What is the difference between synthesis and decomposition reactions?

2 Answers
May 18, 2018

Well, they are opposites of each other...


Synthesis reactions are chemical reactions where two elements combine to make a product. The newly formed product is always a compound, while the reactants can be elements or compounds.

An example is the burning of charcoal (carbon) in excess oxygen to make carbon dioxide.


Decomposition reactions are chemical reactions where a reactant produces another product, usually two or more. This is when the bonds in the compound are broken apart to form new compounds. Usually, these reactions are endothermic, as heat is used to break apart those chemical bonds.

The one reaction that always comes to my mind when I hear about decomposition reactions is the thermal decomposition of limestone.


May 18, 2018

Synthesis forms conmpex molecules but decomposition reactions form simpler molecules at the end


Example photosynthesis:
#6CO_2 + 6H_2O -> C_6H_12O_6 + 6O_2#

As you might notice glucose (#C_6H_12O_6#) is a complex molecule formed after the reaction.

On the other hand decomposition reactions provide simpler molecules:

Example: Ferric hydroxide udecomposes in the presence of heat (therefore endothermic reaction), giving ferric oxide and water at the end:

#2Fe(OH)_3 -> Fe_2O_3 + H_2O#

In the presence of light, hydrogen peroxide (#H_2O_2#) decomposes into water and oxygen. Note that water and oxyen gas are simpler moleculer compared to the hydrogen peroxide:

#2H_2O_2 -> 2H_2O + O_2#