Chemical Reactions and
Stoichiometry
Ms. Grobsky
Wrapping Up the Types of Chemical Reactions…
• We just got done investigating the different types of chemical
reactions
• We can now answer the two questions that are a central focus
in chemistry:
– When two substances are mixed, will a chemical reaction
occur?
• Yes, if at least one of the four driving forces are present:
–
–
–
–
Transfer of electrons
Formation of a gas
Formation of a precipitate (solid)
Formation of water
– If a chemical reaction occurs, what will the products be?
• Products can be predicted based on the type of chemical reaction
present:
– SR, synthesis, decomposition, combustion, double replacement – precipitation,
double replacement - neutralization
Making the Connection…
• In this unit, you have also learned that chemical
reactions follow the Law of Conservation of
Matter
– All atoms present going in MUST COME OUT!
– This was done by balancing the chemical equation for
the reaction using coefficients
• Coefficients represent number of moles
• Remember, chemical equations tell us what
amounts of reactants to mix and what amounts of
products to expect for a particular reaction
– Kind of like a chemist’s recipe
• Therefore, it is necessary to be able to quantitatively
calculate the quantity of reactants needed to make the
products in a reaction
So, What is Stoichiometry?
• When you know the quantity of one substance
in a reaction, you can calculate the quantity of
any other substance consumed or created in
the reaction
– Quantity meaning the amount of a substance in
grams, liters, molecules, or moles
• This process – the calculation of quantities in
chemical reactions by looking at a chemical
equation - is called stoichiometry
– Greek for “measuring elements”
Example
N2(g) + 3H2(g)  2NH3(g)
• This equation can be read as:
“1 mole of N2 reacts with 3 moles of H2 to produce 2 moles of NH3”
• N2 and H2 always react in this mole ratio
– So, if you start with 10 moles of nitrogen gas and 30
moles of hydrogen gas, you will form 20 moles of
ammonia
In General…
• Use the coefficients from the balanced chemical
equation to make mole ratios
– Ratios of balanced coefficients = mole ratios
– Makes connections between reactants and products
• Using this information, you can calculate the
amounts of the reactants involved and the
amount of product you might expect
• Keep in mind that any calculation done with this
process is a theoretical value
– The real world isn’t always perfect
Converting From Mole to Mole Practice!
• The following reaction shows the
synthesis of aluminum oxide:
3O2(g) + 4Al(s)  2Al2O3(s)
• If you wanted to produce 24 moles of
product, how many moles of each
reactant would you need?
Given: 24 moles of Al2O3
Unknown: ____ moles of Al
____ moles of O2
• Need 2 separate equations to solve for
the unknowns:
3O2(g) + 4Al(s)  2Al2O3(s)
24 mol Al2O3
24 mol Al2O3
4 mol Al
2 mol Al2O3
3 mol O2
2 mol Al2O3
= 48 mol Al
= 36 mol O2
Practice for You…
1. How many moles of hydrogen will be
produced if 0.44 mol of CaH2 reacts
according to the following equation?
CaH2 + 2H2O  Ca(OH)2 + 2H2
(.89 mol H2)
2. Iron will react with oxygen to produce
Iron III oxide. How many moles of Iron III
oxide will be produced if 0.18 mol of Iron
reacts?
(.090 mol Fe2O3)
USING STOICHIOMETRY TO
CONVERT FROM MASS TO MASS
OR OTHER UNITS
A LOOK AT MASS – MASS CALCULATIONS
• No lab balance measures moles directly
– Generally, mass is the unit of choice
• From the mass of 1 reactant or product,
the mass of any other reactant or product
in a given chemical equation can be
calculated
– You must have a balanced reaction equation!
• As in mole-mole calculations, the
unknown can be either a reactant or a
product
Basic Steps to Solve Mass-Mass
Stoichiometry Problems
1. Balance the equation
2. Get to moles
- Convert mass to moles
3. Set up mole ratios by looking at
coefficients in equation
- Use mole ratios to calculate moles of desired
compound
4. Convert moles to grams, if necessary
PRACTICE!
Acetylene gas (C2H2) is produced by adding
water to calcium carbide (CaC2):
CaC2 + 2H2O  C2H2 + Ca(OH)2
How many grams of C2H2 are produced by
adding water to 5.00 g CaC2?
MASS – MASS CALCULATIONS 1
• What do we know?
– Given mass = 5.0 g CaC2
• What are we asked for?
– grams of C2H2 produced
Step 1 - “Get to Moles!”
In this case that can be done by using the
Molar Mass of your given compound:
5.0 g CaC2
1 mol CaC2
64.0 g CaC2
= .07813 mol
CaC2
Step 2 - Convert from moles of our given to
moles of unknown using the mole ratio:
1 mol C2H2
.07813 mol
CaC2 1 mol CaC
2
= .07813 mol
C2H2
Step 3 - Since we are asked for mass of our
unknown in this problem, we need to use our
molar mass of our unknown and convert our
newly calculated moles into grams:
26.0 g C2H2
.07813 mol
C2H2 1 mol C H
2 2
General Summary of 3 Steps:
1. Get to Moles
2. Mole Ratio
3. Get to desired final unit
= 2.03 g C2H2
Expanding Stoichiometry to Other Units
• Recall, the coefficients in a balanced
reaction equation indicates the relative
numbers of moles of reactants and
products
• We can expand our stoichiometric
calculations to include any unit of measure
that is related to the mole
– The given quantity can be expressed in numbers of
particles, units of mass, or volumes of gases at STP!
• The problems can include mass-volume,
volume-volume, and particle-mass
calculations
General Steps to Solving ALL Stoich
Problems
• For any typical stoichiometry problems,
follow the steps below:
1. Convert given quantity to moles
2. Then, use the mole ratio from the
balanced equation to convert from the
moles of given to the number of moles of
the unknown
3. The moles of the unknown are converted
to the units that the problem requests
Practice!
• How many molecules of O2 are
produced when a sample of
29.2 g of H2O is decomposed by
electrolysis?
2H2O  2H2 + O2
MORE MOLE EXAMPLES
• What do we know?
– 29.2 g H2O
• What are we asked for?
– Molecules of O2
Mass A  Mols A  Mols B 
29.2 g 1 mol H2O
H2O
18.0 g H2O
Molecules B
1 mol O2
2 mol H2O
6.02x1023
molecules O2
1 mol O2
= 4.88 x 1023
molecules O2
Practice!
• The last step in the production of
nitric acid is the reaction of NO2
with H2O:
3NO2+H2O2HNO3 + NO
How many liters of NO2 must react
with water to produce 5.00x1022
molecules of NO at STP?
MORE MOLE EXAMPLES
• What do we know?
– 5.0x1022 molecules NO
• What are we asked for?
– Liters of NO2 at STP
Molecules A Mols A Mols B Volume B
5.0x1022 molecules
NO
1 mol NO
3 mol NO2
6.02x1023 molecules
NO
1 mol NO
22.4 L NO2
1 mol NO2
= 5.58 L NO2