BEHS Chemistry

Predicting Types of Chemical Reactions

COMPOSITION Reactions R + S ----> RS
For example: 2Na + Cl2 ---> 2NaCl
Any time there are just two elements listed as the reactants (left side of arrow) you have a composition reaction.

DECOMPOSITION Reactions RS ----> R + S
For example: 2MgO --->2 Mg + O2
Any time there is just one compound as the reactant you have a decomposition reaction.

There are five different ways that componds can decompose, the way that they do depends on their composition

The Rules:
a.. metallic carbonates, when heated, decompose into metallic oxides and carbon dioxide
Any metal compound that that ends with CO3 is a carbonate.
For example: CaCO3 ---> CaO + CO2

b. metallic hydroxides, when heated, decompose into metallic oxides and water
Any metal compound that that ends with OH is a hydroxide.
For example: Ca(OH)2 ---> CaO + H2O

c. metallic chlorates, when heated, decompose into metallic chlorides and oxygen
Any metal compound that that ends with ClO3 or ClO4 is a chlorate.
For example: Ca(ClO3)2 ---> CaCl2 + 3O2

d. some acids, when heated, decompose into nonmetallic oxides and water
All compounds that begin with Hydrogen are acids. (see exceptions in naming schemata)
For example: H2SO3 ---> SO2 + H2O (These are very rare)

e. some oxides, when heated, decompose
Any compound that ends with the second elemnt being oxygen, is called an oxide.
For example: 2Na2O ---> 4Na + O2

SINGLE-REPLACEMENT Reactions T + RS ----> TS + R
Any time you see a single element reacting with a compound it is a single replacement reaction.
The single element always* changes place with the first element in the compound. *(except for Halogens, see "d")

There are four ways elements replace elements in these reactions.

a. replacement of a metal in a compound by a more active metal
For example: K + NaCl ---> KCl + Na

b. replacement of hydrogen in water by metals
For example: Mg + H2O ---> MgO + H2

c. replacement of hydrogen in acids by metals
For example: Mg + 2HCl ---> MgCl2 + H2

*d. replacement of a halogen by a more active halogen
For example: F2 +2 HCl --->2HF + Cl2

Any time that you have two compounds reacting you have a double replacement reaction.
The first elements trade places with eachother.
For Example: CuSO4 + MgCO3 ---> MgSO4 + CuCO3

COMBUSTION Reactions CxHy + O2 ----> CO2 + H2O
All combustion reactions follow the same format. They will all start with a hydrocarbon (an element composed
of only hydrogen and carbon) and oxygen.
For Example: C6H12 + 9O2 --->6CO2 + 6H2O


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