiting Reactant
•The limiting reactant can be
found by determining which
of the starting amounts
yields the least amount of
product
Limiting Reactant Example Problem
▫ Sodium chloride can be prepared by the reaction of sodium with
chlorine gas:
2Na (s) + Cl2 (g)  2NaCl (s)
▫ Suppose that 6.70 mol Na reacts with 3.2 mol Cl2
 How many moles of NaCl will be formed in this
reaction?
6.4 mol NaCl
 What is the limiting reactant? Cl2
Limiting Reactant Practice Problem
• 2.70 mol C2H4 is reacted with 6.30 mol of O2. Calculate the amount
in moles of water that forms, and identify the limiting reactant.
C2H4 (g) + 3O2 (g) 2CO2 (g) + 2H2O (g)
4.2 mol H2O
Limiting Reactant Example Problem
• Urea is prepared by reacting ammonia with carbon dioxide:
2 NH3 (g) + CO2 (g) → (NH2)2CO(aq) + H2O(l)
In one process, 637.2 g of NH3 are treated with 1142 g of CO2.
Calculate the mass of urea, (NH2)2CO, formed.
1122 g urea
Limiting Reactant Practice Problem
• Calculate the number of grams of NH3 produced by the
reaction of 5.40 g of hydrogen with 8.25 g of nitrogen.
The balanced equation is:
N2 + 3 H2 2 NH3
9.97 g NH3
Percent Yield
• Theoretical yield is the amount of
product that would result if all the
limiting reagent reacted
• Actual yield is the amount actually
obtained in lab from a reaction
▫ Actual yield is almost always less than
theoretical
Percent Yield
• Percent yield describes proportion of actual yield
to theoretical yield.
actual yield
Percent Yield 
x 100%
theoretical yield
Percent Yield Example Problem
• When 84.8 g of iron (III) oxide reacts with an excess of
carbon monoxide, 54.3 g of iron is produced. What is the
percent yield of this reaction?
Fe2O3 (s) + 3CO (g)  2 Fe (s) + 3CO2 (g)
91.6%
Percent Yield Practice Problem
• If 25 grams of carbon dioxide gas can be produced in the
following reaction, how many grams of sodium
hydroxide should be produced? If 10.7 grams of sodium
hydroxide are actually produced, what is the percent
yield?
NaHCO3  NaOH + CO2
22.7 g NaOH / 41.1%