Synthesis is one of the four main types of chemical reactions, and it occurs when two or more substances -- either elements or compounds -- combine to yield a new compound. That means the reaction involves more than one reactant and generally just one product that contains each element from the reactants. Many significant chemical reactions are synthesis reactions.
Synthesis of Metal Oxides
One important synthesis reaction that occurs in nature is that of a metal and an oxygen molecule to form a metal oxide. This reaction is also an oxidation reaction and is the first step in the corrosion of a metal. Since oxygen is a natural component of air, it reacts with the top surface of metals to form a new layer of metal oxide. To combat this phenomenon, some materials are made with a protective layer of metal oxide already coating the surface. One example of the synthesis of a metal oxide is 2Mg + O2 -> 2MgO, in which magnesium reacts with oxygen to produce magnesium oxide.
Synthesis of Metallic Hydroxides
The second step in the corrosion process is also a synthesis reaction. In this step, the metal oxide reacts with water to form a metallic hydroxide. The most well-known reaction of this type is the formation of rust. After iron has reacted with an oxygen molecule, the newly formed iron oxide reacts with water to yield hydrated iron oxide, which is another name for rust. Another example is the reaction of magnesium oxide with water to form magnesium hydroxide, which is given by the equation MgO + H2O -> Mg(OH)2.
Synthesis of Salts
Salts are ionic compounds formed when an element from the far left of the periodic table -- the main group of metals -- combine with nonmetals from the far right of the periodic table. For example, the equation 2Na + Cl2 -> 2NaCl represents the reaction of sodium and chloride to form the salt sodium chloride. This reaction may take place when sodium is in its solid state with chlorine gas, but a similar reaction occurs when sodium and chlorine are dissolved in water. In this case, the reactants are ions, and the equation is Na+ + Cl- -> NaCl.
Synthesis of Ammonia
The synthesis of ammonia is a significant reaction whose equation is N2 + 3H2 -> NH3. Ammonia is important because it is used in fertilizer, but the synthesis reaction was developed by Fritz Haber -- which is why it is also known as the Haber Process -- during World War II to manufacture explosives. Haber was a German chemist with the dubious title, "Father of Chemical Warfare." The Haber Process must be carried out at high pressure and temperature and requires a catalyst, a substance that increases the rate of a reaction without actually being a reactant itself.