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Balancing Chemical
Equations
Balancing Chemical
Equations
Trial and Error Algebra
• Balancing chemical equations is
based on the conservation on matter
– We must finish our chemical reaction
with as many atoms of each element
as when we started
– The mass of the parts equals the mass
of the whole
Balancing Chemical
Equations
Balancing Chemical
Equations
• Why balance chemical equations?
• H2 + O2
– Not saying how much of each element was
going to react would run the risk of violating
the conservation of mass
• Saying that the mass of what we end up with is
different than the mass of what we started with.
– The same as a recipe that doesn’t specify
how much of each ingredient is needed to
make the chili
H2O
– This is an unbalanced equation
• This means that there are UNEQUAL
numbers of least one atom on each side of
the arrow
• In this case there are more oxygen atoms
on the left (reactant) side than the right
(product) side
• Almost like units,
Balancing Chemical
Equations
Balancing Chemical
Equations
• H2 + O2
• H2 + O2
H2 O
H2 O
– How many hydrogen atoms are there on the
reactant side?
– How many oxygen atoms are there on the
reactant side?
– How many hydrogen atoms are there on the
product side?
– How many oxygen atoms are there on the
product side?
– The amount of hydrogen atoms are balanced!
– The amount of oxygen atoms are unbalanced!
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Balancing Chemical
Equations
Balancing Chemical
Equations
• An equation is balanced by changing
coefficients
• Add a coefficient and count the atoms
using the distributive property!
– Only the coefficients can be changed,
NEVER a subscript
– 2H2O
• Hydrogen atoms = 2 x 2 = 4
• Oxygen atoms = 2 x 1 = 2
– No subscript = 1
• Total atoms = 6
Balancing Chemical
Equations
Balancing Chemical
Equations
• Charcoal used in a
grill is carbon
• The carbon reacts
with oxygen to
produce carbon
dioxide
• Count the atoms using the distributive
property!
– 2CH4O2
•
•
•
•
Hydrogen atoms = 2 x 4 = 8
Oxygen atoms = 2 x 2 = 4
Carbon atoms = 2 x 1 = 2
Total atoms = 14
Balancing Chemical
Equations
Balancing Chemical
Equations
• The chemical equation for this
reaction
• To balance the example equation
– C + O2
CO2
– Says the same thing as the sentence
– And adds quantitative meaning as well
– Is this reaction balanced?
– H2 + O2
H2 O
• Hydrogen is balanced, but oxygen is not
• Both must be balanced
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Balancing Chemical
Equations
Balancing Chemical
Equations
• Placing a coefficient of two in front of
the water molecule balances
oxygen.
• To balance, place a coefficient of
two in front of the hydrogen on the
left side.
• H2 + O2
2 H2O
2 H2 + O2 2H2O
– This balances the equation
• However, this causes hydrogen to
become unbalanced
Things You CANNOT Do
Things You CANNOT Do
•
•
You cannot change a subscript
You cannot change the oxygen's
subscript in water from one to two, as
in:
• H2 + O2
H2O2
– True, this balances the equation, but
H2O2 (hydrogen peroxide) is a
completely different substance from
H2 O
You cannot place a coefficient in
the middle of a formula
–
–
The coefficient goes at the beginning
of a formula, not in the middle, as in
•
H2 + O2
–
H22O
This is the nonsensical raving of a lunatic
mind
Things You CANNOT Do
Things You CANNOT Do
•
• 4 H2 + 2 O2
Like fractions, your answer is in
simplest terms
–
For example, this equation is
balanced:
•
•
4 H2 + 2 O2
4 H2O
However, all the coefficients have the
common factor of two
4 H2O
• Divide through to eliminate common
factors like this
• Divide each coefficient by two gets
• 2H2 + O2
2H2O
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Things You CANNOT Do
Balancing Chemical
Equations
• The equation is correctly balanced,
but it is not the BEST answer
• The best answer is in simplest terms
• Do not argue this with your teacher if
you happen to make this mistake
• Step Uno:
Balancing Chemical
Equations
Balancing Chemical
Equations
• Al + O2
Al2O3
– Those people are obtuse; ignore them
– You're drawing those boxes so that
you'll be sure not to mess around with
the formulas to balance the equation
– While they all suffer in the pits of
academic purgatory, you'll be laughing
from the honor roll
– Draw boxes around all the
chemical formulas
• Everything in the box is
“untouchable”
– This is frequently skipped by
people because they feel that it's a
stupid and or silly thing to do
Step Two-o:
– Make an element
inventory
– How many atoms
of each type in the
reactant and
product side
– Al + O2
Al2O3
Element
Reactant Product
Al
1
2
O
2
3
Balancing Chemical
Equations
Balancing Chemical
Equations
• Step Three-o:
Step Four-o:
– Play with your coefficients
• Numbers placed in front of the boxes
• Remember, the stuff inside the box is
“untouchable”
4 Al
+ 3 O2
2 Al2O3
– Make an atom
inventory
– How many atoms of
each type in the
reactant and
product side
– 4Al + 3O2
2Al2O3
Element
Reactant Product
Al
4
4
O
6
6
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Balancing Chemical
Equations
2 Al + ___
3 Br2
___
1 Al2Br6
___
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