Mr. Shields
Regents Chemistry
U11 L01
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Stoichiometry
Before we begin …
What exactly does Stoichiometry mean?
Stoichiometry (definition)
The study of quantitative relationships
Between the amounts of reactants used &
Products formed in a chemical reaction.
For example: CH4 + 2O2  CO2 + 2H20
i.e
1 part
+ 2 parts = 1 part + 2 parts
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Now we need to discuss what a chemical reaction is.
(We’ll Get into the number stuff later)
Chemical Reactions occur all around us every day.
Can you think of some?
1. Rusting metal
2. Burning wood in your fireplace
3. Using electricity from batteries
4. Running your car engine
5. Thinking / feeling
6. The browning of paper in sunlight
7. Cooking
8. Leaves changing color
9. Oxygenation of Blood
10.Food digestion & Growing
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And these are only a few of many possible examples
Chemical equations
Let’s consider the rusting of iron.
We can state what happens in words. How would we
describe this process?
Iron reacts with Oxygen to form Rust.
How would we describe the burning of
Gas in your cars engine?
Gasoline reacts with oxygen to form carbon dioxide
Water, and heat
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What is the common thread in these reactions ?
One or more substances are being converted into
other substances
When we write chemical reactions …
FOR EXAMPLE:
Fe + O2  Fe203 (rust)
Those things listed on the left side of the
chemical equation are known as …
- REACTANTS
Substances on the right side of the equation are…
- PRODUCTS
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Reactants are substances we start with and
Products are substances we end up with & which
have different physical and chemical properties
from the reactants
For example “Gasoline reacts with oxygen to form
carbon dioxide, Water, and heat”?
What are the reactants and what are the products?
Note that the properties of these substances before
reaction are different than the properties of the
substances after reaction
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How do we separate reactants and products in
chemical reactions ?
Yep … By using an arrow (  )
This arrow is more important than just separating
Reactants from Products. What else does it tell us?
It tells us the DIRECTION of the reaction path
going this way guys 
Or … even this way 
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In our example …
“Iron reacts with Oxygen to form rust”
How do we write this, not in words, but as a
Chemical equation?
Exactly … Fe + O2  Fe2O3 What are the reactants
And what are the products
The arrow is read as “yields”
Look at this equation closely. What’s wrong with it?
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Conservation of Mass
In our example “Fe + O2  Fe2O3” we have 1 atom
Of iron yielding 2 atoms of iron and 2 atoms of
Oxygen yielding 3 atoms of oxygen.
What law does this equation violate and what does
The law state?
The law of conservation of mass:
“Matter can neither be created nor destroyed in
ordinary chemical reactions”
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Conservation of Mass
Since our example of a chemical reaction
Fe + O2  Fe2O3
violates the law of conservation of mass the
Equation is said to be “NOT BALANCED”
The equation as written tells us WHAT WE NEED or
have produced but NOT HOW MUCH we need of
each reactant or how much of each
Product is produced
THIS IS KNOWN AS A “SKELETON EQUATION”
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Let’s look at this equation a little more closely.
Fe + O2  Fe2O3
But instead of dealing with atoms and molecules
Let’s discuss this equation in terms of MOLES
-Do you remember what a mole is ??
-1 mole of anything = 6.02 x 1023
Mole
In terms of MOLES the above equation says:
1 mole of Fe reacts with 1 mole of oxygen to yield
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1 mole of Iron (III) oxide.
1 mole Fe + 1 mole O2  1 mole Fe2O3
But we know this can’t be! Why?
For one, 1 mole of iron can’t create 2 moles of iron
Why are there 2 moles of iron ??
Now Let’s look at this equation from a mass point of view:
1 mole
1 mole
1 mole
Fe +
of Fe
= 55.85 g
of O2
= 31.98 g
of Fe2O3 = 159.64 g
O2  Fe2O3,
55.85g + 31.98g =
159.64g ???
No way!
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Fe +
O2

55.85g + 31.98g =
(87.83g)
Fe2O3,
159.64g
The mass of the
Reactants DOES NOT
Equal the mass of the
Products!
So whether we look at moles or mass this chemical
Eqn. doesn’t obey the Law of Conservation of Mass!
To do so the # of atoms on the reactant side MUST
EQUAL the number of atoms on the product side
#R atom #P
1
2
Fe
O
2
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To satisfy the Law of Conservation
Of mass we will need to BALANCE
This equation.
We need to make the #R atoms & #P
Atoms equal
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Note… Coefficients in balanced chemical
equations must always be WHOLE
NUMBERS!
4Fe +
3O2

2Fe2O3
And Not fractions
2Fe +
1 ½ O2

Fe2O3
But we can use this as an intermediate step to obtain the above! How?
In our next lesson we’ll see how we
go about balancing chemical equations!
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