balancing chemical equations...balancing equations 1. list the elements in alphabetical order 2....
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Balancing Chemical Equations
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Essential Standard 2.2Understand chemical bonding and
chemical interactions.
Learning Objective 2.2.4
Exemplify the law of conservation of
mass by balancing chemical
equations.
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I Can StatementsAt the end of this lesson, you should be
able to say, with confidence:
• I can state what the Law of Conservation
of Mass is and explain what it means.
• I can balance chemical equations.
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Glucose & Oxygen
Chemical ReactionsChemical Reactions are a chemical change
in which one or more substances are
converted into new substances
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Reactants and Products
Reactants are the substances that
react and products are the substances
that are produced.
CO2 + H2O C6H12O6 + O2
Reactants Products
Chemical reactions are expressed as
chemical equations.
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Conservation of MassThe Law of Conservation
of Mass states that in a
chemical reaction, matter
is not created or destroyed,
only transformed.
In this example, the mass of the
candle and oxygen gas before
the candle is burned is the same
as the mass of the candle and
carbon dioxide gas, after the
candle is burned.
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Rearrangement of AtomsDuring a chemical reaction, no new atoms
are added, they are only rearranged.
CO2 + H2O C6H12O6 + O2
1 Carbon
2 Hydrogen
3 Oxygen
The above reaction could not take place
because you have more carbon, hydrogen,
and oxygen atoms in the products than were
in the reactants.
6 Carbon
12 Hydrogen
8 Oxygen
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Balanced ReactionsIn order to know which reactions would take
place, we have to balance the equations.
The number of each type of atom on the
reactant side must equal the number for each
type of atom on the produce side.
6CO2 + 6H2O C6H12O6 + 6O2
6 Carbon
12 Hydrogen
18 Oxygen
6 Carbon
12 Hydrogen
18 Oxygen
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Balanced ReactionsWhat this means is that in order for the
photosynthesis reaction to occur, there
must be 6 CO2 molecules and 6 H2O
molecules, in order to produce 1 glucose
molecule and 6 O2 molecules.
6CO2 + 6H2O C6H12O6 + 6O2
6 Carbon
12 Hydrogen
18 Oxygen
6 Carbon
12 Hydrogen
18 Oxygen
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CoefficientsThe numbers written in front of molecule are
called coefficients.
Coefficients are used to balance equations.
6CO2 + 6H2O C6H12O6 + 6O2
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Coefficients & SubscriptsWhen balancing chemical equations, you can
change the coefficients, but you cannot
change the subscripts.
6CO2
The 6 indicates that there are 6 molecules
of CO2
CO2 CO2 CO2 CO2 CO2 CO2
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SubscriptsThe subscript, 2, signifies that within a CO2,
molecule there will always be 2 oxygen atoms
for every carbon atom. (it’s a ratio of 1:2)
6CO2
Even with the 6 coefficient added, the ratio of
atoms within the molecule does not change.
6 C 12 O
It is still a 1:2 ratio.
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Balancing Equations
1. List the elements in alphabetical order
2. Write down the number of atoms per element
2. Find a common numerator and write it into the
reaction as a coefficient
3. Change the number for all elements that are
affected
4. Do the same for the remaining elements
There are five basic steps to balance any
chemical equation:
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Balancing EquationsThis chemical reaction occurs when propane
is burned in the presence of oxygen and water
and carbon dioxide is produce.
Propane Oxygen Water Carbon
Dioxide
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First StepList the elements for the reactants and the
products in alphabetical order.
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Second StepWrite down the number of atoms per element.
Notice that the oxygen atoms from the H2O and CO2
were added together on the product side.
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Third StepFind a common numerator and add it to the
reaction as a coefficient.
Beginning with the first element, C, a common
numerator for 3 and 1 would be 3.
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Third StepFind a common numerator and write it into the
reaction as a coefficient.
After the 3 is written as a coefficient, change the number
of carbons on the product side to 3.
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Fourth StepChange the number for all the elements that
are affected.
The oxygen atom was also affected, so now there are
7 oxygen atoms on the product side.
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Fifth StepDo the same for the rest of the elements.
A common numerator of 8 and 2 is 4, so a 4 is written
on the product side as a coefficient in front of H2O.
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Fifth StepDo the same for the rest of the elements
The oxygen atom in the H2O was also affected, so
now there are 10 oxygen atoms on the product side.
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Fifth StepDo the same for the rest of the elements
A common numerator of 10 and 2 is 5, so a 5 is written
on the reactant side as a coefficient in front of O2.
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Check Your WorkThe equation is balanced when the number of
each type of atom on the reactant side equals the
number of each type of atom on the product side.
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Practice
HgO Hg + O2
Balance the following equation:
Seriously, try it.
Then, check your answer.
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Practice
HgO Hg + O2
1 Hg
1 O
1. List the elements in alphabetical order
1 Hg
2 O
2. Write down the number of atoms per element
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Practice
HgO Hg + O2
1 Hg
1 O
1 Hg
2 O
2 is a common numerator of 1 and 2, so write in
a 2 as a coefficient for HgO on the reactant side.
Hg is already
balanced, so move
on to O.
3. Find a common numerator and add it to the
equation as a coefficient
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Practice
2 HgO Hg + O2
4. Change the number for all the elements
affected
2 Hg
2 O1 Hg
2 OOn the reactant side, both the Hg and the O
were affected.
The equation is now balanced.
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Last ThingIf there is an odd number of any of the atoms,
multiply them together to find the common
numerator.
3H + O H2O
3 H
1 O2 H
1 O
3 x 2 = 6.
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Last ThingBut to get the numerator of 6, you multiply the
3 on the reactant side by 2.
6H + O 3H2O
(2) 3 H
1 O
(3) 2 H
1 O
Then, multiple the 2 on the product side by 3.
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Then, Fix the RestBut to get the numerator of 6, you multiply the
3 by 2 on the reactant side.
6H + 3O 3H2O
6 H
3 O
6 H
3 O
The equation is now balanced.
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The End