CHEMISTRY: INTRODUCTION TO STOICHIOMETRY
Recipe for Success!
Chemistry is like cooking! How? Ingredients in a recipe make a certain quantity of food just like reactants
in a chemical reaction make a certain quantity of products.
The Ultimate Chocolate Chip Cookie
1 cup butter
1 cup sugar
1 cup brown sugar
2 eggs
<
>
4 Na (s) +
02
(g)
-
2 Na
0 (s)
2
1 tsp vanilla
2 cups flour
2 /2 cups oatmeal (thoroughly blended)
1 tsp baking soda
1 tsp salt
1 tsp baking powder
12 oz chocolate thips (one bag)
4-6oz
finely grated chocolate bar
1 V
2 cups chopped nuts (optional)
p
A balanced chemical equation shows us the “recipe” to use to make our product. The coefficients in the
equation tell us about the ratios or relative amounts of reactants and products So, by doubling,
tripling, or halving the reactants the resulting amount of products will be proportional. Look at the
cookie recipe above as an example. The recipe shows us that it takes one cup of butter to make one
batch of cookies or to make one batch of cookies we need one cup of butter. This proportion can be
shown as
1 batch cookies
=
1 c butter
1 batch cookies
Can you think of another proportion from the cookie recipe? Write one below.
1
c
butter
Now let’s look at the chemical equation. It shows us that 4 moles of solid sodium react with 1 mole of
oxygen gas to form 2 moles of solid sodium oxide. So 4 moles of sodium react with 1 mole of oxygen
or 1 mole of oxygen reacts with 4 moles of sodium
4 mol Na
1 mol 02
1 mol O
4 mol Na
This is called a mole ratio. Mole ratios show the relationships between reactant(s) and product(s) in a
balanced chemical equation Stoichiometry is the name given to the process of using these ratios in
chemistry to calculate the amount of a desired product or given reactant.
MOLE TO MOLE STOICHIOMETRY
Let’s look at a real life example.
Airbags, like the ones in your car, depend on stoichiometric precision for their design. Airbags that
under—inflate may not protect the occupants of the vehicle, but airbags that over—inflate can cause injury
to the occupants or may even rupture, making them useless. (Why would an airbag under-inflate or
over—inflate?) To make certain that airbags inflate to just the right amount the designers must use
principles of stoichiometry and chemical reactions. In other words, they have to make sure they use the
proper ingredients in the proper proportions.
, with
3
For most current systems, the “ingredients” (reactants) are a solid mixture of sodium azide, NaN
in the
which
nitrogen,
is
produced
an oxidizer. The gas (product) that inflates the bag is almost entirely
following decomposition reaction.
2 (g)
3 (s)
2 Na (s) + 3 N
2 NaN
Remember
this equation reads, “2 mo/es of solid sodium azide decomposes to form 2 mo/es of solid
sodium and 3 mo/es of nitrogen gas”. What are the ratios of reactant(s) and product(s)? List them below.
-
...
This reaction alone cannot inflate the bag fast enough (it must inflate within 0.1 second after impact),
and the sodium metal produced is a dangerously reactive substance. Oxidizers are included as a
reactant so that they can immediately react with the sodium metal. This is exothermic which raises the
temperature more than a hundred degrees so that the gas fills the bag faster.
3 Na20 (s) + 2 Fe (s)
3 (s)
0
2
6 Na (5) + Fe
Can you interpret this equation into words? What ratios are given in this equation?
-
Even sodium oxide is unsafe because it is an extremely basic (pH>>7) substance. Eventually, it reacts
with carbon dioxide, and moisture from the air to form sodium hydrogen carbonate, or baking soda, the
white residue often found on the occupants after an accident.
3 (5)
O (g)
2
2 NaHCO
0 (s) + 2 CO
2
Na
2 (g) + H
Can you interpret this equation into words? What ratios are given in this equation?
-
Now that we know the balanced equations for all of the reactions involved, we have our “recipes” to work
with. Let’s see what this stoichiometry stuff is all about.
2 3
NaN (s)
-
2 Na (s)
+
2 (g)
3 N
What if we need to produce 6 moles of nitrogen gas, N
2 (g)? How many moles of NaN
3 (s) would be
eeded to start the reaction? Look at our “recipe”
2 (g) are produced from 2 moles of
3 moles of N
3 (5) so
NaN
3
? mol NaN
=
6m’N
2 mol NaN
3
3 m2
‘1’
=
1
what we’re
okin for
the ratio from
our balanced
equation
4 mol NaN
3
amount
needed
6 Na (s) + 2
Fe (s) 4 3 Na
3
0
0
2
(5) +
2 Fe (s)
If we have 22 moles of sodium to begin with, how many moles of iron would be produced?
i-low many moles of sodium oxide would be produced if 8 moles of iron are produced
7
0 (s)
2
Na
+
2 CO
2 (g)
+
0 (g) 4 2 NaHCO
2
H
3
(5)
How many moles of water are needed to react with 1 7 moles of carbon dioxide?
How many moles of sodium hydrogen carbonate are produced from 8.4 moles of sodium oxide?
STOICHIOMETRY:
MOLE-MOLE PROBLEMS
Name
3
2NH
2
N + 3H
1. 2
How many moles of hydrogen are needed to completely react with Iwo moles
of nitrogen?
2KCI +
2KC10
2. 3
How many moles of oxygen are produced by the decomposition of six moles
of potassium chlorate?
2
2 + H
ZnC1
3. Zn + 2HCI
How many moles of hydrogen are produced from the reaction of three moles
of zinc with an excess of hydrochloric acid?
0
2
2 + 4H
3CC
4. 3
C + °2
8
H
How many moles of oxygen are necessary to react completely with four moles of
)?
8
H
3
propane (C
4
)
3
3 + AIPO
3KN0
5. K
0 + Al(N0
4
P
3
How many moles of potassium nitrate are produced when Iwo moles of potassium
phosphate react with Iwo moles of aluminum nitrate?