Chapter 11: Stoichiometry
Section 11.1:
Defining Stoichiometry
Objectives:
 Identify
the quantitative relationships in
a balanced chemical equation
 Determine the mole ratios from a
balanced chemical equation
What is stoichiometry?
 From
observing the demonstration,
what do you conclude is the reason the
reaction stops?
 How would you be able to determine the
amount of oxygen that was used up?
 How would you be able to determine the
amounts of products formed?
 Answers to both questions depends on
the change in mass of the _________ .
What is stoichiometry?
A
chemist knows that a chemical reaction
stops when one of the reactants is used
up.
 Therefore, the amounts of other
reactants used or products formed
depends on the amount of reactant that
is used up.
 Stoichiometric calculations allow us to
use this information to determine the
amounts of other reactants used or
products formed in a reaction.
What is stoichiometry?
 Stoichiometry
is the study of
quantitative relationships between
amounts of reactants used and the
products formed by a chemical reaction.
 It is based on the Law of Conservation
of Mass: the amount of matter present
at the end of a reaction is the same as
was present at the beginning.
 The total mass of the reactants equals
the mass of the products.
Interpretation of Chemical Equations
•The coefficients in a chemical equation
can be interpreted several ways.
•We have learned to interpret them in
terms of representative particles.
What happens is you multiply each
coefficient by 6.02 x 1023 particles?
Each coefficient is now “number of
moles”! We can, therefore, interpret
coefficients as numbers of moles!
Interpret this equation:
C3H8(g) + 5O2(g)
3CO2(g) + 4H2O(g)
The equation in terms of molecules = ?
The equation in terms of moles = ?
The equation in terms of mass = ?
Interpret this equation:
C3H8(g) + 5O2(g)
3CO2(g) + 4H2O(g)
The equation in terms of molecules = ?
The equation in terms of moles = ?
The equation in terms of mass = ?
44 g + 160 g = 132 g + 72 g
204 g =
204 g
Practice Problems
 Interpret
each equation in terms of
particles, moles, and mass. Show that
the Law of Conservation of Mass is
obeyed.
 4Fe
+ 3O2 --> 2Fe2O3
 N2 + 3H2 --> 2NH3
Mole Ratios
 In
the following reaction, how many
moles of Al will react with 3 moles of
Br2? How many moles of AlBr3 are
formed?
 2Al(s)
 The
+ 3Br2(l) → 2AlBr3(s)
relationships above can be
expressed as MOLE RATIOS.
 A Mole Ratio is a ratio between the
numbers of moles of any two
substances in a balanced chemical
equation.
Mole Ratios
+ 3Br2(l) → 2AlBr3(s)
 Mole ratios are:
2 mol Al and 3 mol Br2
3 mol Br2
2 mol Al
 2Al(s)
2 mol Al and 2 mol AlBr3
2 mol AlBr3
2 mol Al
3 mol Br2 and
2 mol AlBr3
2 mol AlBr3
3 mol Br2
Mole Ratios
 Mole
ratios are used as conversion
factors in stoichiometric calculations
 With the chemical equation and the
mole ratios, you can calculate the
amount used of any reactant in the
equation and the maximum amount of
product you can obtain.
Practice Problems
 Balance
the equation and determine all
the possible mole ratios.
 ZnO
+ HCl --> ZnCl2 + H2O
 Al + O2 --> Al2O3