Reaction Classification
There are 5 general types of reactions
1. Decomposition
2. Synthesis
3. Double displacement
4. Single displacement
5. Combustion
Decomposition

Decomposition = one compound  two (or more pieces).
AB  A + B
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
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Pieces can be elements or simpler compounds
i. Element examples:
1. HgO  Hg + O2
2. H2O  H2 + O2
3. MgCl2  Mg + Cl2
4. FeS  Fe + S
ii. Simpler compound examples
1. CaCO3  CaO + CO2
2. Na2CO3  Na2O + CO2
3. KClO3  KCl + O2
4. Ba(ClO3)2  BaCl2 + O2
iii. Acids and bases
1. (base) Ca(OH)2  CaO + H2O
2. (base) NaOH  Na2O + H2O
3. (acid) HNO3  N2O5 + H2O
4. (acid) H3PO4  P2O5 + H2O
Notice how, in every case so far, there is only one substance on the lefthand (reactant) side. This is always the case in a decomposition reaction.
Rules for figuring out what the products are for decomposition
i.
ii.
iii.
iv.
v.
All binary compounds (example i)  elements
All carbonates (1, 2 in example ii)  the oxide and carbon dioxide.
Chlorates (like 3, 4 in example ii)  binary salt and oxygen.
Bases(like 1, 2 in example iii)  the oxide of the metal plus water.
Acids (like 3, 4 in example iii)  the oxide of the nonmetal plus water.
Single Displacement

Single displacement, one element replaces another element in a compound.


One reactant is always an element. It does not matter if the element is written
first or second on the reactant side. The other reactant will be a compound.
Two possibilities:
1. Cations switch.
AX + Y ---> YX + A


Element Y replaced A (in the compound AX) to form a new compound
YX and the free element A. Remember that A and Y are both cations
(postively-charged ions) in this example.
Examples
1.
2.
3.
4.
Cu + AgNO3 ---> Ag + Cu(NO3)2
Fe + Cu(NO3)2 ---> Fe(NO3)2 + Cu
Ca + H2O ---> Ca(OH)2 + H2
Zn + HCl ---> ZnCl2 + H2
2. Anions switch places:
A + XY ---> XA + Y
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Element A has replaced Y (in the compound XY) to form a new
compound XA and the free element Y. Remember that A and Y are
both anions (negatively-charged ions) in this example.
Examples
1. Cl2 + NaBr ---> NaCl + Br2
2. Br2 + KI ---> KBr + I2

How to predict single displacement reactions:
1.
2.
3.
4.
Decide if the reactant element is normally positive or negative
Identify the opposite charged portion of the compound and its charge
Write a formula using information from step one & two.
Write the left over element as the second product
Double Displacement
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
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Double displacement, the cations and anions of two different compounds switch
places.
Both reactants are compounds, each with a cation part and an anion part.
Diatomic elements do not count; they are included in the single displacement
category.
AB + XY ---> AY + XB


A and X are the cations (postively-charged ions) in this example, with B and Y
being the anions (negatively-charged ions).
Examples:
1.
2.
3.
4.

KOH + H2SO4 ---> K2SO4 + H2O
FeS + HCl ---> FeCl2 + H2S
NaCl + H2SO4 ---> Na2SO4 + HCl
AgNO3 + NaCl ---> AgCl + NaNO3
Examples (Acids and Bases):
1. CaCO3 + HCl ---> CaCl2 + CO2 + H2O
2. K2SO3 + HNO3 ---> KNO3 + SO2 + H2O
3. NH4Cl + NaOH ---> NaCl + NH3 + H2O
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Notice how, one of the two product compounds decomposes.
According to rule above, reaction 1 should have been
CaCO3 + HCl ---> CaCl2 + H2CO3


H2CO3 is a product compound in this eqtn.
Certain compounds (H2CO3, H2SO3, or NH4OH) will decompose as products.
CaCO3 + HCl ---> CaCl2 + H2CO3  CaCl2 + CO2 + H2O

How to predict double displacement reactions
1. Identify the cations and anions in each compound
2. Pair up each cation with the anion from the OTHER compound
3. Write two new (CORRECT!!) formulas using the pairs from step two.
Synthesis
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
Synthesis are, at this introductory level, almost always the reverse of a
decomposition reaction.
Two pieces  one, more complex compound.
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Complex means the product compound has more atoms than the reactant
molecules. Usually!!
Pieces can be elements or simpler compounds.
A + B ---> AB


Examples: two elements are combining
1. Mg + O2 ---> MgO
2. H2 + O2 ---> H2O
3. K + Cl2 ---> KCl
4. Fe + O2 ---> Fe2O3
Examples: two compounds making a more complex compound (or a
compound and an element joining together):
1.
2.
3.
4.

CaO + CO2 ---> CaCO3
Na2O + CO2 ---> Na2CO3
KCl + O2 ---> KClO3
BaCl2 + O2 ---> Ba(ClO3)2
Advanced Examples:
 two products. Here's an example:
1. CO2 + H2O ---> C6H12O6 + O2

Rxn’s that don’t happen:
2. H2 + O2 ---> H2O2

Predicting synthesis reactions
 Opposite of decomposition
1.
2.
3.
4.
5.

Union of elements  binary compounds
Metallic oxide + carbon dioxide  carbonates
binary salt + oxygen  Chlorates
Metallic oxide + water  Base
Non-metallic oxide + water  Acid
a. CaO + H2O ---> Ca(OH)2
b. Na2O + H2O ---> NaOH
c. N2O5 + H2O ---> HNO3
d. P2O5 + H2O ---> H3PO4
How to predict synthesis reactions
1. Ask yourself what type of decomposition produces these products
2. Write the reactant formula using the compounds from step one
Combustion



Combustion, at its most general, can mean the reaction of oxygen gas (O2)
with anything.
However, we define combustion as the reaction of oxygen with a compound
containing carbon and hydrogen.
A common synonym for combustion is burn.
CxHy + O2 ---> CO2 + H2O
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
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Examples:
1. CH4 + O2 ---> CO2 + H2O
2. C2H6 + O2 ---> CO2 + H2O
3. C6H12O6 + O2 ---> CO2 + H2O
4. C2H5OH + O2 ---> CO2 + H2O
Notice that some compounds contain carbon, hydrogen AND oxygen.
The products are all the same, in every reaction.
Variations include NO2 and SO2 Like this:
1. C21H24N2O4 + O2 ---> CO2 + H2O + NO2
2. C2H5SH + O2 ---> CO2 + H2O + SO2

There are complexities with combustion as you get deeper into it.
o i.e. Not enuf O2  CO instead of CO2

How to predict combustion reactions:
1. Identify the reaction as combustion
2. Know that the combustion products . . . are always CO2 and H2O
(possibly SO2 or NO2