Lesson 3: Chemical Reactions and Equations

1.Single Replacement

In these reactions, one of the elements in the compound is replaced by another element. 

Cu(s) +AgNO₃(aq)--> Ag(s) +Cu(NO₃)₂(aq) 

2. Double Replacement

In these reactions, both the metal and nonmetal will be replaced between two compounds. 

Mg(OH)₂(s) + HCl(aq)--> MgCl₂(aq) +HOH(l) 

3. Synthesis Reaction

A synthesis reaction, in the simplest sense, involves elements as reagents and the formation of a compound (a substance composed of more than one element) as the product, often as the only product. 

Fe  +  S  --->  FeS
Iron + Sulfur = Ion sulfide

4. Decomposition 

Decomposition reactions are chemical reactions in which chemical species break up into simpler parts. 

E.g. 2NH₃ → N₂ + 3H₂

Compounds don't need to break down into elements in a decomposition reaction. 
E.g. 
(NH₄)₂CO₃(s) →2NH₃(g)+CO₂(g) +H₂O(g)

5. Hydrocarbon Combustion 

When hydrocarbons such as gasoline, methane, and sucrose are burned (combusted), they always produce energy, carbon dioxide gas, and water vapor. The act of burning is actually just a rapid reaction with oxygen. 

E.g. methane gas is burned (combusted).
CH₄(g) + 2O₂(g) →CO₂(g) + 2H₂O(g) +Energy

A Final Note...
There are other categories of chemical reactions. The two broadest categories are the acid-base reactions and oxidation-reduction reactions.
You will learn more about the former in the following units, and the latter in Chemistry 30.