Chemical reactions in which energy is absorbed
are endothermic. Energy is required for the reaction to occur. The energy absorbed is often heat energy or electrical energy.
Adding electrical energy to metal oxides can separate them into the pure metal and oxygen. Adding electrical energy to sodium
chloride can cause the table salt to break into its original sodium and chlorine parts.
Chemical reactions in which energy is released
are exothermic. The energy that is released was originally stored in the chemical bonds of the reactants. Often the heat given
off causes the product(s) to feel hot. Any reaction that involves combustion (burning) is an exothermic chemical reaction.
However, they may also be divided into 7 parts:
Synthesis: Two or more simple compounds combine to form a more
Single-replacement: Only one element switches places with an element from
Double-replacement: Onions and cations from different molecules switch places
and form two completely different compounds.
Combustion: Oxygen combines with another compound to form water and CO2.
[Exothermic: produce heat]
Decomposition: Complex molecule breaks down into smaller ones.
Acid-Base: When an acid and a base are mixted together in the exact proportions
to form a neutral substance. [salt]
Precipitation: When two soluble substances are mixted together
to form an insoluble compound which settles out as a solid [precipitate]