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Balancing Chemical Equations
If you are starting with a word equation, be
sure to write each chemical formula
correctly.
 You must remember that elements
ending in - gen or - ine are diatomic!
Count how many of each type of atom is
present on each side of the equation.
Begin by balancing elements that are found in
one substance on each side.
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Balance hydrogen and oxygen last as they
sometimes balance out at the end.
If there is an odd number of an element on
one side and an even number on the other,
the odd number will need to be evened out so
try using a coefficient of 2 for that substance.
If there are polyatomic ions that remain
together as a unit during the reaction, count
the polyatomic ion as one unit.
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You are allowed to change coefficients but
never change subscripts!
 Subscripts come from the valence
(oxidation) numbers of the elements and
once a chemical formula is correctly
written, the subscripts become etched in
stone!
After balancing, put in the correct physical
states by using the solubility chart.
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States of Elements and Compounds
Ionic Compounds
 NaCl(s), NH4Cl(s), MgCl2(s)
Nonpolar Covalent Compounds
 H2(g), F2(g), Br2(l), CO2(g), O2(g), I2(s)
Polar Covalent Compounds
 H2O(l), C12H22O11(s), NH3(g), SO2(g)
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Elements
 Na(s), Al(s), Cu(s)
Types of Bonds (difference in
electronegativities)
 0.0 – 0.3
nonpolar covalent
 0.4 – 1.7
polar covalent
 1.8 – 4.0
ionic
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Synthesis Reactions
A synthesis or a direct combination reaction
has the general form:
A + B → AB
2H2(g) + O2(g) → 2H2O(g)
 Two elements in their elemental (free)
states form a compound.
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Important Notes to Remember
 Note that hydrogen and oxygen because
they end in - gen are diatomic.
 The reactant states are determined by
what state hydrogen and oxygen are at
room temperature (25°C, 298 K).
 Note that the subscripts are not changed
in attempting to balance the equation.
 Water can be in either the liquid or
gaseous state depending on the ambient
temperature.
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Decomposition Reactions
A decomposition or an analysis reaction has
the general form:
AB → A + B
2H2O(l) → 2H2(g) + O2(g)
 A compound is decomposed into its
constituent elements.
 See important notes for synthesis
reactions.
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Typical Decomposition Reactions
When metallic carbonates are heated,
metallic oxides and carbon dioxide are
formed.
 Metallic carbonates are ionic
compounds containing the carbonate
polyatomic (CO32-) ion.
 Metallic oxides are ionic compounds
containing the oxide (O2-) ion.
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Δ
 BaCO3(s) →
BaO(s) + CO2(g)
When metallic hydroxides are heated,
metallic oxides and water are formed.
 Metallic hydroxides are ionic
compounds containing the hydroxide
(OH-) ion.
Δ
 Ba(OH)2(s) →
BaO(s) + H2O(g)
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When metallic chlorates are heated,
metallic chlorides and oxygen are formed.
 Metallic chlorates are ionic
compounds containing the chlorate
(ClO3-) ion.
Δ
 2KClO3(s) → 2KCl(s) + 3O2(g)
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When some acids are heated, nonmetallic
oxides and water are formed.
 Nonmetallic oxides are covalent
(molecular) compounds containing oxide
(O2-).
 Examples are NO2, SO3, and CO2.
Δ
 H2SO3(aq) →
H2O(l) + SO2(g)
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When some metallic oxides are heated,
a metal and oxygen are formed.
Δ 2Hg(l) + O (g)
 2HgO(s) →
2
Some decomposition reactions are produced
by electrolysis (an electric current).
 2H2O(l) → 2H2(g) + O2(g)
 2NaCl(l) → 2Na(l) + Cl2(g)
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Single Replacement Reactions
A single replacement reaction has the
general form:
A + BC → AC + B
Zn(s) + Cu(NO3)2(aq) → Zn(NO3)2(aq) + Cu(s)
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Important Notes to Remember
 Note that elements (Zn, Cu) in their free
or uncombined state have no charge.
 Not all single replacement reactions
occur.
 The elemental metal, Zn, must be a more
reactive metal to replace the metal in the
ionic compound.
 To determine if a single replacement
occurs, check the Activity Series of the
Elements.
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More Single Replacement Reactions
Mg + HNO3 → ?
Mg(s) + 2HNO3(aq) → Mg(NO3)2(aq) + H2(g)
This reaction occurs because Mg is above H
on the Activity Series indicating that it is a
more active metal.
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More Single Replacement Reactions
Ag + Pb(NO3)2 → ?
Ag + Pb(NO3)2 → NR
This reaction does not occur because Ag is
below Pb on the Activity Series indicating that
it is a less active metal.
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Double Replacement Reactions
A double replacement reaction has the
general form:
AB + CD → AD + CB
Ba(OH)2(s) + H2SO4(aq) → CaSO4(s) + 2H2O(l)
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Important Notes to Remember
Not all double replacement reactions occur.
For such a reaction to occur, one of the
following must take place:
 A precipitate may be formed
(i.e. AgCl(s)).
 A gas may be formed (i.e. CO2(g))
 Water may be formed (i.e. H2O(l)).
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Combustion Reactions
Organic compounds (compounds of carbon)
contain either carbon and hydrogen
(hydrocarbons) or carbon, hydrogen, and
oxygen.
CO and CO2 are not considered organic
compounds.
When such compounds burn completely in
excess oxygen, combustion occurs and the
products are always CO2(g) and H2O(l).
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Combustion Reactions
When such compounds burn without
sufficient oxygen, combustion occurs and the
products are always CO(g) and H2O(l).
When balancing combustion reactions, follow
these steps in order:
 balance the carbons.
 balance the hydrogens.
 balance the oxygens.
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Balancing Combustion Reactions
C6H12O6 + O2 →
C6H12O6 + O2 → CO2 + H2O
C6H12O6 + O2 → 6CO2 + H2O
C6H12O6 + O2 → 6CO2 + 6H2O
C6H12O6 + 6O2 → 6CO2 + 6H2O
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)
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Balancing Combustion Reactions
C8H18 + O2 →
C8H18 + O2 → CO2 + H2O
C8H18 + O2 → 8CO2 + H2O
C8H18 + O2 → 8CO2 + 9H2O
25
C8H18 +
O2 → 8CO2 + 9H2O
2
2C8H18(g) + 25O2(g) → 16CO2(g) + 18H2O(l)
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What Must Balance In A Reaction?
ammonium carbonate + calcium choride → ?
ammonium carbonate + calcium choride →
ammonium chloride + calcium carbonate
(NH4)2CO3 + CaCl2 → NH4Cl + CaCO3
(NH4)2CO3(aq) + CaCl2(aq) →
2NH4Cl(aq) + CaCO3(s)
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Atoms
2 N atoms → 2 N atoms
8 H atoms → 8 H atoms
1 C atom
→ 1 C atom
3 O atoms → 3 O atoms
1 Ca atom → 1 Ca atom
2 Cl atoms → 2 Cl atoms
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Mass
(NH4)2CO3(aq) + CaCl2(aq) →
2NH4Cl(aq) + CaCO3(s)
96.11 g + 110.98 g = 107.00 g + 100.09 g
207.09 g = 207.09 g
Moles
2 mol reactant ≠ 3 mol product
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