A + B -> AB
Synthesis Reactions occur when two elements join together to produce one, more
complex compound. Complex simply means that the product compound has more
than the reactant molecules.
2Mg + O2 ---> 2MgO 2H2 + O2 ---> 2H2O
2K + Cl2 ---> 2KCl 2Fe + 3O2 ---> 2Fe2O3
Synthesis can also occur when two compounds make a more complex compound or
a compound and an element join together.
2Ca + 3CO2 ---> 2CaCO3 Na2O +CO2 ---> Na2CO3
AB --> A + B
A Decomposition Reaction is a reaction in which two compounds exchange parts with each other and
form two different compounds.
2HgO ---> 2Hg + O2
2H2O ---> 2H2 + O2
MgCl2 ---> Mg + Cl2
FeS ---> Fe + S
Decomposition Reactions also occur when one compound is split into two similar compounds.
CaCO3 ---> CaO + CO2
Na2CO3 ---> Na2O + CO2
2KClO3 ---> 2KCl + 3O2
Ba(ClO3)2 ---> BaCl2 + 3O2
A +BX --> B + AX
When one element replaces another element in a compound, a single replacement (also known as
displacement) reaction has taken place. When this happens a new compound is formed and an element
is released. The general formula for this reaction is A + BX --> AX + B.
A single replacement reaction will only take place if a more reactive element is replacing a less
Usually a single replacement reaction consists of a solid being added to a solution.
Just because a formula is written doesn't necessarily mean the reaction can take place. Single
Replacement reactions can not be reversed. Meaning, the reaction AX + B --> A + BX will not take
place. For example:
This equation works: Fe + 2HCl --> FeCl2 + H2
Whereas the reverse of this equation
will not take place:
H2 + FeCl2 --> Fe + 2HCl
Here are some more example equations:
Cu + 2AgNO3 --> Cu(NO3)2 + 2Ag
2Na + 2H2O --> 2NaOH + H2
AX + BY --> AY + BX
A double replacement reaction (also known as an exchange reaction or precipitation reaction) takes
place when two compounds interchange elements to form two new compounds.
The general formula for this reaction is AX + BY --> AY + BX.
In order for a double replacement reaction to take place, both of the reactants must be soluble in water.
To tell if a compound is soluble, we have to read the solubility rules, shown below. If a compound
contains at least one of the ions that is proven soluble, then the compound will be at least moderately
Next, if both reactants are soluble we must look at the products. One product must be soluble and one
must be insoluble. The insoluble product is the part of the product that will 'fall out' of the solution when
the reaction takes place. Usually a precipitate is a solid, but it doesn't have to be.
Solubility Rules for Ionic Compounds (p.s. DON'T PANIC! - you don't NEED to know this for my class)
|Ammonium, Group 1A, NH4+, Li+, Na+, K+, Rb+, Cs+||
|All ammonium and Group 1A (alkali metal) salts are soluble|
|Chlorides, Bromides, Iodides, Cl-, Br-, I||
|All common chlorides, bromides and iodides are soluble except AgCl, Hg2Cl2, |
PbCl2; AgBr, Hg2Br2, PbBr2; AgI, Hg2I2, PbI2
|Most sulfates are soluble; exceptions include CaSO4, SrSO4, BaSO4, and |
|All Chlorates are soluble|
|All perchlorates are soluble|
|All phosphates are insoluble except those of NH4+ and Group 1A elements |
(alkali metal cations)
Usually a double replacement reaction will take place when two solutions are mixed together.
Like single replacement reactions, not all reactions will necessarily take place. If all of the conditions
for the reaction are not met, the reaction will not happen. Likewise, the reverse of a double
replacement reaction will not take place either. Here is an example:
This reaction will take place: MgSO4 + Na2C2O4 --> MgC2O4 + Na2SO4
Whereas this reaction will not: MgC2O4 + Na2SO4 --> MgSO4 + Na2C2O4
Here are some more examples:
AgNO3 + KCl --> KNO3 + AgCl
BaCl2 + Na2SO4 --> BaSO4 + 2NaCl
Types of Reactions